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Surface creep and slip-behavior segmentation along the northwestern Xianshuihe fault zone of southwestern China determined from decades of fault-crossing short-baseline and short-level surveys
Accepted Manuscript Surface creep and slip-behavior segmentation along the northwestern Xianshuihe fault zone of southwestern China determined from decades of fault-crossing short-baseline and short-level surveys
Jing Zhang, Xue-ze Wen, Jian-ling Cao, Wei Yan, Yong-lin Yang, Qin Su PII: DOI: Reference:
S0040-1951(17)30456-0 doi:10.1016/j.tecto.2017.11.002 TECTO 127668
To appear in:
Tectonophysics
Received date: Revised date: Accepted date:
31 July 2017 31 October 2017 3 November 2017
Please cite this article as: Jing Zhang, Xue-ze Wen, Jian-ling Cao, Wei Yan, Yong-lin Yang, Qin Su , Surface creep and slip-behavior segmentation along the northwestern Xianshuihe fault zone of southwestern China determined from decades of fault-crossing short-baseline and short-level surveys. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Tecto(2017), doi:10.1016/j.tecto.2017.11.002
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ACCEPTED MANUSCRIPT Surface creep and slip-behavior segmentation along the northwestern Xianshuihe fault zone of southwestern China determined from decades of fault-crossing shortbaseline and short-level surveys Jing Zhanga, Xue-ze Wenb,, Jian-ling Caoa, Wei Yanb,c, Yong-lin Yangd, Qin Sud CEA Key Laboratory of Earthquake Prediction (Institute of Earthquake Science, China Earthquake Administration), Beijing 100036, China
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State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China
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China Earthquake Networks Center, Beijing 100045, China
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Institute of Geodetic Engineering, Earthquake Administration of Sichuan Province, Ya’An, Sichuan 625000, China
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Corresponding author at: State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, A1 Huayanli, Chaoyang District, Beijing 100029, China. Tel: +86 10 62009138. E-mail address: [email protected] (X. Wen)
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ACCEPTED MANUSCRIPT ABSTRACT This study examines the ~200-km-long northwestern Xianshuihe fault zone (NWXFZ), southwestern China, using more than three decades of geodetic observations from fault-crossing short-baseline and short-leveling surveys at seven sites. These data enable estimates of creep rates and depths, and examination of the long-term slip behavior. The surface motion of the NWXFZ is dominated by sinistral creep, although sinistral, transverse, and vertical slip components show spatio-temporal
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variations. Combining these slip variations with data of earthquake rupture, coseismic slip, seismicity,
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fault geometry, and far-fault movement velocity, and using the velocity-and-state friction theory, our
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analysis indicates that the surface slip behavior of the NWXFZ is segmented along strike. The 1973 rupture section of this fault zone exhibits spatio-temporally variable slip behavior, showing timedecaying post-1973 afterslip on the northwestern and southeastern parts of the rupture at depths above
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5.87.0 km with average sinistral-creep rates of 1.3 and 3.5 mm/yr, respectively, but being relocked in the central part of the rupture. The 1923/1981 rupture section is generally in locking state, with
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postseismic and interseismic sinistral-creep at 1.1 mm/yr on its central part at depths above 2.0–2.8 km. The 1893 rupture section has been tightly locked without creep since at least the early 1980s. The thickness of the shallow velocity-strengthening (or creep) layer and the restraining bend geometry of
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the NWXFZ are the key factors that control spatio-temporal variations in surface creep rates. Two surface-observed locked fault portions are located within two different restraining bends in the
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NWXFZ, both of which act as compressive asperities and hence have enabled the long-term locking of these portions. Creep along the NWXFZ has also been affected to varying degrees by M6.5Mw9.2
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earthquakes at distances of 503800 km from the fault zone. Most of these effects have been removed
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from our estimates of the long-term average creep rates. Keywords: Fault-crossing short-baseline and short-level surveys; average surface creep rate; creep depth;
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along-strike variable slip behavior; restraining bend fault geometry.
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ACCEPTED MANUSCRIPT 1. Introduction Short-range geodetic surveys across active faults using baseline and leveling arrays, alignment arrays, trilateration networks, and creepmeters have provided highly accurate observations that have enabled the study of various types of near-field surface-fault movements, including pre-seismic slip, inter-seismic creep, co-seismic slip, and dynamically triggered slip, as well as post-seismic slip or creep (e.g., Allen and Smith, 1966; Allen et al., 1972; Burford and Harsh, 1980; Galehouse and Lienkaemper, 2003; Lee et al., 2001;
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Lisowski and Prescott, 1981; Savage, 1979; Sun and Wu, 2007; Thurber and Sessions, 1998; Yu and Liu, 1989;
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Zhang, 1981). These observations can provide key insights into how spatial and temporal variations in fault movements relate to long-term slip behavior along faults, enabling more precise estimates of the future
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seismic potential of individual faults (e.g., Galehouse and Lienkaemper, 2003; Lienkaemper et al., 2013, 2014). In addition, near-fault geodetic surveys can be helpful in monitoring possible earthquake precursors (e.g., Allen and Smith, 1966; Bakun and Lindh, 1985; Sun and Wu, 2007; Thurber and Sessions, 1998). Near-
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fault geodetic surveys have been conducted since the mid-1970s along the NW-trending Xianshuihe fault zone of southwestern China using high-precision, very short-range (lengths of tens of meters to >200 m) repeated
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baseline and leveling measurements at location-fixed fault-crossing sites (Fig. 1). These measurements are usually termed ‘short-baseline’ or ‘short-leveling’ surveys in China, although very few of these studies have been reported in international publications.
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The >300 km Xianshuihe fault zone (XFZ) of southwestern China forms part of the ~1400 km long leftlateral Xiashuihe–Xiajiang fault system that extends from the southern part of Yunnan Province northwest
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through Sichuan Province into Qinghai Province (e.g., Allen et al., 1991; Wang et al., 1998). The fault zone represents a highly active sinistral-slip boundary between the Bayanhar and Sichuan–Yunnan blocks within
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the eastern Tibetan plateau region (Fig. 1; Zhang et al., 2003). The XFZ is divided into northwestern and south–central (or southern) segments by a geometric discontinuity near Bamei. These segments have geologically averaged slip rates of 8–12 and 8–10 mm/yr, respectively (Wen et al., 1990, 1996; Xu et al.,
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2003). At least 10 major earthquakes have occurred along this fault zone during the past ~300 years (Wen et al., 2008). The two most recent major events were the 6 February 1973 M 7.6 Luhuo, Sichuan earthquake and
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the 24 January 1981 M6.9 Daofu, Sichuan earthquake. These events led the Seismological Bureau (now the Earthquake Administration) of Sichuan Province to establish 10 fault-crossing repeat survey sites along the fault zone using short (i.e., spans of tens of meters to >200 m) baseline and leveling arrays. Eight of these sites lie along the northwestern segment of the fault zone, with the remaining two located on the south–central segment (Fig. 1). To date, more than three decades of highly precise observations have been acquired from repeat surveys at these sites. These data represent an important resource for studying the characteristics of the near-field surface movement along active strike-slip faults. The two sites located on the south–central segment of the XFZ at Zheduotang and Tuanjie only have short leveling-arrays that are unsuitable for monitoring the predominantly strike-slip movements within the fault zone. As such, we have restricted this study to the ~200 km long northwestern segment of the XFZ (i.e., the NWXFZ), which is located between the area northwest of the Zhuwo site to the area southeast of the Laoqianning site (Fig. 1). 3
ACCEPTED MANUSCRIPT This study summarizes more than three decades of repeat surveys using fault-crossing short-baseline and short-leveling arrays at seven of the eight sites along the NWXFZ; the Daofu site has been excluded because this site does not span the main fault trace (Fig. 1). Here, we analyze the observed surface-fault movements and estimate the long-term average strike-slip, horizontal transverse, and vertical-motion rates at these sites by considering and reducing as much as possible the effects of local, regional, and even remote earthquakes with various magnitudes and epicentral distances. Combining surface-observed fault motion series data with a
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velocity- and state-dependent friction law enables the determination of creep depths. This study also examines spatio-temporal variations in creep rates and the relationship between these creep rates and historical ruptures,
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coseismic surface slip, fault geometry, microseismicity, and far-field motion velocities to analyze the long-
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term, macroscopic (i.e., geodetically observed rather than experimental or modeled) slip behavior along fault sections that have experienced a variety of ruptures during the last major earthquakes along the NWXFZ. This research provides useful insights into near-field surface creep and how these data can constrain seismogenic
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coupling or locking fraction estimates along the NWXFZ, thereby enabling the determination of the seismic potential of different sections of the NWXFZ. This study also demonstrates how the surface-slip behavior of a
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major active strike-slip fault within a continent plate setting varies both along strike and over time.
Fig. 1. (a) Map showing the Xianshuihe fault zone (XFZ) and the locations of the fault-crossing short-baseline and shortleveling arrays/sites along the fault zone. (b) Map showing the position of the XFZ within the active fault system in southwestern China and the surrounding regions.
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ACCEPTED MANUSCRIPT 2. Data and Methods 2.1 Data and methods of fault-crossing surveying 2.1.1 Data, survey sites, and arrays The data used in this paper are the result of over three decades of repeated surveys using short-baseline and short-leveling arrays across the main fault trace at seven sites along the NWXFZ (Zhuwo, Gelou, Xuxu, Xialatuo, Goupu, Longdengba, and Laoqianning; Figs 1 and 2). Repeated surveying was undertaken at
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irregular intervals of 1–8 months between 1976 and 1987, and surveys since March 1987 have been
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undertaken at regular intervals of every 10 days or less at Xialatuo and every 1–2 months at the other sites. Field surveying and data pre-processing are both undertaken by the Institute of Geodetic Engineering,
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Earthquake Administration of Sichuan Province, China.
Six of the seven sites contain a triangular-shaped survey array that consists of two measuring lines crossing the main fault at different angles and a third that does not cross the fault. Repeated short-baseline
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surveys are conducted along the lines crossing the fault and repeated short-leveling surveys are undertaken along all three lines. The array at Longdengba has three measuring lines that cross the fault (Fig. 2, also see
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Figure s1 in Supplementary Material 1 online). Each measuring line within a given array has two endpoints that are separated by several pass points. A measuring line for the short-baseline surveying, any span between pass points, or between an endpoint and an adjacent pass point is generally designed to be ~24 m in length.
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Each endpoint is made of a reinforced concrete monument with a base buried to a depth of 1.50 m below the ground surface. The benchmarks of the baseline and level surveys are set to be the top and waist (near the
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ground surface) of these monuments, respectively (Figure s2 in Supplementary Material 1). Some of the endpoint monuments are built on bedrock, whereas others (and the majority of the pass points) are located on
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firm Quaternary sediments.
2.1.2 Measurement methods and accuracies The fault-crossing geodetic measurements along the NWXFZ use two conventional surveying techniques,
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namely invar-wire baseline measurements and precise leveling. Short-baseline measurements use a 24 m long invar wire (Secrétan, France) to measure the horizontal distance between two endpoints on either side of the
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fault following the technical procedures and accuracy requirements outlined in the Specifications of Crossfault Geodetic Measurement (State Seismological Bureau, 1991). Distance variations are monitored via repeated baseline measurements, and the decades of repeated surveys yield uncertainties at the seven sites on these short baseline measurements of 0.50–0.58 10–6 (see Table s1 in Supplementary Material 1 for the uncertainty values). Short-level measurements use a leveling instrument to measure the height difference between endpoints on different sides of the fault using the accuracy requirements outlined in the Specifications of Cross-fault Geodetic Measurement (State Seismological Bureau, 1991), which are the same as for the first-order leveling surveys. Variations in height differences are repeatedly monitored and repeated measurements at the seven sites yield uncertainties of 0.13–0.25 mm (see Table s1 in Supplementary Material 1).
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Fig. 2. General topographic features and geological setting of the seven short-baseline and short-leveling cross-fault survey sites along the northwestern Xianshuihe fault zone (NWXFZ). North is towards the top of each map, the left 6
ACCEPTED MANUSCRIPT column shows digital elevation model (DEM) images obtained using the GeoTaos software package and its internal DEM dataset (https://staff.aist.go.jp/xinglin-lei/Lei00.htm), and the right column shows geological and sketch surveyarray maps of the sites. Geological maps are at a scale of 1200,000 and are from the NGAC Digital Library (http://geodata.ngac.cn/Map/List, last accessed on November 6, 2016), in which major fault traces have been updated according to field investigations by Allen et al. (1991), Li et al. (1997), and the second author of this study. Also see Figures s1 to s3 and Table s1 in Supplementary Material 1 for more details about the survey sites, arrays, and faults.
Accidental errors during surveys were minimized by performing both short-baseline and short-level
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measurements during hours that are favorable for high-quality and steady surveying. The surveying crew
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consists of experienced members of the Institute of Survey Engineering, Earthquake Administration of Sichuan Province, China. The presence of clear variations at a given site on successive measurement days
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requires a field or on-site investigation to confirm whether the observed variation is independent of possible environmental changes and meteorological factors, as house building, excavation or landfilling, and extreme
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rainfall or flooding events at or near the site can cause these variations; alternatively, a resurvey is conducted. The survey crew also checks observational data, the invar wire, and the leveling instrument and its staff during surveys to ensure that equipment readings accurately represent measured variations. These procedures
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mean that any measured variations that might be caused by non-tectonic factors of unknown origin are noted so that the reliability of observations can be properly assessed after the survey.
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2.2 Formulas for calculating fault-movement components For each of the survey arrays of two fault-crossing measured lines as shown in Fig.2, fault-movement
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variations in the three components, A (i.e., strike-slip), B (i.e., horizontal transverse), and C (i.e., vertical),
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D1 sin 2 D2 sin 1 sin 1 cos 2 cos 1 sin 2
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D1 A cos 1 sin 1
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1 H1 H 2 2 ,
(1)
(2) (3)
are the clockwise rotation angles from baselines 1 and 2 to the fault, respectively; and D1
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where 1 and
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between successive measurement days are calculated trigonometrically from the formulae (Bo et al,1998):
and D2 are variations in the lengths of baselines 1 and 2, respectively, between successive measurements. A values of >0 suggest that variations in the strike-slip component are sinistral, whereas A values of <0 are indicative of dextral motion. In addition, B values of >0 represent extension in the horizontal transverse direction and B values of <0 are indicative of shortening in this direction. The
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equation (3) denote changes in height differences between the endpoints of leveling lines 1 and 2, respectively, both of which span the fault, meaning that C values of >0 are indicative of the upward movement of the fault’s footwall (i.e., a normal faulting component) and C values of <0 are indicative of the downward movement of the footwall (i.e., a reverse faulting component). 7
ACCEPTED MANUSCRIPT 2.3 Methods for estimating average fault movement rates Analyzing the observed time series of cumulative fault movements in the three component directions outlined above enables an estimate of the average rate of each movement component at every site over various time periods (see section 3 for site-by-site details). This includes a long-term average rate computed for relatively long periods during which earthquake effects were weak or relatively insignificant (see subsection 2.4 for the criteria and approach used for determining the length of this period). Average rates are
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estimated using two methods, as follows. (1) Simple averages for all three movement components are computed by dividing the total displacement
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by the total time using the approaches of Galehouse and Lienkaemper (2003) and Lienkaemper et al. (2014).
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(2) Non-linear averages are used that can only be computed for strike-slip components. Previous research determined that the observed cumulative sinistral-movement of the NWXFZ at the Xialatuo site is logarithmic increasing over time (Allen et al., 1991; Du et al., 2010), indicating that these movements have temporal
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characteristics that follow velocity- and state-dependent friction laws (Kato et al., 2007). Consequently, we used a non-linear method developed from velocity-state friction theory to estimate average sinistral-creep
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rates for all sites, enabling the identification of variation trends in sinistral movement rates and the related creep depths at each site (see section 4). The first step in this process is to fit the surface-observed sinistralcreep series for each site by inverting equation (4) using the Levenberg–Marquardt algorithm to obtain the
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best-fitting parameters , , γ, and v0 as follows:
(4)
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U t ln t 1 v0t
Equation (4) is rewritten from an equation derived from the velocity- and state-dependent friction laws of
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Scholz (1990) and Marone et al. (1991). Here, U is the cumulative slow-slip/creep (in mm) and t is the time (in days) elapsed since the last earthquake. The parameter is associated with friction, effective normal stress, the thickness of the shallow velocity-strengthening layer, and the shear modulus (Marone et al., 1991). The
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physical meanings of the other parameters (, γ, and v0) are outlined by Scholz (1990) and Marone et al.
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(1991). This enables the estimation of average sinistral-creep rates CR for each site, as follows:
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U t 2 U t1 , t2 t1
(5)
where Ut1 and Ut2 (in mm) are displacements accumulated by times t1 and t2, respectively, and are calculated from a specific equation (4) that is fitted for each site (see Table 1 in section 3).
2.4 Reduction of earthquake effects on estimated movement rates Earthquakes can affect geodetically observed rates of fault movement (e.g., Galehouse, 1990; Galehouse and Lienkaemper, 2003; Lienkaemper et al., 1997, 2001, 2014) and anomalous creep events occur when the shallow creeping zone of a fault responses to external stress perturbations caused by earthquakes (e.g., Kanu and Johnson, 2011; Wei et al., 2015). Wei et al. (2015) used observations and simulations to suggest that the triggering of creep events on the Superstition Hills fault of southern California occurred during Mw>7 8
ACCEPTED MANUSCRIPT earthquakes within 150 km of the fault, Mw>6 earthquakes within 80 km of the fault, and Mw>5 earthquakes within 20 km of the fault, with a stress change amplitude of around 0.6 MPa required to trigger a surface slip event of 0.01 mm. Since estimated average creep rates from near-fault geodetic observation should be representative of the long-term mean of stable surface sliding along active faults and useful in assessing fault slip behavior and seismic potential, it is necessary to exclude or remove wherever possible significant effects of earthquakes from the observed fault motions (Galehouse and Lienkaemper, 2003; Lienkaemper et al., 1997,
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Lienkaemper et al., 2014).
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Fig. 3. Map showing epicentral locations (black stars) and focal mechanisms of nine earthquakes that have to some degree affected geodetically observed movement of the NWXFZ. The approximate geometric center of the NWXFZ is shown as an open star, and distances between this location and the epicenters of individual earthquakes can be assessed using the dashed concentric circles. Focal mechanism solutions are from the Global Centroid Moment Tensor Catalog (www.globalcmt.org/CMTsearch.html, last accessed on October 15, 2014) barring the 1989 M6.5 event, for which the mechanism solution is from the Seismological Bureau of Sichuan Province (1993). See Figs 4 to 6 and Supplementary Material 3 for the effects of these nine earthquakes on the observed movement series of the NWXFZ.
Our analysis indicates that at least nine earthquakes between the early 1980s and the middle of 2014 affected the observed movements along the NWXFZ. These include two Mw9.0–9.2 megathrust events at epicentral distances of ~3000 and ~4000 km, and seven M6.5–8.1 events at epicentral distances of ~50 to
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ACCEPTED MANUSCRIPT ~1400 km (Fig. 3). Here, we determine creep responses to these earthquakes at the seven sites along the NWXFZ using the following lines of evidence and experiential criteria. (1) Direct and indirect evidences from relevant geodetic studies and dynamic simulation modeling, which may demonstrate that displacements or stress variations related to one or more of the nine earthquakes in Fig. 3 have reached or affected the NWXFZ already (e.g., Cai et al., 2012; Liu et al., 2014; Papadimitriou et al., 2004; Wang et al., 2013; Zhang et al., 2013).
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(2) Anomalous sudden, rapid, or transient variations in the amount or rate of fault movement within one or more observed time series that are temporally correlated with a single earthquake event but are also
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independent of any meteorological effects (e.g., Allen et al., 1972; Lienkaemper et al., 2001, 2014; McGill et
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al., 1989; Titus et al., 2006; Wei et al., 2011, 2015).
(3) The observed creep response on a fault to an earthquake may persist for years. This is exemplified by the creep response of the southern Hayward fault to the 1989 Loma Prieta, California, earthquake, where the
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nearly zero creep that appeared immediately following this earthquake was followed by a rapid and significant creep event over the next several years before the creep along the fault returned to a steady state (Galehouse, 1990; Kanu and Johnson, 2011; Lienkaemper et al., 1997, 2001).
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A supplementary investigation was undertaken by comparing monthly rainfall and average temperature time series data from two local meteorological stations with observed fault-movement series from four of the
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seven sites along the NWXFZ. These data are given in Supplementary Material 2 and suggest that meteorological factors, including rare rainfall events and extreme temperature fluctuations, have insignificant
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effects on observed fault movements (unlike the nine earthquakes shown in Fig. 3) and are therefore considered negligible in the majority of cases.
We minimized or remove significant effects of the nine earthquakes outlined above on the long-term
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average rate estimates of three component movements of the NWXFZ by using the following approach: For every motion component of these sites, we estimate an long-term average rate for a selected and relatively
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long period, during which the earthquake effects were insignificant or can be excluded; in most cases, such periods were before the Wenchuan, China, earthquake of May 12, 2008. This means that any significant anomalous fault motions that appeared within these selected periods and are related to the earthquakes (Fig.3)
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and extreme weather events, or to unknown causes, have been excluded from the calculation of long-term average rates of the observed fault movement (also see Supplementary Materials 2 and 3).
3. Fault movement series and average rates This section focuses on the analysis of surface-observed fault movement time series from the repeated fault-crossing surveys over a period of more than three decades. These series data were used to estimate the long-term average rates of the three components of movement along the NWXFZ at the seven sites, using the methods outlined in subsection 2.3 in addition to excluding the significant effects of earthquakes, extreme weather events, and unknown causes based on the relevant criteria and approaches described in subsection 2.4.
3.1 Zhuwo Site 10
ACCEPTED MANUSCRIPT The Zhuwo site spans the northwestern part of the surface rupture created by the 1973 M7.6 Luhuo earthquake in Sichuan Province (Figs 1 and 2). The three components of surface fault movement observed at this site during the ~34 years from June 1980 to June 2014 are shown as curves labeled zw-s, zw-t, and zw-v in Fig. 4. The cumulative sinistral-creep rate decays over time and can be fitted well by a fitting curve based on Equation (4) (Table 1). The vertical component series also has similar time-decaying characteristics. Such characteristics suggest that the observed surface fault movement at this site represents the afterslip of the 1973
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rupture (i.e., post-1973 creep) that suggests to a great extent the existence of velocity-strengthening friction behavior (Kato et al., 2007; Marone et al., 1991; Scholz, 1990).
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Applying the lines of evidence, experiential criteria and approaches summarized in subsection 2.4 allows
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the identification of the effects of significant earthquakes on the observed fault movement at the Zhuwo site, as indicated by the zw-s, -t, and -v curves in Fig. 4 (also see Supplementary Material 3 for the identification). Our analysis suggests that the observed sinistral-creep component of fault movements at this site were
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insignificantly influenced by earthquakes for the ~26 years between June 1982 and May 2008, whereas the transverse component was insignificantly influenced for the 18.5 years between May 1983 and November 2001. Almost no earthquake effects are apparent in the vertical component series obtained over the 34 years
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between June 1980 and June 2014. This yields average creep rates for sinistral, transverse, and vertical components of 1.26, 0.08, and 0.71 mm/yr, respectively, for these corresponding time periods. This result
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means that the observed fault movements at the Zhuwo site are dominated by sinistral creep, which is accompanied with a clear normal faulting component (positive vertical motion rate of 0.71 mm/yr that is
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indicative of the relative uplift of the footwall of the fault, in this case the southwestern block of the fault) and a very slight horizontal–transverse stretching component (of 0.08 mm/yr) in a SWNE direction. This normal faulting is likely associated with the existence of an active NE–SW trending horst in the southwestern block
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of the NWXFZ at the Zhuwo site (see a-2 in Fig. 2 and Profile a in Figure s3 in Supplementary Material 1).
3.2 Gelou Site
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The Gelou site crosses the central part of the of the 1973 surface rupture zone (Figs 1 and 2). Faultcrossing short-baseline surveying started at this location in May 1982 with short-level surveying beginning in
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January 1986. The observed fault movement at this site is quite slow, but still involves time-decaying sinistral creep accompanied by even slower to near-zero vertical and horizontal transverse motions (gl-s, -t, and -v in Fig. 4). Applying the lines of evidence, criteria, and approaches outlined in subsection 2.4 indicates that the observed fault motion at this site before 2008 was unaffected by earthquakes or other factors barring a dextral slip event of –2.3 mm with an unknown cause between March and May 1987, which was excluded from our average rate estimates. However, the fault movement of this site has been significantly affected by 34 earthquakes between 2008 and the present day, including the 2008 M8.0 Wenchuan earthquake, the 2011 Mw9.0 Tohoku-Oki, Japan earthquake, the 2013 M7.0 Lushan, China earthquake, and perhaps the 2010 M7.1 Yushu, China earthquake (Fig. 4; for a detailed analysis of the earthquake effects, see Supplementary Material 3). These indicate that the sinistral and transverse components of fault motion at the Gelou site were least influenced by earthquakes during the ~26 years between June 1982 and May 2008, with the vertical 11
ACCEPTED MANUSCRIPT component being least influenced during the ~22 years between June 1986 and May 2008. The resulting estimates of long-term average motion rates for these three components during these periods are 0.09, 0.02, and –0.13 mm/yr, respectively, suggesting that the fault at this site has almost entirely re-locked during the
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period after the 1973 M7.6 Luhuo earthquake.
Fig. 4. Time series of surface displacements along the NWXFZ at the Zhuwo, Gelou, and Xuxu sites. These displacements were calculated using equations (1)–(3) and observations from the fault-crossing short-baseline and short-leveling surveys. Round, square and triangular symbols represent individual measurements of strike-slip, horizontal transverse, and vertical components, respectively, and all measurements are shown in the diagram. Positive strike-slip movements represent faultparallel sinistral-slip, whereas positive transverse movements indicate fault-normal extension, and positive vertical movements denote relative upward motion of footwall block. Earthquake dates are shown as dotted lines and the significant effects of earthquakes and unknown causes on these observed series are labeled and excluded from average rate estimates, with corresponding analyses given in Supplementary Material 3. Equation (4) was used to fit sinistral-creep series and to estimate the non-linear rates of this motion component. Red solid curves show the fitting of the data over the 12
ACCEPTED MANUSCRIPT fitting periods and dashed red curves represent inferred parts from the fitted curves.
3.3 Xuxu site The Xuxu site also spans the central part of the 1973 surface rupture zone (Figs 1 and 2). Short-leveling and short-baseline surveys commenced here in May and June 1980, respectively. The observed fault movement at this site before May 2008 consisted of very slow sinistral creep with minor amounts of reverse faulting that caused the footwall of the fault (i.e., the southwestern block of the fault) to move down relative
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to the hangingwall. However, the strike-slip and transverse components of movement have been significantly
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affected by earthquakes since May 2008 and since November 2001, respectively, although the vertical component has remained generally unaffected during the entirety of the 34 year measuring period (gl-s, -t, and
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-v in Fig. 4). The lines of evidence, criteria, and approaches outlined in subsection 2.4 were used to identify and exclude significant earthquake effects in these data (for a detailed analysis of these effects, see
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Supplementary Material 3), yielding estimated average fault movement rates of 0.16 mm/yr for the sinistral component for the ~28 years before May 2008, –0.15 mm/yr for the transverse motion for the ~21 years before November 2001, and –0.09 mm/yr for the vertical component for the entire 34.1 year measuring period.
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These results suggest that the fault at the Xuxu site is also nearly completely relocked. Table 1. Results of curve fitting, based on Equation (4), of surface-observed sinistral-creep series at all sites
Sites
fitting
Fitting equation, parameters and root-mean-square errors (RMS)
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Selected periods for
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(y m d―y m d)
U t ln t 1 v0t
RMS (mm)
1982/06/18―2008/05/07
U(t)=50.123ln(0.010/50.123t+1)24.7440.0006t
0.20
Geluo
1982/08/06—2008/05/07
U(t)= 4.376ln(16.894/4.376t+1)40.7410.0001t
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Xuxu
1980/06/13—2008/05/08
U(t)= 0.622ln(-3.049E-05/0.622t+1)1.3682+0.0005t
0.14
Xialatuo
1976/06/18—2014/07/05
U(t)= 43.519ln(0.114/43.519t+1)66.8088+0.0024t
0.66
Goupu
1983/05/20—2013/03/12
U(t)= 6.921ln(1004.503/6.921t+1)80.6284+0.0013t
0.76
Longdengba
1988/05/31—2008/05/09
U(t)= 0.7ln(-1.306E-05/0.7t+1)5.3335+0.0002t
0.12
Laoqianning
1985/01/28—2001/11/11
U(t)= 0.413ln(-1.208E-10/0.413t+1)13.8211+0.0004t
0.26
AC
CE
PT
Zhuwo
Notes: The fitting root-mean-square (RMS) error
N
RMS
(Ui i 1
obs
Ui fit ) 2
N
. See subsection 2.3 for the fitting method.
3.4 Xialatuo site The Xialatuo site has been surveyed since 1976 and spans the southeastern part of the 1973 surface rupture zone as well as the northwestern-most part of the surface rupture of the 1923 M7.3 Luhou–Daofu earthquake (Figs 1 and 2). The observed fault movement at this site is dominated by relatively fast sinistral creep accompanied by NE-dipping normal faulting (xlt-s, -t, and -v in Fig. 5). The sinistral-creep displacement logarithmically increases over time, indicating that this displacement relates to afterslip associated with the 1973 event (Allen et al., 1991; Du et al., 2010; Kato et al., 2007). The observational 13
ACCEPTED MANUSCRIPT dataset of this site spans a longer period than those used in previous research, allowing that the sinistral-creep series can be well fitted to a curve using Equation (4). The fitting curve indicates that the observed fault movements at this site do represent surface afterslip associated with the 1973 M 7.6 Luhuo earthquake (xlt-s
AC
CE
PT
ED
M
AN
US
CR
IP
T
in Fig. 5; also see Table 1).
Fig. 5. Time series of surface displacements along the NWXFZ at the Xialatuo and Goupu sites. The symbol explanation is as for Fig.4. All measurements are shown. The good fit of the red curves modeled using Equation (4) to the data indicate that the observed dominant fault movement is sinistral creep. In addition, the movement at these sites most likely represents post-seismic slip associated with the 1973 and 1981 earthquakes, with interseismic creep most likely also present at the Goupu site after 2000. See Table 1 for the results of curve fitting.
14
ACCEPTED MANUSCRIPT
The average movement rate of this site excludes a rapid ~1.5 mm transverse shortening during July 1998 (xls-t in Fig. 5) because this may relate to either an extreme episode of rainfall in July 1998 after a four-year drought or may be associated with the M7.5 Manyi, China earthquake of November 8 1997 (see Figure s4 and the discussion in Supplementary Material 2). However, we do not exclude probable responses to earthquakes (e.g., pulses, jumps, and rate variations) in the strike-slip series as these responses are too small in amplitude to notably affect the sinistral-creep motion at this site. In addition, these small responses usually self-recover
T
soon after they occur. Finally, the effects of earthquakes on the vertical component of this site are negligible
IP
and require no exclusions (xlt-s and -v in Fig. 5).
CR
The analysis outlined above indicates that earthquakes had minor or negligible effects on the three components of fault movement at the Xialatuo site during the ~38 years between June 1976 and July 2014, barring a rapid transverse shortening of ~1.5 mm in July 1998 that was removed from our rate calculation.
US
This yields average creep rates at this site, for the entire ~38-year period, of 3.47, 0.32, and 0.47 mm/yr for
AC
CE
PT
ED
M
AN
the sinistral, transverse, and vertical components, respectively.
Fig. 6. Photographs of visible surface creep along the NWXZ at Xialatuo, taken on July 11, 2013. (a) View to the NNW of a concrete pavilion located astride the main fault at the Xialatuo site, which was installed in 1984 (also see Figure s1 in Supplementary Material 1 for the location of the pavilion). (b) and (c) Views to the W and SSE, respectively, of sinistral offsets as a result of fault creep along the gap-line (the main fault trace) between two concrete pillars in the pavilion. The entirety of the floor of the two parts of the pavilion, including the gap-line, was covered by a thin cement plaster layer prior to the 1990s that was later destroyed.
Visible surface creep of the NWXZ has also been identified at the Xialatuo site, at the location of a concrete pavilion installed as a memorial to the victims of the 1973 M7.6 earthquake by the former
15
ACCEPTED MANUSCRIPT Seismological Bureau of Sichuan Province in 1984 (see Fig. 2 for the location of this pavilion). The pavilion was made of two separate but symmetrical parts that were each placed on a different side of the 1973 main surface rupture (Fig. 6a). In 1986, minor en echelon cracks appeared on the thin cement plaster floor of the pavilion along the gap line (i.e., the position of the main fault) as a result of fault creep between the two parts of the base of the monument (Allen et al., 1991). The cumulative offset of this sinistral-creep along the gap line was measured to be ~4 cm in November 2007 (Du et al., 2010) but appeared to be larger during a field
T
visit to the site in the summer of 2013 (Fig. 6b and c).
IP
3.5 Goupu site
CR
The Goupu site spans the central portion of the surface rupture zone of the 1981 M6.9 Daofu, Sichuan, earthquake, which occurred along the NWXFZ (Fig. 1). Fault-crossing surveys began at this location in May 1983, and the observed fault movement is dominated by sinistral creep combined with a smaller component of
US
reverse faulting that is evident as a result of negative but minor vertical and transverse components (gp-s, -t, and -v in Fig. 5). This location is characterized by initially fast sinistral creep that gradually decelerated over
AN
time before reaching a relatively steady rate in ~2000 that has continued to the present day. The sinistral creep series can be well fitted to a curve using Equation (4), indicating that the motion of this component is associated with post-1981 afterslip and subsequent interseismic creep (gp-s in Fig. 5 and Table 1).
M
Applying the lines of evidence, criteria, and approaches outlined in subsection subsection 2.4 enables an analysis of earthquake effects on observed fault motion at the Goupu site, suggesting that of the nine events
ED
shown in Fig. 3, the M7.0 Lushan, Sichuan, earthquake of April 20, 2013 had the greatest effect on the observed fault movement at this site (see the corresponding analysis given in Supplementary Material 3). This
PT
yields estimated average sinistral, transverse, and vertical component creep rates of 1.09, −0.35, and −0.40 mm/yr, respectively, for the site during the ~30 years between May 1983 and March 2013.
CE
3.6 Longdengba Site
The Longdengba site crosses the south–central part of the surface rupture zone of the 29 August 1893 M≥7 south Daofu earthquake, which occurred along the NWXFZ between Laoqianning and the southeastern
AC
part of Daofu (Allen et al., 1991; Li et al., 1998; Wen et al., 2008; Fig. 1). Fault-crossing surveys have been undertaken here since 1985, yielding maximum sinistral, transverse, and vertical component cumulative displacements of 1.25, 3.00, and 0.73 mm, respectively (ldb-s, t, and v in Fig. 7). These small cumulative displacements suggest that even minor motion variations associated with earthquake effects would impact the estimates of long-term average rates at this site. The observed surface fault motion at the Longdengba site has been affected by a number of earthquakes, as follows: the 1981 M6.9 Daofu earthquake within the central NWXFZ affected strike-slip and vertical components before May and before January 1986, respectively, whereas the Wenchuan earthquake of May 2008 and the Tohoku earthquake of March 2011 have affected strike-slip and transverse components, respectively (ldb-s, -t, and -v in Fig. 7; see the corresponding analysis in Supplementary Material 3). As such,
16
ACCEPTED MANUSCRIPT average motion rates are estimated to be 0.06 mm/yr for sinistral-creep between February 1986 and May 2008, 0.10 mm/yr for transverse motion between May 1985 and January 2011, and –0.03 mm/yr for vertical motion between January 1986 and June 2014. These data indicate that the fault at this site has been tightly locked for
CE
PT
ED
M
AN
US
CR
IP
T
at least the last three decades.
Fig. 7. Time series of surface displacements along the NWXFZ at the Longdengba and Laoqianning sites. The symbol
AC
explanation is as for Fig.4. All measurements are shown. The very slow sinistral movements at these sites can be well fitted to a curve using Equation (4) too, yielding the red solid fitting curves. Also see Table 1 for the fitting results.
3.7 Laoqianning site The Laoqianning site is close to the southeastern-most end of the NWXFZ and probably crosses the surface rupture of the 1893 M≥7 earthquake (Fig. 1), which was discovered ~2 km northwest of the site (Allen et al., 1991). Fault-crossing surveys have been undertaken here since 1979, yielding total cumulative sinistral, transverse, and vertical displacements of 2.07, 2.01, and 2.88 mm, respectively (lqn-s, -t, and -v in Fig. 7). Applying the lines of evidence, criteria, and approaches outlined in subsection 2.4 suggests that the fault
17
ACCEPTED MANUSCRIPT movement at this site has been significantly affected by earthquakes. The 1981 Daofu M6.9 earthquake significantly impacted the strike-slip component, as well as having less significant impacts on transverse and vertical components in measurements taken before February 1985. The six earthquakes since November 2001 have significantly affected strike-slip movements at this site (see the corresponding analysis in Supplementary Material 3). Taking these effects into consideration, we determined average movement rates at this site to be 0.15 mm/yr for the sinistral creep component between January 28, 1985 and November 11, 2001, –0.06
T
mm/yr for the transverse component between September 8, 1979 and November 11, 2001, and 0.08 mm/yr for the vertical component between September 7, 1979 and May 11, 2014. These low motion rates indicate that
IP
the fault at the Laoqianning site has been tightly locked for at least the last 30 years.
CR
3.8 Estimated average creep rates
The analyses outlined above enable the determination of movement rates for various periods for the three components at the seven sites of the NWXFZ using the methods described in subsection 2.3; these results are
US
summarized in Table 2. Long-term average rates (shown in bold in Table 2) have been estimated over those selected and relatively long periods (i.e., decades) in which earthquake effects, or (in rare cases) weather and
AN
unknown-causal effects, on the observed fault motion were either insignificant (have been ignored) or excludable (have been removed from the long-term average rates), as analyzed in subsections 3.1 to 3.7 and in Supplementary Materials 2 and 3.
M
The data given in Table 2 indicate that the average strike-creep rates estimated using two different methods, i.e., the simple average and the nonlinear average, respectively, are similar for the relatively long
ED
periods shown in bold digits for each site, but are either somewhat or markedly different for relatively short periods associated with earthquake effects. The latter include strike-slip creep estimates for the Laoqianning
PT
site for periods after November 2001 and the Gelou and Xuxu sites for periods after May 2008. In addition, strike-slip creep rates estimated using the nonlinear method are more steady and predictable than those estimated by the simple average method (Table 2; Figs 4, 5, and 7). This indicates that the effects of
CE
earthquakes need to be determined and minimized (as much as possible) during estimates of the long-term average rate of fault movement using fault-crossing geodetic observations. Furthermore, Figs 4, 5, and 7, as
AC
well as the corresponding analyses presented in subsections 3.1–3.7, indicate that both sinistral and transverse fault motion components are relatively sensitive to earthquake effects, whereas vertical components appear to be much less sensitive.
18
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Table 2. Fault-crossing short-baseline and short-leveling survey-based estimated creep rates along the NWXFZ for different periods at seven sites Strike-slip component Sites
Horizontal transverse component
Vertical component
Cumulative Simple Non-linear Cumulative Simple Cumulative Simple Periods Periods sinistral slip average rate average rate movement average rate movement average rate ymd―ymd ymd―ymd (mm) (mm/a) (mm/yr) (mm) (mm/yr) (mm) (mm/yr) 1980/06/14―1981/04/30 1.6±0.22 1.83±0.25 2.17±0.41 1980/06/26―1989/07/19 9.84±0.24 1.08±0.03 1980/06/14―1983/05/03 -3.43±0.18 -1.19±0.06 1982/06/18―2001/11/08 26.49±0.22 1.36±0.01 1.38±0.02 1989/07/19―2001/11/08 7.80±0.24 0.63±0.02 1983/05/03―2001/11/08 1.39±0.18 0.08±0.01 2001/12/17―2008/05/07 4.79±0.22 0.75±0.03 0.89±0.06 2001/12/18―2008/05/07 3.87±0.24 0.60±0.04 Zhuwo 2001/12/17―2008/05/07 2.78±0.18 0.43±0.03 2008/06/11―2014/06/10 3.45±0.22 0.57±0.04 0.75±0.06 2008/06/11―2014/06/10 2.64±0.24 0.44±0.04 2008/06/11―2014/06/10 3.72±0.18 0.62±0.03 1982/06/18―2008/05/07 1980/06/26―2014/06/10 31.95±0.22 1.23±0.01 1.26±0.01 24.05±0.24 0.71±0.01 1982//08/06—1987/03/12 2.3±0.13 0.5±0.03 0.34±0.07 1986/01/15—2008/05/07 1982/08/06—2008/05/07 0.49±0.06 0.02±0.00 -2.92±0.13 -0.13±0.01 1982/08/06—2008/05/07 5.33±0.13 0.21±0.01 0.15±0.01 Gelou 2008/06/11—2011/03/09 0.22±0.06 0.08±0.02 2008/05/07—2011/03/09 0.10±0.13 0.04±0.05 2008/06/11—2013/01/08 0.73±0.13 0.16±0.03 0.08±0.07 2011/03/09—2013/06/08 -2.81±0.06 -1.25±0.02 2011/03/09—2013/06/08 1.04±0.13 0.46±0.04 2013/01/08—2013/06/08 3.85±0.13 9.31±0.09 0.07±0.22 1980/06/13—2008/05/08 4.43±0.17 0.16±0.01 0.16±0.01 1980/06/13—2001/11/09 -3.35±0.42 -0.15±0.02 Xuxu 1980/05/07—2014/06/11 -2.99±0.22 -0.09±0.01 2008/06/11—2014/06/12 -1.51±0.17 -0.25±0.03 0.15±0.04 2001/12/18—2014/06/12 7.01±0.42 0.56±0.03 1976/06/18—1980/06/30 30.39±0.20 7.53±0.05 7.89±0.24 1976/06/18—1980/09/01 2.94±0.39 0.7±0.09 1976/06/20—1980/06/29 5.02±0.15 1.25±0.04 1980/06/30—1997/06/10 62.86±0.20 3.71±0.01 3.71±0.06 1981/04/24—1998/06/10 6.72±0.39 0.39±0.02 1980/06/29—1997/06/15 8.09±0.15 0.48±0.01 Xialatuo 1997/06/20—2008/05/10 25. 88±0.20 2.38±0.02 2.30±0.09 1998/07/20—2008/05/10 2.90±0.39 0.30±0.04 1997/06/15—2008/05/11 2.78±0.15 0.25±0.01 2008/05/20—2014/07/05 10.7±0.20 1.75±0.03 1.98±0.15 2008/05/20—2014/07/05 2.28±0.39 0.37±0.06 2008/05/12—2014/07/06 1.96±0.15 0.32±0.02 1976/06/18—2014/07/05 130.51±0.20 3.43±0.01 3.47±0.02 1976/06/18—2014/07/05 12. 05±0.39 0.32±0.01 1976/06/20—2014/07/06 17.79±0.15 0.47±0.00 1983/05/20—1986/06/14 10.21±0.12 3.32±0.04 2.38±0.35 1983/05/20—1986/06/14 -0.84±0.07 -0.27±0.02 1983/05/19—1986/06/14 -2.64±0.15 -0.86±0.05 1986/06/14—1991/05/18 5.88±0.12 1.19±0.03 1.39±0.22 1986/06/14—1991/05/18 -3.51±0.07 -0.71±0.01 1986/06/14—1991/05/18 -2.42±0.15 -0.49±0.03 Goupu 1991/05/18—2001/11/10 10.9±0.12 1.04±0.01 0.94±0.10 1991/05/18—2001/11/10 -5.73±0.07 -0.55±0.01 1991/05/18—2001/11/10 -4.63±0.15 -0.44±0.01 2001/11/10—2013/03/12 8.61±0.12 0.76±0.01 0.74±0.09 2001/11/10—2013/03/12 -0.32±0.07 -0.03±0.01 2001/11/10—2013/03/12 -2.12±0.15 -0.19±0.01 1983/05/20—2013/03/12 35.59±0.12 1.19±0.00 1.09±0.04 1983/05/20—2013/03/12 -10.4±0.07 -0.35±0.00 1983/05/19—2013/03/12 -11.81±0.15 -0.4±0.01 1985/05/31—1988/05/19 -0.32±0.10 -0.11±0.03 0.06±0.06 1985/05/31—1988/05/19 -0.54±0.10 -0.18±0.03 1985/07/12—1988/05/19 1.60±0.15 0.57±0.05 Longdengba 1986/04/22—2008/05/09 1.62±0.10 0.07±0.00 0.06±0.01 1985/05/31—2011/01/09 2.57±0.10 0.10±0.00 1986/01/30—2014/06/14 -0.78±0.15 -0.03±0.01 2011/01/09—2014/06/13 0.12±0.10 0.03±0.03 0.05±0.06 2011/01/09—2014/06/13 0.42±0.10 0.12±0.03 2011/01/09—2014/06/14 -0.13±0.15 -0.04±0.04 1985/01/28—2001/11/11 2.08±0.31 0.12±0.01 0.15±0.02 1979/09/08—2001/11/11 -1.39±0.13 -0.06±0.01 1979/09/07—2014/05/11 2.88±0.21 0.08±0.01 Laoqianning 2001/12/20—2008/05/09 -0.34±0.31 -0.05±0.05 0.15±0.07 2001/12/20—2008/05/09 1.4±0.13 0.22±0.02 2001/11/11—2008/05/10 1.26±0.21 0.19±0.03 2008/06/12—2014/06/13 -1.6±0.31 -0.27±0.05 0.15±0.08 2008/06/12—2014/06/13 1.9±0.13 0.32±0.02 2008/06/13—2014/06/14 0.47±0.21 0.08±0.03 Notes: Bold values represent selected and relatively long periods and the associated cumulative displacements and long-term average rates for all three components estimated for these periods, in which effects of earthquakes on the observed fault movement were either insignificant or have been excluded. Strike-slip rates are also estimated using a non-linear method based on Equations (4) and (5). The uncertainties of average rates of the three components were estimated by propagating errors associated with the short-baseline and short-leveling measurements and, for non-linear estimated rates, the RMS errors from the fitting to Equation (4) (see Table s1 in the online Supplementary Material 1 for measurement errors and Table 1 for RMS errors). Positive and negative strike-slip rates represent sinistral and dextral creep, respectively, and positive and negative transverse motions represent extension and shortening in a fault-normal direction, respectively, and positive and negative values of vertical motion represent creep associated with normal and reverse faulting components, respectively. The real accuracy of the estimated fault movement rates and their errors should be limited to one digit after the decimal point, rather than to two digits after the point as listed in this table for a result of the calculation. Periods ymd―ymd
T P
I R
C S U
N A
D E
M
T P E
C C
A
19
ACCEPTED MANUSCRIPT 4. Creep depth estimates The surface-observed sinistral-creep series at the seven sites along the NWXFZ (especially at Zhuwo, Xialatuo, and Goupu) have a time-dependent characteristic that manifests as a logarithmic increase in cumulative creep displacement or an exponential decrease in creep rate over time. These series accurately fit curves based on Equation (4) (Figs 4, 6, and 7; Table 1) and therefore represent either earthquake afterslip or interseismic slow slip at shallow depths governed by a velocity-strengthening mechanism (Marone et al., 1991; Scholz, 1990). Similar time-dependent creep series have been reported for other active strike-slip fault systems, including the Ismetpasa segment of the North Anatolian fault, Turkey
SC RI PT
(Bilham et al., 2016; Cakir et al., 2005; Cetin et al., 2014; Kunako et al., 2013), and the central and southernmost segments of the San Andreas fault, California (e.g., Marone et al., 1991; Wei et al., 2015). In this section, we use the surface-observed sinistral-creep series to estimate the depths of fault creep at all sites along the NWXFZ from the approach of Marone et al (1991) based on a velocity- and statedependent friction law. At such depths the slip behavior of the studied fault zone theoretically transits from shallow velocity-strengthening friction to deep velocity-weakening friction.
Surface-observed sinistral-creep series data were fitted to Equation (4) using a nonlinear inversion
NU
approach based on the Levenberg–Marquardt algorithm, yielding the best-fitting values of parameters ,
, γ, and v0 for each site (Table 1). Equation (4) is derived from a velocity- and state-dependent friction
MA
law (Marone et al., 1991; Scholz, 1990), in which =n'(a–b)/G/h, is a function associated with the friction rate parameter (a–b), the effective normal stress n', and the thickness h (in km) of the shallow velocity-strengthening layer of the crust, assuming a constant shear modulus G (Marone et al., 1991). The thickness h is also the depth of shallow fault creep. As such, the relationship curves between (a–b) and h
ED
can be plotted using a rearranged definition of (Fig. 8): (a – b) = ( G)/(n' h)
(6)
PT
and by substituting the inverted values of parameter (from Table 1) into Equation (6) for each of the sites where n' = 1.0 + 13.0 h (in MPa) and the shear modulus G = 30 GPa (after Kaneko et al., 2013).
CE
Laboratory measurements of the friction rate parameter (a–b) range between 0.001 and 0.005 (Marone et al., 1990, 1991) and are shown as the vertical distance between the two horizontal dotted lines in Fig. 8. Chang et al. (2013) presented the results of numerical modeling of the depth-variable fault friction of
AC
aseismic afterslip associated with the 2004 Parkfield, California earthquake based on a multi-layer dynamic model. This research narrowed the range of possible (a–b) values down to ~0.004 to 0.007 for depths of 0–1 km, 0.002 to 0.004 for depths of 1–3 km, and 0.001 to 0.003 for depths of 3–7 km. These new (a–b) values from Chang et al. (2013), which are shown as gray-shadowed rectangles in Fig. 8, were used to estimate the ranges of creep depth h from Equation (6) for the individual sites in the study area, with the results shown in the table insets in Fig. 8. The maximum estimate of h is restricted to 7 km as a result of the distribution model of (a–b) values used during this study. The data shown in Fig. 8 suggest that the estimated depths h of shallow fault creep are highly variable from site to site along the NWXFZ, range between 0.3 and 7 km, and positively correlate with the average surface sinistral-creep rates determined at these sites. For instance, the Xialatuo, Goupu, and Longdengba sites have different creep depths (i.e., h values) of 5.8 to 7, 2.0 to 2.8, and 0.4 to 0.6 km, which
20
ACCEPTED MANUSCRIPT correspond to average sinistral-creep rates of 3.47, 1.09, and 0.06 mm/yr, respectively. Experimental and modeling studies suggest that the maximum values of the depth of shallow creep associated with active faults are restricted by the thickness of the sediment cover (e.g., Marone et al., 1990, 1991; Scholz, 1998; Wei et al., 2013). A seismic profile within the study area suggests that the thickness of poorly consolidated sediments (i.e., the topmost layer of the crust with P-wave velocities of ≤5.6 km/s) in the NWXFZ region varies between 1 and 7 km (Wang et al., 2007), suggesting that the range of the estimated creep depth h in Fig.8 is reasonable and that the distribution model of (a–b) values we used during this study (from Chang et al., 2013) is applicable to the study area. The next section focuses on the probable relationship between
PT
ED
MA
NU
SC RI PT
along-strike variations in shallow-creep depth and surface-observation-based fault creep rates.
Fig. 8. Friction rate parameter (a–b) versus shallow creep depth h for all seven sites along the NWXFZ. Each estimate of h is a range of uncertainty represented by a distance shadowed by a gray rectangular box on a specific depth curve.
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The estimated h values for all sites are listed in the inset tables along with the best estimates of derived from Equation (4) and listed in Table 1, as well as long-term average sinistral-creep rates (Table 2).
AC
5. Spatio-temporal variations in surface creep and influencing factors The long-term average creep rates for all three component directions estimated in section 3 and given in Table 2 are now shown in Fig. 9a. These along-strike distributions of creep rates, combined with the creep time series given in Figs 4, 5, and 7, indicate that the geodetic-observed surface and near-field creep along the NWXFZ is highly spatially and temporally variable. Similar along-strike variations in creep rate have been reported for the Hayward fault in California (e.g., Galehouse and Lienkaemper, 2003; Simpson et al., 2001). Here, we first summarize these spatio-temporal variations in creep rate and their implication for the segmentation of surface slip behavior along the fault zone, and then discuss the probable factors that control these variations by combining additional data such as coseismic surface slip, the elapsed time since previous major earthquakes (Fig. 9b and c), historical ruptures since 1893 (Fig. 9c), relocated earthquake distribution (Fig. 9d and e), and fault geometry (Fig. 9e). The shallow-creep depths estimated in Fig. 8 are 21
ACCEPTED MANUSCRIPT also shown in Fig. 9d.
5.1 Surface creep distribution and segmentation of slip behavior Along the NWXFZ, the southeastern and northwestern parts (or the two peripheral parts) of the 1973 rupture section have been occurring time-decaying sinistral-creep with the largest and second largest average rates, 3.47mm/yr and 1.26mm/yr, respectively, accompanied with much smaller average transverse and vertical motion rates (indicating secondary normal faulting), as observed at the Xialatuo and Zhuwo sites (Fig 9a). The sinistral-creep series at these two sites have similar time-dependent decaying patterns (Figs 4 and 5), a characteristic that follows Equation (4), which is based on a velocity-strengthening friction
SC RI PT
law (Marone et al., 1991), indicating that these sites have been recording the afterslip following the 1973 rupture, which occur within the shallow velocity-strengthening layer of the fault at depths above 6.2–7 km and above 5.8–7 km, respectively (Fig. 8). However, the central part of the 1973 rupture section has recorded little post-1973 afterslip since the early 1980s, despite the fact that very slow time-decaying sinistral creep (at 0.15–0.16 mm/yr) has occurred on this fault part at very shallow depths (above 0.4–1.5 km) (Fig. 8 and Fig. 9d), as observed at the Geluo and Xuxu sites (Fig. 4). This suggests that the central part of the 1973 rupture section has re-coupled rapidly after the rupture, and is now almost relocked.
NU
Overall, the 1973 rupture section of the NWXFZ shows spatially and temporally variable surface slip behavior: long-lasting and time-decaying post-1973 afterslip on the southeastern and northwestern parts but little post-1973 afterslip and rapidly relocking on the central part.
MA
The third largest average rate of surface sinistral-creep along the NWXFZ (1.09 mm/yr at the Goupu site) is located within the central part of the 1923/1981 rupture section, which is accompanied with much smaller average transverse and vertical movement rates that are indicative of a reverse-creep component
ED
(Fig. 9a). The observed sinistral-creep at the Goupu site occurs at a depth above 2.0–2.8 km (Figs 8 and 9d) and has the time-dependent decaying characteristics before 2000 at least (Fig. 5), indicating that this site has recorded the post-1981 afterslip, at least up to the year 2000. The surface sinistral-creep rate also
PT
gradually decreases from northwest to southeast along the ~70km-long 1923/1981 rupture section, from 3.47 mm/yr at Xialatuo to 1.09 mm/yr at Goupu and perhaps even lower farther to the southeast (Fig. 9a).
CE
Length-averaged sinistral-creep rates along the 1973 and 1923/1981 rupture sections of the NWXFZ are <2 mm/yr (Fig. 9a), much smaller than typical average geological slip rates (812 mm/yr; Wen et al.,
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1996) and the average far-field sinistral displacement rate of ~10 mm/yr derived from fault-parallel GPS velocity differences across the same two fault sections (Du et al., 2010; Lü et al., 2003). This indicates that the surface slip behavior along these two NWXFZ sections is generally not completely creeping or completely locked, but instead is in locking state with time-decaying post-seismic creep/afterslip and subsequent likely interseismic creep. This inference is supported by the study of Wang et al. (2009), who made a joint inversion using InSAR and GPS data and yielded an average locking depth of 3–6 km for a portion of the NWXFZ that is roughly equivalent to the total extent of the two NWXFZ sections of the 1973 and 1923/1981 ruptures, outlined in our segmentation model (Fig. 9a). The 1893 rupture section is located within the ~40 km long southeastern-most portion of the NWXFZ and has very slow sinistral creep (0.05 and 0.15 mm/yr) along with very slow transverse and vertical motions at the Longdengba and Laoqianning sites (Figs 7 and 9a). Combining these observations with the modeling results shown in Fig. 8 suggests that this fault section is now completely locked up, at least at
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ACCEPTED MANUSCRIPT very shallow depths (0.3–0.6 km). This completely locked section of the fault formed at least as early as the early 1980s (Fig. 7), some 90 years after the 1893 rupture. This means that on this fault section the transfer of the slip behavior from healing to coupling and finally tight locking was relatively fast, given the size of the 1893 surface co-seismic slip (1.6–2.9 m; Li et al., 1998; Papadimitriou et al., 2004; Fig. 9b) and the farfield sinistral-displacement rate (~8 mm/yr from an average fault-parallel GPS velocity difference across the same NWXFZ section; Lü et al., 2003). The section below discusses possible reasons for the relatively
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rapid interseismic coupling along this fault section.
Fig. 9. Distributions of creep rate, slip behavior, coseismic surface slip, historical ruptures, seismicity, and fault geometry along the NWXFZ. (a) Average creep rates of all three components (from Table 2); the sinistral-creep 23
ACCEPTED MANUSCRIPT rate is shown with the estimates from the non-linear method. (b) The 1973 coseismic surface slips from Wen (1995; solid green lines) and from Papadimitriou et al. (2004) (and hence from field investigations by R. Weldon and X. Wen in 1995 and 1996) (dashed green lines). The 1923 surface slips (solid and dashed orange lines) are from Li et al. (1998), with dashed orange lines indicating uncertain surface slips. The 1981 and 1893 rupture data are from Li et al. (1998; solid lines) and Papadimitriou et al. (2004; dashed lines). (c) Pattern of historical ruptures and seismic gaps (modified from Wen et al., 2008) where horizontal lines represent the extent of historical ruptures and dashed lines indicate inferred rupture extents. (d) Depths and uncertainties of shallow creep at all sites (shown as purple boxes) are derived from Fig. 8. A 95% cutoff depth in seismicity is used to infer the bottom boundary of the seismogenic layer (after Smith-Konter et al., 2011). The relocated seismic data are for the period
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between Jan. 1981 and Dec. 2012, and are from a final research report by Jiang et al. (2014). Relocations were performed by Fang Lihua using a simplex algorithm and an updated velocity model, and have the following uncertainties of hypocentral locations: <1.5 km for 40% of the events and <3.0 km for 70% of the events in the horizontal direction, <2.0 km for 20% of the events and <4.0 km for 40% of the events in the depth direction. Areas A–D are delineated with blue dashed curves and indicate fault patches with seismicity distributions, whereas E–G indicate fault patches lacking seismicity. The 1981 aftershock area is shown after Yi et al. (2001) with aftershock hypocenters excluded. (e) Map view of the main fault trace and its geometry (Allen et al., 1991)
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with the relocated epicenters from Jiang et al. (2014).
5.2 Discussion: factors affecting spatio-temporal variations in surface creep
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This section focuses on the factors that probably influenced or controlled spatio-temporal variations (especially the marked along-strike variations) in surface-observed sinistral-creep rates along the NWXFZ (Fig. 9a) using the additional information in Fig. 9b–e as well as by combining velocity- and statedependent friction theories. The along-strike distribution of shallow creep (or velocity-strengthening) is
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shown in Fig. 9d as a purple dotted curve that connects estimated shallow-creep depths (purple boxes) for all of the sites. Figure 9d also shows several fault areas with various rates of seismicity since January 1981
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(labeled ‘A’ to ‘G’) within the deeper and velocity-weakening seismogenic layer based on the relocated earthquake catalog of Jiang et al. (2012). These areas (including the 1981 aftershock area) indicate fault patches with various degrees of interseismic coupling, including locked patches (e.g., areas E–G, all of
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which are free of seismicity), as the surface-observed sinistral-creep rates over these areas (03.47 mm/yr) are much lower than the far-field sinistral motion rates along the NWXFZ (810 mm/yr; Du et al., 2010; Lü
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et al., 2003). Geodetically observed locking segments of faults may also show microseismicity, as exemplified by the Anza section of the San Jacinto fault in California (e.g., Jiang and Fialko, 2016; Wang, et al., 2013). Theoretically, during interseismic periods a fault plane within the shallow-creep layer can produce stable sliding that is controlled by velocity-strengthening friction, whereas fault planes within the underlying seismogenic layer may create unstable sliding that is associated with microseismicity and governed by velocity-weakening friction (e.g., Marone et al., 1990, 1991; Scholz, 1998). This theoretical framework means that surface-observed creep rates are not simply translations of the sliding rates of faults within the underlying seismogenic layer. 5.2.1 Along-strike variations in shallow-creep depth The shallow creep depth, or the thickness of the shallow velocity-strengthening layer, is markedly variable along the strike of the NWXFZ, ranging from 0.4 to 7 km (including uncertainties; Fig. 9d). The analysis outlined in section 4 suggests the presence of a rough positive correlation between shallow-creep 24
ACCEPTED MANUSCRIPT depth and surface-observed average sinistral-creep rate at all of the sites (Fig. 8). This means that the thickness of the shallow velocity-strengthening (or creep) layer and its along-strike change probably control the spatio-temporal variations in surface-observed sinistral-creep rates (Fig. 9a and d). This is examined further below. A number of observations, including laboratory experiments focusing on fault friction, field investigations of fault zones, statistical analyses of the depth distribution of earthquakes along faults with and without well-developed gouge zones, and dynamic simulations, all suggest that the vertical thickness of the shallow velocity-strengthening layer (or the depth of the shallow-creep layer) within strike-slip fault
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zones is closely associated with the degree of development of shallow unconsolidated gouge and is restricted by thicknesses of poorly consolidated sediment (e.g., Kaneko et al., 2013; Marone and Scholz, 1988; Wei et al., 2013). A shallow velocity-strengthening layer with a well-developed gouge zone may act to arrest co-seismic slip from an underlying velocity-weakening layer during major earthquakes, yielding a slip deficit that upon relaxation produces afterslip during postseismic periods, and to produce slow stable sliding (shallow interseismic creep) under tectonic loading during interseismic periods (Marone and Scholz, 1988; Marone et al., 1991). Therefore, where thicker shallow velocity-strengthening layers exist, more co-
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seismic slip from underlying velocity-weakening layers are arrested and longer duration and larger cumulative amount (or higher rate) of afterslip yield. This theory matches very well the near-fault geodetic and seismo-geological observations on the NWXFZ.
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The northwestern and southeastern parts of the 1973 rupture section have relatively large thicknesses of the shallow velocity-strengthening layer and hence relatively high average sinistral-creep rates (longlasting and time-dependent afterslip), but both have only relatively small coseismic surface slip during the
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1973 rupture; however, the central part of the same rupture section has a very small thickness of the shallow velocity-strengthening layer and hence very low average sinistral-creep rates ( with little afterslip), but had relatively large coseismic surface slip during the rupture (Fig. 9a and b, also see Figs 4, 5, and 8).
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This complementary relation between the 1973 surface coseismic sinistral-slips and the average post-1973 sinistral-creep rates actually suggests a causal relationship between the long-term sinistral-creep rates and
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the thicknesses of the shallow velocity-strengthening layer along the NWXFZ. This in turn indicates that the marked spatial and temporal variations in sinistral-creep rate along the NWXFZ are likely related to variations in the thickness (and along-strike variations) of the shallow velocity-strengthening (or creep)
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layer. This probable correlation is summarized and illustrated in Fig. 10, which uses the data shown in Fig. 9 and data presented in this section. The analysis outlined above is based solely on the estimates of shallow creeping depths shown in Fig. 8, meaning that these analyses are based on the theoretical framework of rate-state friction. These estimates therefore depend on assuming the depth-varying friction properties of the fault within the brittle crust and hence on the validity of Equations (4) and (6) in the real NWXFZ as well as whether the simulation- and experiment-derived values of the friction rate parameters ab are applicable to the real fault. These dependencies would result in large uncertainties in the estimated creep depths and the physical meaning of these depths. As such, our above analysis of the relationship between surface-observed creep rates and shallow creep depths is only one scenario based on the assumed depth-varying frictional properties of faults. Another possible scenario would be fault creeping with no limit of depth (e.g., Avouac, 2015), or some
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intermediate model (Kelin Wang, 2017, personal communication, 13 Sept.), within the brittle crust.
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Fig. 10. Sketch 3D model of the NWXFZ that summarizes and illustrates the probable correlation between the spatial distribution of surface-observed sinistral-creep rates, estimated shallow-creep depths, inferred fault patches with various degrees of interseismic coupling, and the relative movement of blocks on either side of the fault zone. The
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distribution of sinistral-creep rate is from Fig. 9a, and the depth distribution of the shallow creep layer is from Figs 8 and 9d. The bottom boundary of the seismogenic layer is inferred from Fig. 9d, and the fault patches within the seismogenic layer are from Fig. 9d based on distinct seismicity distributions. The sinistral shear rates along the ductile
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shear zone are from the far-field fault-parallel GPS velocity differences across various sections of the NWXFZ, and the fault geometry is from Fig. 9e. Paired arrows on the surface fault trace indicate the locations of the survey sites as well
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as the average sinistral-creep rates at these sites.
5.2.2 Restraining bend geometries of the NWXFZ Subsection 5.1 indicates that the NWXFZ contains two surface-observed locked portions, namely the
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central part of the 1973 rupture section and the entirety of the 1893 rupture section. These fault portions have thin (0.41.5 km and 0.30.6 km thick, respectively) shallow creep (or velocity-strengthening) layers
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(Figs 9a and d, 10), but show rapid or relatively rapid post-seismic coupling of the fault. So, what is the key factor controlling the rapid coupling and the long-term locking behavior of these two fault portions? Allen et al. (1991) used geological mapping to identify two restraining bends along the NWXFZ; i.e., a clockwise change in strike of ~9° over ~12 km centered near Gelou along the central part of the 1973 rupture section and a clockwise change in strike of ~10° over ~20 km (a length updated during this study) centered near Longdengba along the 1893 rupture section (Figs 9e, 10). These authors suggested that both restraining bends may act as geometrical asperities during major earthquake cycles, a hypothesis that is now supported by new data given during this study. Firstly, a comparison of the coseismic surface slip shown in Fig. 9b and the fault geometry shown in Fig. 9e indicates that coseismic slip increased rather than decayed or terminated when passing through the restraining bends near Gelou or near Londengba during the 1973 or 1893 earthquakes. In addition, these restraining bends may not only be present within the thin and shallow velocity-strengthening layer but also within the underlying velocity-weakening seismogenic 26
ACCEPTED MANUSCRIPT layer. This is indicated by the fact that these restraining bends correspond to two distinct parts of the fault (patches ‘E’ and ‘G’) that are free of seismicity at depths above ~14 and 9–16 km, respectively (Figs 9d, 10). This complete lack of seismicity implies that almost no detectable unstable sliding has occurred within fault patches E and G since at least January 1981, an indicative that these two fault patches respectively along two restraining bends have been completely locked within the whole depth of the seismogenic layer. These suggest that the two restraining bends near Gelou and Londengba have the property of large-scale and persistent compressive asperities. The faults surrounding these types of asperities have a relatively large frictional resistance to sliding and undergo relatively fast stressing, both of which allow a more rapid
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increase in strain on the asperity-associated fault parts during interseismic periods, and hence allows more moment release and larger coseismic slip during earthquakes along these fault parts (e.g., Ruff, 1992; Scholz, 1990). Therefore, the two compressive asperities (in the form of the restraining bends near Gelou and near Londengba) along the NWXFZ are likely the key factors that controlled the rapid fault coupling along the central part of the 1973 rupture and the relatively rapid fault coupling along the 1893 rupture, respectively, and have resulted these parts/portions of the NWXFZ the in long-term locking behavior (as per the surface observations).
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Although it remains unclear what have caused the two surface-observed locked parts/portions of the NWXFZ to be associated with thin shallow creep (or velocity-strengthening) layers (Figs 9d and e, 10), a dynamic numerical simulation by Kaneko et al. (2013) suggested that the frictional properties of surface-
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observed locked segments of a strike-slip fault are dominated by velocity-weakening conditions at shallow depths (figure 11 in Kaneko et al., 2013). This is most likely the case for the two surface-observed locked parts/portions with restraining bend geometries located near Gelou and Londengba, as both of them have
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very thin to negligible velocity-strengthening layers at shallow depths. Therefore, distinct restraining bend
6. Conclusions
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geometries can be a significant control on the shallow slip behavior of strike-slip faults.
The ~200 km long northwestern Xianshuihe fault zone (NWXFZ) of southwestern China has been the
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subject of more than three decades of fault-crossing geodetic observations. These data indicate that the near-field surface movement of the fault zone is dominated by sinistral creep but that creep rates in all three component directions (i.e., strike-slip, transverse, and vertical) show marked spatial and temporal variations.
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Combining these data with additional information on historical ruptures, coseismic surface slip, relocated seismicity, fault geometry, and far-fault movement velocity, as well as using an approach derived from the velocity- and state-dependent friction theory, our analysis indicates that these variations reflect the alongstrike segmentation of surface slip behavior. The 1973 rupture section of the NWXFZ shows spatiotemporal variable surface-slip behavior, where long-lasting and time-dependent post-1973 afterslip has occurred along the northwestern and southeastern parts of the section at depths above 5.8–7.0 km with sinistral-creep rates of 1.26 and 3.47 mm/yr, respectively, and nearly locked with little post-1973 afterslip but very slow sinistral-creep (0.16 mm/yr) along the central part of the section at depths above 0.4–1.5 km. The 1923/1981 rupture section of the NWXFZ is generally in locking state with shallow sinistral creep at 1.09 mm/yr on its central part at depths above 2.0–2.8 km, which contains the post-1981 afterslip, and probably the subsequent interseismic creep. The 1893 rupture section has been completely locked since at
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ACCEPTED MANUSCRIPT least the early 1980s, indicating that this section underwent relatively rapid interseismic coupling. Our study also suggests that the thickness (and along-strike variations in thickness) of the shallow velocity-strengthening (or creep) layer is an important control on the temporal and spatial variations in surface-observed creep rates along the NWXFZ. This is not just because the shallow-creep depths and surface-observed sinistral-creep rates show a positive correlation at all of the sites in the study area, but also because a complementary relationship is present between average sinistral-creep/afterslip rates and the coseismic surface slip along the 1973 rupture section of the NWXFZ. The other factor affecting the spatial and temporal variations in surface-observed creep rates is the geometry of restraining bends in the fault.
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The two surface-observed locked fault portions, namely the central part of the 1973 rupture section and the entirety of the 1893 rupture section, are located along two distinct restraining bends within the NWXFZ associated with clockwise rotations in fault strike of ~910° over distances of ~1220 km near Gelou and Longdengba, respectively. These restraining fault bends would act as persistent and compressive asperities and have enabled these two fault portions to have long-term locking behavior.
At least nine earthquakes, including five M6.5–8.0 events at epicentral distances of 50–300 km, two M7.5–8.1 events at distances of 1000–1400 km, and two Mw9.0–9.2 megathrust events at distances of
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3000–4000 km, have affected, to various extents, the geodetically observed motion of the NWXFZ. It is necessary to exclude or deduce significant earthquake effects in the average rate estimation of surface fault
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movement.
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ACCEPTED MANUSCRIPT Acknowledgments The authors thank Kelin Wang, Xinglin Lei and an anonymous reviewer for their comments, suggestions, criticisms and advices that enable us to have significantly improved earlier versions of the manuscript. The authors also thank the Institute of Geodetic Engineering, Earthquake Administration of Sichuan Province, for providing the fault-crossing survey data, Fang Lihua for sharing relocated seismicity data, He Chang-rong for information related to friction theory, Liu Jing for introducing the authors to relevant publications, and Li Fei-fei for helping in the construction of two figures in Supplementary Material 1. This research was supported by the National Key Technology Research and Development
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Program of China (Grant No. 2012BAK19B01-02) and the State Key Laboratory of Earthquake Dynamics
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of China (Grant No. LED2015A02).
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Wang, L.F., Liu, J., Zhao, J., Zhao, J.G., 2013. Coseismic slip and post-seismic relaxation of the 2011 M 9.0 Tohoku–Oki earthquake and its influence on China Mainland. Earthquake 33(40), 238– 247 (in Chinese).
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Wang, H., Wright, T.J., Biggs, J., 2009. Interseismic slip rate of the northwestern Xianshuihe fault from InSAR data. Geophy Res. Lett. 36, L03302, doi:10.1029/2008GL036560. Wei, M., Kaneko, Y., Liu, Y., McGuire, J.J., 2013. Episodic fault creep events in Cali-fornia controlled by
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shallow frictional heterogeneity. Nat. Geosci.6, 1–5. http://dx.doi.org/10.1038/ngeo1835. Wei, M., Liu, Y., Kaneko, Y., McGuired, J. J. Bilham, R., 2015. Dynamic triggering of creep events in the Salton Trough, Southern California by regional M≥5.4 earthquakes constrained by geodetic
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observations and numerical simulations. Earth Planet. Sci. Lett 427 1–10. http://dx.doi.org/10.1016/j.epsl.2015.06.044.
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Wei, M., Sandwell, D., Fialko, Y., Bilham, R., 2011. Slip on faults in the Imperial Valley triggered by the 4 April 2010 Mw 7.2 El Mayor-Cucapah earthquake revealed by InSAR. Geophys. Res. Lett. 38,
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L01308. http://dx.doi.org/10.1029/2010GL045235. Wen, X. Z., 1995. Quantitative estimates of seismic potential on active faults. Seismological Press, Beijing, pp. 150 (in Chinese).
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Wen, X.Z., Allen, C.R., Luo, Z.L., Qian, H., Zhou, H.W., Huang, W.S., 1990. Segmentation and geometric features of Xianshuihe Holocene fault zone and their seismotectonic implication. Acta Seismologica Sinica 3(4), 437–452. Wen, X.Z., Han, W.B., Li T.S., Lü, Y.P., Zhou, R.J., He, Y.L., Weldon, R., Stimac, J., 1996. Slip-rate and recurrence interval of the Luhuo/Daofu segment of the Xianshuihe fault, Sichuan, PRC. EOS Trans. Am. Geophys. Union 77: 693–694. Wen, X.Z., Ma, S.L., Xu, X.W., He Y.N., 2008. Historical pattern and behavior of earthquake ruptures along the eastern boundary of the Sichuan-Yunnan faulted-block, southwestern China. Phys. Earth Planet. Inter. 168, 16–36. Yi, G.X., Fan, J., Wen, X.Z., 2005. Study on faulting behavior and fault-segments for potential strong earthquake risk along the central-southern segment of Xianshuihe fault zone based on current
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ACCEPTED MANUSCRIPT seismieity. Earthquakes 25(1), 58–66 (in Chinese). Yu, S.B., Liu, C.C., 1989. Fault creep on the central segment of the Longitudinal Valley fault, eastern Taiwan. Proc. Geol. Soc. China 32(3), 209–231. Zhang, C., 1981. Fault-crossing geodetic measurements and near-field movement of faults. Earthquake 1981(5), 24–25 (in Chinese). Zhang, P.Z., Deng, Q.D.,Zhang, G.M., Ma, J., Gan, W.J., Wang. M., Mao, F.Y., Wang. Q., 2003. Active tectonic blocks and strong earthquake activities in the continent of China. Sci. China (Ser. D)
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46(Suppl), 13–24. Zhang, Q.Z., Tang, W.Q., Liu, Y.P., Li J., 2013. Co-seismic displacement of the Ms 7.0 Lushan earthquake
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based on GPS monitoring. Earth Sci. Frontiers 20(6), 42–47 (in Chinese).
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Graphical Abstract
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Highlights Decades of fault-crossing geodesy show fault afterslip and slow creep and locking.
Along-strike variation in slip behavior is related to creep depth and fault geometry.
Time-decaying afterslip occurs on fault sections with relatively large creep depths.
Two surface-locked fault sections are located along restraining fault bends.
Observed fault motion response to M6.5–9.2 earthquakes at distances of 50–
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4000 km.
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Jing Zhang, Xue-ze Wen, Jian-ling Cao, Wei Yan, Yong-lin Yang, Qin Su PII: DOI: Reference:
S0040-1951(17)30456-0 doi:10.1016/j.tecto.2017.11.002 TECTO 127668
To appear in:
Tectonophysics
Received date: Revised date: Accepted date:
31 July 2017 31 October 2017 3 November 2017
Please cite this article as: Jing Zhang, Xue-ze Wen, Jian-ling Cao, Wei Yan, Yong-lin Yang, Qin Su , Surface creep and slip-behavior segmentation along the northwestern Xianshuihe fault zone of southwestern China determined from decades of fault-crossing short-baseline and short-level surveys. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Tecto(2017), doi:10.1016/j.tecto.2017.11.002
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ACCEPTED MANUSCRIPT Surface creep and slip-behavior segmentation along the northwestern Xianshuihe fault zone of southwestern China determined from decades of fault-crossing shortbaseline and short-level surveys Jing Zhanga, Xue-ze Wenb,, Jian-ling Caoa, Wei Yanb,c, Yong-lin Yangd, Qin Sud CEA Key Laboratory of Earthquake Prediction (Institute of Earthquake Science, China Earthquake Administration), Beijing 100036, China
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State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China
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China Earthquake Networks Center, Beijing 100045, China
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Institute of Geodetic Engineering, Earthquake Administration of Sichuan Province, Ya’An, Sichuan 625000, China
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Corresponding author at: State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, A1 Huayanli, Chaoyang District, Beijing 100029, China. Tel: +86 10 62009138. E-mail address: [email protected] (X. Wen)
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ACCEPTED MANUSCRIPT ABSTRACT This study examines the ~200-km-long northwestern Xianshuihe fault zone (NWXFZ), southwestern China, using more than three decades of geodetic observations from fault-crossing short-baseline and short-leveling surveys at seven sites. These data enable estimates of creep rates and depths, and examination of the long-term slip behavior. The surface motion of the NWXFZ is dominated by sinistral creep, although sinistral, transverse, and vertical slip components show spatio-temporal
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variations. Combining these slip variations with data of earthquake rupture, coseismic slip, seismicity,
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fault geometry, and far-fault movement velocity, and using the velocity-and-state friction theory, our
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analysis indicates that the surface slip behavior of the NWXFZ is segmented along strike. The 1973 rupture section of this fault zone exhibits spatio-temporally variable slip behavior, showing timedecaying post-1973 afterslip on the northwestern and southeastern parts of the rupture at depths above
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5.87.0 km with average sinistral-creep rates of 1.3 and 3.5 mm/yr, respectively, but being relocked in the central part of the rupture. The 1923/1981 rupture section is generally in locking state, with
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postseismic and interseismic sinistral-creep at 1.1 mm/yr on its central part at depths above 2.0–2.8 km. The 1893 rupture section has been tightly locked without creep since at least the early 1980s. The thickness of the shallow velocity-strengthening (or creep) layer and the restraining bend geometry of
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the NWXFZ are the key factors that control spatio-temporal variations in surface creep rates. Two surface-observed locked fault portions are located within two different restraining bends in the
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NWXFZ, both of which act as compressive asperities and hence have enabled the long-term locking of these portions. Creep along the NWXFZ has also been affected to varying degrees by M6.5Mw9.2
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earthquakes at distances of 503800 km from the fault zone. Most of these effects have been removed
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from our estimates of the long-term average creep rates. Keywords: Fault-crossing short-baseline and short-level surveys; average surface creep rate; creep depth;
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along-strike variable slip behavior; restraining bend fault geometry.
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ACCEPTED MANUSCRIPT 1. Introduction Short-range geodetic surveys across active faults using baseline and leveling arrays, alignment arrays, trilateration networks, and creepmeters have provided highly accurate observations that have enabled the study of various types of near-field surface-fault movements, including pre-seismic slip, inter-seismic creep, co-seismic slip, and dynamically triggered slip, as well as post-seismic slip or creep (e.g., Allen and Smith, 1966; Allen et al., 1972; Burford and Harsh, 1980; Galehouse and Lienkaemper, 2003; Lee et al., 2001;
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Lisowski and Prescott, 1981; Savage, 1979; Sun and Wu, 2007; Thurber and Sessions, 1998; Yu and Liu, 1989;
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Zhang, 1981). These observations can provide key insights into how spatial and temporal variations in fault movements relate to long-term slip behavior along faults, enabling more precise estimates of the future
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seismic potential of individual faults (e.g., Galehouse and Lienkaemper, 2003; Lienkaemper et al., 2013, 2014). In addition, near-fault geodetic surveys can be helpful in monitoring possible earthquake precursors (e.g., Allen and Smith, 1966; Bakun and Lindh, 1985; Sun and Wu, 2007; Thurber and Sessions, 1998). Near-
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fault geodetic surveys have been conducted since the mid-1970s along the NW-trending Xianshuihe fault zone of southwestern China using high-precision, very short-range (lengths of tens of meters to >200 m) repeated
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baseline and leveling measurements at location-fixed fault-crossing sites (Fig. 1). These measurements are usually termed ‘short-baseline’ or ‘short-leveling’ surveys in China, although very few of these studies have been reported in international publications.
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The >300 km Xianshuihe fault zone (XFZ) of southwestern China forms part of the ~1400 km long leftlateral Xiashuihe–Xiajiang fault system that extends from the southern part of Yunnan Province northwest
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through Sichuan Province into Qinghai Province (e.g., Allen et al., 1991; Wang et al., 1998). The fault zone represents a highly active sinistral-slip boundary between the Bayanhar and Sichuan–Yunnan blocks within
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the eastern Tibetan plateau region (Fig. 1; Zhang et al., 2003). The XFZ is divided into northwestern and south–central (or southern) segments by a geometric discontinuity near Bamei. These segments have geologically averaged slip rates of 8–12 and 8–10 mm/yr, respectively (Wen et al., 1990, 1996; Xu et al.,
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2003). At least 10 major earthquakes have occurred along this fault zone during the past ~300 years (Wen et al., 2008). The two most recent major events were the 6 February 1973 M 7.6 Luhuo, Sichuan earthquake and
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the 24 January 1981 M6.9 Daofu, Sichuan earthquake. These events led the Seismological Bureau (now the Earthquake Administration) of Sichuan Province to establish 10 fault-crossing repeat survey sites along the fault zone using short (i.e., spans of tens of meters to >200 m) baseline and leveling arrays. Eight of these sites lie along the northwestern segment of the fault zone, with the remaining two located on the south–central segment (Fig. 1). To date, more than three decades of highly precise observations have been acquired from repeat surveys at these sites. These data represent an important resource for studying the characteristics of the near-field surface movement along active strike-slip faults. The two sites located on the south–central segment of the XFZ at Zheduotang and Tuanjie only have short leveling-arrays that are unsuitable for monitoring the predominantly strike-slip movements within the fault zone. As such, we have restricted this study to the ~200 km long northwestern segment of the XFZ (i.e., the NWXFZ), which is located between the area northwest of the Zhuwo site to the area southeast of the Laoqianning site (Fig. 1). 3
ACCEPTED MANUSCRIPT This study summarizes more than three decades of repeat surveys using fault-crossing short-baseline and short-leveling arrays at seven of the eight sites along the NWXFZ; the Daofu site has been excluded because this site does not span the main fault trace (Fig. 1). Here, we analyze the observed surface-fault movements and estimate the long-term average strike-slip, horizontal transverse, and vertical-motion rates at these sites by considering and reducing as much as possible the effects of local, regional, and even remote earthquakes with various magnitudes and epicentral distances. Combining surface-observed fault motion series data with a
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velocity- and state-dependent friction law enables the determination of creep depths. This study also examines spatio-temporal variations in creep rates and the relationship between these creep rates and historical ruptures,
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coseismic surface slip, fault geometry, microseismicity, and far-field motion velocities to analyze the long-
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term, macroscopic (i.e., geodetically observed rather than experimental or modeled) slip behavior along fault sections that have experienced a variety of ruptures during the last major earthquakes along the NWXFZ. This research provides useful insights into near-field surface creep and how these data can constrain seismogenic
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coupling or locking fraction estimates along the NWXFZ, thereby enabling the determination of the seismic potential of different sections of the NWXFZ. This study also demonstrates how the surface-slip behavior of a
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major active strike-slip fault within a continent plate setting varies both along strike and over time.
Fig. 1. (a) Map showing the Xianshuihe fault zone (XFZ) and the locations of the fault-crossing short-baseline and shortleveling arrays/sites along the fault zone. (b) Map showing the position of the XFZ within the active fault system in southwestern China and the surrounding regions.
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ACCEPTED MANUSCRIPT 2. Data and Methods 2.1 Data and methods of fault-crossing surveying 2.1.1 Data, survey sites, and arrays The data used in this paper are the result of over three decades of repeated surveys using short-baseline and short-leveling arrays across the main fault trace at seven sites along the NWXFZ (Zhuwo, Gelou, Xuxu, Xialatuo, Goupu, Longdengba, and Laoqianning; Figs 1 and 2). Repeated surveying was undertaken at
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irregular intervals of 1–8 months between 1976 and 1987, and surveys since March 1987 have been
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undertaken at regular intervals of every 10 days or less at Xialatuo and every 1–2 months at the other sites. Field surveying and data pre-processing are both undertaken by the Institute of Geodetic Engineering,
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Earthquake Administration of Sichuan Province, China.
Six of the seven sites contain a triangular-shaped survey array that consists of two measuring lines crossing the main fault at different angles and a third that does not cross the fault. Repeated short-baseline
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surveys are conducted along the lines crossing the fault and repeated short-leveling surveys are undertaken along all three lines. The array at Longdengba has three measuring lines that cross the fault (Fig. 2, also see
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Figure s1 in Supplementary Material 1 online). Each measuring line within a given array has two endpoints that are separated by several pass points. A measuring line for the short-baseline surveying, any span between pass points, or between an endpoint and an adjacent pass point is generally designed to be ~24 m in length.
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Each endpoint is made of a reinforced concrete monument with a base buried to a depth of 1.50 m below the ground surface. The benchmarks of the baseline and level surveys are set to be the top and waist (near the
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ground surface) of these monuments, respectively (Figure s2 in Supplementary Material 1). Some of the endpoint monuments are built on bedrock, whereas others (and the majority of the pass points) are located on
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firm Quaternary sediments.
2.1.2 Measurement methods and accuracies The fault-crossing geodetic measurements along the NWXFZ use two conventional surveying techniques,
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namely invar-wire baseline measurements and precise leveling. Short-baseline measurements use a 24 m long invar wire (Secrétan, France) to measure the horizontal distance between two endpoints on either side of the
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fault following the technical procedures and accuracy requirements outlined in the Specifications of Crossfault Geodetic Measurement (State Seismological Bureau, 1991). Distance variations are monitored via repeated baseline measurements, and the decades of repeated surveys yield uncertainties at the seven sites on these short baseline measurements of 0.50–0.58 10–6 (see Table s1 in Supplementary Material 1 for the uncertainty values). Short-level measurements use a leveling instrument to measure the height difference between endpoints on different sides of the fault using the accuracy requirements outlined in the Specifications of Cross-fault Geodetic Measurement (State Seismological Bureau, 1991), which are the same as for the first-order leveling surveys. Variations in height differences are repeatedly monitored and repeated measurements at the seven sites yield uncertainties of 0.13–0.25 mm (see Table s1 in Supplementary Material 1).
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Fig. 2. General topographic features and geological setting of the seven short-baseline and short-leveling cross-fault survey sites along the northwestern Xianshuihe fault zone (NWXFZ). North is towards the top of each map, the left 6
ACCEPTED MANUSCRIPT column shows digital elevation model (DEM) images obtained using the GeoTaos software package and its internal DEM dataset (https://staff.aist.go.jp/xinglin-lei/Lei00.htm), and the right column shows geological and sketch surveyarray maps of the sites. Geological maps are at a scale of 1200,000 and are from the NGAC Digital Library (http://geodata.ngac.cn/Map/List, last accessed on November 6, 2016), in which major fault traces have been updated according to field investigations by Allen et al. (1991), Li et al. (1997), and the second author of this study. Also see Figures s1 to s3 and Table s1 in Supplementary Material 1 for more details about the survey sites, arrays, and faults.
Accidental errors during surveys were minimized by performing both short-baseline and short-level
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measurements during hours that are favorable for high-quality and steady surveying. The surveying crew
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consists of experienced members of the Institute of Survey Engineering, Earthquake Administration of Sichuan Province, China. The presence of clear variations at a given site on successive measurement days
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requires a field or on-site investigation to confirm whether the observed variation is independent of possible environmental changes and meteorological factors, as house building, excavation or landfilling, and extreme
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rainfall or flooding events at or near the site can cause these variations; alternatively, a resurvey is conducted. The survey crew also checks observational data, the invar wire, and the leveling instrument and its staff during surveys to ensure that equipment readings accurately represent measured variations. These procedures
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mean that any measured variations that might be caused by non-tectonic factors of unknown origin are noted so that the reliability of observations can be properly assessed after the survey.
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2.2 Formulas for calculating fault-movement components For each of the survey arrays of two fault-crossing measured lines as shown in Fig.2, fault-movement
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variations in the three components, A (i.e., strike-slip), B (i.e., horizontal transverse), and C (i.e., vertical),
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D1 sin 2 D2 sin 1 sin 1 cos 2 cos 1 sin 2
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(2) (3)
are the clockwise rotation angles from baselines 1 and 2 to the fault, respectively; and D1
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where 1 and
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between successive measurement days are calculated trigonometrically from the formulae (Bo et al,1998):
and D2 are variations in the lengths of baselines 1 and 2, respectively, between successive measurements. A values of >0 suggest that variations in the strike-slip component are sinistral, whereas A values of <0 are indicative of dextral motion. In addition, B values of >0 represent extension in the horizontal transverse direction and B values of <0 are indicative of shortening in this direction. The
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equation (3) denote changes in height differences between the endpoints of leveling lines 1 and 2, respectively, both of which span the fault, meaning that C values of >0 are indicative of the upward movement of the fault’s footwall (i.e., a normal faulting component) and C values of <0 are indicative of the downward movement of the footwall (i.e., a reverse faulting component). 7
ACCEPTED MANUSCRIPT 2.3 Methods for estimating average fault movement rates Analyzing the observed time series of cumulative fault movements in the three component directions outlined above enables an estimate of the average rate of each movement component at every site over various time periods (see section 3 for site-by-site details). This includes a long-term average rate computed for relatively long periods during which earthquake effects were weak or relatively insignificant (see subsection 2.4 for the criteria and approach used for determining the length of this period). Average rates are
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estimated using two methods, as follows. (1) Simple averages for all three movement components are computed by dividing the total displacement
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by the total time using the approaches of Galehouse and Lienkaemper (2003) and Lienkaemper et al. (2014).
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(2) Non-linear averages are used that can only be computed for strike-slip components. Previous research determined that the observed cumulative sinistral-movement of the NWXFZ at the Xialatuo site is logarithmic increasing over time (Allen et al., 1991; Du et al., 2010), indicating that these movements have temporal
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characteristics that follow velocity- and state-dependent friction laws (Kato et al., 2007). Consequently, we used a non-linear method developed from velocity-state friction theory to estimate average sinistral-creep
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rates for all sites, enabling the identification of variation trends in sinistral movement rates and the related creep depths at each site (see section 4). The first step in this process is to fit the surface-observed sinistralcreep series for each site by inverting equation (4) using the Levenberg–Marquardt algorithm to obtain the
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best-fitting parameters , , γ, and v0 as follows:
(4)
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U t ln t 1 v0t
Equation (4) is rewritten from an equation derived from the velocity- and state-dependent friction laws of
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Scholz (1990) and Marone et al. (1991). Here, U is the cumulative slow-slip/creep (in mm) and t is the time (in days) elapsed since the last earthquake. The parameter is associated with friction, effective normal stress, the thickness of the shallow velocity-strengthening layer, and the shear modulus (Marone et al., 1991). The
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physical meanings of the other parameters (, γ, and v0) are outlined by Scholz (1990) and Marone et al.
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(1991). This enables the estimation of average sinistral-creep rates CR for each site, as follows:
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where Ut1 and Ut2 (in mm) are displacements accumulated by times t1 and t2, respectively, and are calculated from a specific equation (4) that is fitted for each site (see Table 1 in section 3).
2.4 Reduction of earthquake effects on estimated movement rates Earthquakes can affect geodetically observed rates of fault movement (e.g., Galehouse, 1990; Galehouse and Lienkaemper, 2003; Lienkaemper et al., 1997, 2001, 2014) and anomalous creep events occur when the shallow creeping zone of a fault responses to external stress perturbations caused by earthquakes (e.g., Kanu and Johnson, 2011; Wei et al., 2015). Wei et al. (2015) used observations and simulations to suggest that the triggering of creep events on the Superstition Hills fault of southern California occurred during Mw>7 8
ACCEPTED MANUSCRIPT earthquakes within 150 km of the fault, Mw>6 earthquakes within 80 km of the fault, and Mw>5 earthquakes within 20 km of the fault, with a stress change amplitude of around 0.6 MPa required to trigger a surface slip event of 0.01 mm. Since estimated average creep rates from near-fault geodetic observation should be representative of the long-term mean of stable surface sliding along active faults and useful in assessing fault slip behavior and seismic potential, it is necessary to exclude or remove wherever possible significant effects of earthquakes from the observed fault motions (Galehouse and Lienkaemper, 2003; Lienkaemper et al., 1997,
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Fig. 3. Map showing epicentral locations (black stars) and focal mechanisms of nine earthquakes that have to some degree affected geodetically observed movement of the NWXFZ. The approximate geometric center of the NWXFZ is shown as an open star, and distances between this location and the epicenters of individual earthquakes can be assessed using the dashed concentric circles. Focal mechanism solutions are from the Global Centroid Moment Tensor Catalog (www.globalcmt.org/CMTsearch.html, last accessed on October 15, 2014) barring the 1989 M6.5 event, for which the mechanism solution is from the Seismological Bureau of Sichuan Province (1993). See Figs 4 to 6 and Supplementary Material 3 for the effects of these nine earthquakes on the observed movement series of the NWXFZ.
Our analysis indicates that at least nine earthquakes between the early 1980s and the middle of 2014 affected the observed movements along the NWXFZ. These include two Mw9.0–9.2 megathrust events at epicentral distances of ~3000 and ~4000 km, and seven M6.5–8.1 events at epicentral distances of ~50 to
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(2) Anomalous sudden, rapid, or transient variations in the amount or rate of fault movement within one or more observed time series that are temporally correlated with a single earthquake event but are also
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independent of any meteorological effects (e.g., Allen et al., 1972; Lienkaemper et al., 2001, 2014; McGill et
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al., 1989; Titus et al., 2006; Wei et al., 2011, 2015).
(3) The observed creep response on a fault to an earthquake may persist for years. This is exemplified by the creep response of the southern Hayward fault to the 1989 Loma Prieta, California, earthquake, where the
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nearly zero creep that appeared immediately following this earthquake was followed by a rapid and significant creep event over the next several years before the creep along the fault returned to a steady state (Galehouse, 1990; Kanu and Johnson, 2011; Lienkaemper et al., 1997, 2001).
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A supplementary investigation was undertaken by comparing monthly rainfall and average temperature time series data from two local meteorological stations with observed fault-movement series from four of the
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seven sites along the NWXFZ. These data are given in Supplementary Material 2 and suggest that meteorological factors, including rare rainfall events and extreme temperature fluctuations, have insignificant
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effects on observed fault movements (unlike the nine earthquakes shown in Fig. 3) and are therefore considered negligible in the majority of cases.
We minimized or remove significant effects of the nine earthquakes outlined above on the long-term
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average rate estimates of three component movements of the NWXFZ by using the following approach: For every motion component of these sites, we estimate an long-term average rate for a selected and relatively
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long period, during which the earthquake effects were insignificant or can be excluded; in most cases, such periods were before the Wenchuan, China, earthquake of May 12, 2008. This means that any significant anomalous fault motions that appeared within these selected periods and are related to the earthquakes (Fig.3)
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and extreme weather events, or to unknown causes, have been excluded from the calculation of long-term average rates of the observed fault movement (also see Supplementary Materials 2 and 3).
3. Fault movement series and average rates This section focuses on the analysis of surface-observed fault movement time series from the repeated fault-crossing surveys over a period of more than three decades. These series data were used to estimate the long-term average rates of the three components of movement along the NWXFZ at the seven sites, using the methods outlined in subsection 2.3 in addition to excluding the significant effects of earthquakes, extreme weather events, and unknown causes based on the relevant criteria and approaches described in subsection 2.4.
3.1 Zhuwo Site 10
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rupture (i.e., post-1973 creep) that suggests to a great extent the existence of velocity-strengthening friction behavior (Kato et al., 2007; Marone et al., 1991; Scholz, 1990).
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Applying the lines of evidence, experiential criteria and approaches summarized in subsection 2.4 allows
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the identification of the effects of significant earthquakes on the observed fault movement at the Zhuwo site, as indicated by the zw-s, -t, and -v curves in Fig. 4 (also see Supplementary Material 3 for the identification). Our analysis suggests that the observed sinistral-creep component of fault movements at this site were
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insignificantly influenced by earthquakes for the ~26 years between June 1982 and May 2008, whereas the transverse component was insignificantly influenced for the 18.5 years between May 1983 and November 2001. Almost no earthquake effects are apparent in the vertical component series obtained over the 34 years
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between June 1980 and June 2014. This yields average creep rates for sinistral, transverse, and vertical components of 1.26, 0.08, and 0.71 mm/yr, respectively, for these corresponding time periods. This result
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means that the observed fault movements at the Zhuwo site are dominated by sinistral creep, which is accompanied with a clear normal faulting component (positive vertical motion rate of 0.71 mm/yr that is
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indicative of the relative uplift of the footwall of the fault, in this case the southwestern block of the fault) and a very slight horizontal–transverse stretching component (of 0.08 mm/yr) in a SWNE direction. This normal faulting is likely associated with the existence of an active NE–SW trending horst in the southwestern block
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of the NWXFZ at the Zhuwo site (see a-2 in Fig. 2 and Profile a in Figure s3 in Supplementary Material 1).
3.2 Gelou Site
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The Gelou site crosses the central part of the of the 1973 surface rupture zone (Figs 1 and 2). Faultcrossing short-baseline surveying started at this location in May 1982 with short-level surveying beginning in
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January 1986. The observed fault movement at this site is quite slow, but still involves time-decaying sinistral creep accompanied by even slower to near-zero vertical and horizontal transverse motions (gl-s, -t, and -v in Fig. 4). Applying the lines of evidence, criteria, and approaches outlined in subsection 2.4 indicates that the observed fault motion at this site before 2008 was unaffected by earthquakes or other factors barring a dextral slip event of –2.3 mm with an unknown cause between March and May 1987, which was excluded from our average rate estimates. However, the fault movement of this site has been significantly affected by 34 earthquakes between 2008 and the present day, including the 2008 M8.0 Wenchuan earthquake, the 2011 Mw9.0 Tohoku-Oki, Japan earthquake, the 2013 M7.0 Lushan, China earthquake, and perhaps the 2010 M7.1 Yushu, China earthquake (Fig. 4; for a detailed analysis of the earthquake effects, see Supplementary Material 3). These indicate that the sinistral and transverse components of fault motion at the Gelou site were least influenced by earthquakes during the ~26 years between June 1982 and May 2008, with the vertical 11
ACCEPTED MANUSCRIPT component being least influenced during the ~22 years between June 1986 and May 2008. The resulting estimates of long-term average motion rates for these three components during these periods are 0.09, 0.02, and –0.13 mm/yr, respectively, suggesting that the fault at this site has almost entirely re-locked during the
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CE
PT
ED
M
AN
US
CR
IP
T
period after the 1973 M7.6 Luhuo earthquake.
Fig. 4. Time series of surface displacements along the NWXFZ at the Zhuwo, Gelou, and Xuxu sites. These displacements were calculated using equations (1)–(3) and observations from the fault-crossing short-baseline and short-leveling surveys. Round, square and triangular symbols represent individual measurements of strike-slip, horizontal transverse, and vertical components, respectively, and all measurements are shown in the diagram. Positive strike-slip movements represent faultparallel sinistral-slip, whereas positive transverse movements indicate fault-normal extension, and positive vertical movements denote relative upward motion of footwall block. Earthquake dates are shown as dotted lines and the significant effects of earthquakes and unknown causes on these observed series are labeled and excluded from average rate estimates, with corresponding analyses given in Supplementary Material 3. Equation (4) was used to fit sinistral-creep series and to estimate the non-linear rates of this motion component. Red solid curves show the fitting of the data over the 12
ACCEPTED MANUSCRIPT fitting periods and dashed red curves represent inferred parts from the fitted curves.
3.3 Xuxu site The Xuxu site also spans the central part of the 1973 surface rupture zone (Figs 1 and 2). Short-leveling and short-baseline surveys commenced here in May and June 1980, respectively. The observed fault movement at this site before May 2008 consisted of very slow sinistral creep with minor amounts of reverse faulting that caused the footwall of the fault (i.e., the southwestern block of the fault) to move down relative
T
to the hangingwall. However, the strike-slip and transverse components of movement have been significantly
IP
affected by earthquakes since May 2008 and since November 2001, respectively, although the vertical component has remained generally unaffected during the entirety of the 34 year measuring period (gl-s, -t, and
CR
-v in Fig. 4). The lines of evidence, criteria, and approaches outlined in subsection 2.4 were used to identify and exclude significant earthquake effects in these data (for a detailed analysis of these effects, see
US
Supplementary Material 3), yielding estimated average fault movement rates of 0.16 mm/yr for the sinistral component for the ~28 years before May 2008, –0.15 mm/yr for the transverse motion for the ~21 years before November 2001, and –0.09 mm/yr for the vertical component for the entire 34.1 year measuring period.
AN
These results suggest that the fault at the Xuxu site is also nearly completely relocked. Table 1. Results of curve fitting, based on Equation (4), of surface-observed sinistral-creep series at all sites
Sites
fitting
Fitting equation, parameters and root-mean-square errors (RMS)
M
Selected periods for
ED
(y m d―y m d)
U t ln t 1 v0t
RMS (mm)
1982/06/18―2008/05/07
U(t)=50.123ln(0.010/50.123t+1)24.7440.0006t
0.20
Geluo
1982/08/06—2008/05/07
U(t)= 4.376ln(16.894/4.376t+1)40.7410.0001t
0.20
Xuxu
1980/06/13—2008/05/08
U(t)= 0.622ln(-3.049E-05/0.622t+1)1.3682+0.0005t
0.14
Xialatuo
1976/06/18—2014/07/05
U(t)= 43.519ln(0.114/43.519t+1)66.8088+0.0024t
0.66
Goupu
1983/05/20—2013/03/12
U(t)= 6.921ln(1004.503/6.921t+1)80.6284+0.0013t
0.76
Longdengba
1988/05/31—2008/05/09
U(t)= 0.7ln(-1.306E-05/0.7t+1)5.3335+0.0002t
0.12
Laoqianning
1985/01/28—2001/11/11
U(t)= 0.413ln(-1.208E-10/0.413t+1)13.8211+0.0004t
0.26
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CE
PT
Zhuwo
Notes: The fitting root-mean-square (RMS) error
N
RMS
(Ui i 1
obs
Ui fit ) 2
N
. See subsection 2.3 for the fitting method.
3.4 Xialatuo site The Xialatuo site has been surveyed since 1976 and spans the southeastern part of the 1973 surface rupture zone as well as the northwestern-most part of the surface rupture of the 1923 M7.3 Luhou–Daofu earthquake (Figs 1 and 2). The observed fault movement at this site is dominated by relatively fast sinistral creep accompanied by NE-dipping normal faulting (xlt-s, -t, and -v in Fig. 5). The sinistral-creep displacement logarithmically increases over time, indicating that this displacement relates to afterslip associated with the 1973 event (Allen et al., 1991; Du et al., 2010; Kato et al., 2007). The observational 13
ACCEPTED MANUSCRIPT dataset of this site spans a longer period than those used in previous research, allowing that the sinistral-creep series can be well fitted to a curve using Equation (4). The fitting curve indicates that the observed fault movements at this site do represent surface afterslip associated with the 1973 M 7.6 Luhuo earthquake (xlt-s
AC
CE
PT
ED
M
AN
US
CR
IP
T
in Fig. 5; also see Table 1).
Fig. 5. Time series of surface displacements along the NWXFZ at the Xialatuo and Goupu sites. The symbol explanation is as for Fig.4. All measurements are shown. The good fit of the red curves modeled using Equation (4) to the data indicate that the observed dominant fault movement is sinistral creep. In addition, the movement at these sites most likely represents post-seismic slip associated with the 1973 and 1981 earthquakes, with interseismic creep most likely also present at the Goupu site after 2000. See Table 1 for the results of curve fitting.
14
ACCEPTED MANUSCRIPT
The average movement rate of this site excludes a rapid ~1.5 mm transverse shortening during July 1998 (xls-t in Fig. 5) because this may relate to either an extreme episode of rainfall in July 1998 after a four-year drought or may be associated with the M7.5 Manyi, China earthquake of November 8 1997 (see Figure s4 and the discussion in Supplementary Material 2). However, we do not exclude probable responses to earthquakes (e.g., pulses, jumps, and rate variations) in the strike-slip series as these responses are too small in amplitude to notably affect the sinistral-creep motion at this site. In addition, these small responses usually self-recover
T
soon after they occur. Finally, the effects of earthquakes on the vertical component of this site are negligible
IP
and require no exclusions (xlt-s and -v in Fig. 5).
CR
The analysis outlined above indicates that earthquakes had minor or negligible effects on the three components of fault movement at the Xialatuo site during the ~38 years between June 1976 and July 2014, barring a rapid transverse shortening of ~1.5 mm in July 1998 that was removed from our rate calculation.
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This yields average creep rates at this site, for the entire ~38-year period, of 3.47, 0.32, and 0.47 mm/yr for
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CE
PT
ED
M
AN
the sinistral, transverse, and vertical components, respectively.
Fig. 6. Photographs of visible surface creep along the NWXZ at Xialatuo, taken on July 11, 2013. (a) View to the NNW of a concrete pavilion located astride the main fault at the Xialatuo site, which was installed in 1984 (also see Figure s1 in Supplementary Material 1 for the location of the pavilion). (b) and (c) Views to the W and SSE, respectively, of sinistral offsets as a result of fault creep along the gap-line (the main fault trace) between two concrete pillars in the pavilion. The entirety of the floor of the two parts of the pavilion, including the gap-line, was covered by a thin cement plaster layer prior to the 1990s that was later destroyed.
Visible surface creep of the NWXZ has also been identified at the Xialatuo site, at the location of a concrete pavilion installed as a memorial to the victims of the 1973 M7.6 earthquake by the former
15
ACCEPTED MANUSCRIPT Seismological Bureau of Sichuan Province in 1984 (see Fig. 2 for the location of this pavilion). The pavilion was made of two separate but symmetrical parts that were each placed on a different side of the 1973 main surface rupture (Fig. 6a). In 1986, minor en echelon cracks appeared on the thin cement plaster floor of the pavilion along the gap line (i.e., the position of the main fault) as a result of fault creep between the two parts of the base of the monument (Allen et al., 1991). The cumulative offset of this sinistral-creep along the gap line was measured to be ~4 cm in November 2007 (Du et al., 2010) but appeared to be larger during a field
T
visit to the site in the summer of 2013 (Fig. 6b and c).
IP
3.5 Goupu site
CR
The Goupu site spans the central portion of the surface rupture zone of the 1981 M6.9 Daofu, Sichuan, earthquake, which occurred along the NWXFZ (Fig. 1). Fault-crossing surveys began at this location in May 1983, and the observed fault movement is dominated by sinistral creep combined with a smaller component of
US
reverse faulting that is evident as a result of negative but minor vertical and transverse components (gp-s, -t, and -v in Fig. 5). This location is characterized by initially fast sinistral creep that gradually decelerated over
AN
time before reaching a relatively steady rate in ~2000 that has continued to the present day. The sinistral creep series can be well fitted to a curve using Equation (4), indicating that the motion of this component is associated with post-1981 afterslip and subsequent interseismic creep (gp-s in Fig. 5 and Table 1).
M
Applying the lines of evidence, criteria, and approaches outlined in subsection subsection 2.4 enables an analysis of earthquake effects on observed fault motion at the Goupu site, suggesting that of the nine events
ED
shown in Fig. 3, the M7.0 Lushan, Sichuan, earthquake of April 20, 2013 had the greatest effect on the observed fault movement at this site (see the corresponding analysis given in Supplementary Material 3). This
PT
yields estimated average sinistral, transverse, and vertical component creep rates of 1.09, −0.35, and −0.40 mm/yr, respectively, for the site during the ~30 years between May 1983 and March 2013.
CE
3.6 Longdengba Site
The Longdengba site crosses the south–central part of the surface rupture zone of the 29 August 1893 M≥7 south Daofu earthquake, which occurred along the NWXFZ between Laoqianning and the southeastern
AC
part of Daofu (Allen et al., 1991; Li et al., 1998; Wen et al., 2008; Fig. 1). Fault-crossing surveys have been undertaken here since 1985, yielding maximum sinistral, transverse, and vertical component cumulative displacements of 1.25, 3.00, and 0.73 mm, respectively (ldb-s, t, and v in Fig. 7). These small cumulative displacements suggest that even minor motion variations associated with earthquake effects would impact the estimates of long-term average rates at this site. The observed surface fault motion at the Longdengba site has been affected by a number of earthquakes, as follows: the 1981 M6.9 Daofu earthquake within the central NWXFZ affected strike-slip and vertical components before May and before January 1986, respectively, whereas the Wenchuan earthquake of May 2008 and the Tohoku earthquake of March 2011 have affected strike-slip and transverse components, respectively (ldb-s, -t, and -v in Fig. 7; see the corresponding analysis in Supplementary Material 3). As such,
16
ACCEPTED MANUSCRIPT average motion rates are estimated to be 0.06 mm/yr for sinistral-creep between February 1986 and May 2008, 0.10 mm/yr for transverse motion between May 1985 and January 2011, and –0.03 mm/yr for vertical motion between January 1986 and June 2014. These data indicate that the fault at this site has been tightly locked for
CE
PT
ED
M
AN
US
CR
IP
T
at least the last three decades.
Fig. 7. Time series of surface displacements along the NWXFZ at the Longdengba and Laoqianning sites. The symbol
AC
explanation is as for Fig.4. All measurements are shown. The very slow sinistral movements at these sites can be well fitted to a curve using Equation (4) too, yielding the red solid fitting curves. Also see Table 1 for the fitting results.
3.7 Laoqianning site The Laoqianning site is close to the southeastern-most end of the NWXFZ and probably crosses the surface rupture of the 1893 M≥7 earthquake (Fig. 1), which was discovered ~2 km northwest of the site (Allen et al., 1991). Fault-crossing surveys have been undertaken here since 1979, yielding total cumulative sinistral, transverse, and vertical displacements of 2.07, 2.01, and 2.88 mm, respectively (lqn-s, -t, and -v in Fig. 7). Applying the lines of evidence, criteria, and approaches outlined in subsection 2.4 suggests that the fault
17
ACCEPTED MANUSCRIPT movement at this site has been significantly affected by earthquakes. The 1981 Daofu M6.9 earthquake significantly impacted the strike-slip component, as well as having less significant impacts on transverse and vertical components in measurements taken before February 1985. The six earthquakes since November 2001 have significantly affected strike-slip movements at this site (see the corresponding analysis in Supplementary Material 3). Taking these effects into consideration, we determined average movement rates at this site to be 0.15 mm/yr for the sinistral creep component between January 28, 1985 and November 11, 2001, –0.06
T
mm/yr for the transverse component between September 8, 1979 and November 11, 2001, and 0.08 mm/yr for the vertical component between September 7, 1979 and May 11, 2014. These low motion rates indicate that
IP
the fault at the Laoqianning site has been tightly locked for at least the last 30 years.
CR
3.8 Estimated average creep rates
The analyses outlined above enable the determination of movement rates for various periods for the three components at the seven sites of the NWXFZ using the methods described in subsection 2.3; these results are
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summarized in Table 2. Long-term average rates (shown in bold in Table 2) have been estimated over those selected and relatively long periods (i.e., decades) in which earthquake effects, or (in rare cases) weather and
AN
unknown-causal effects, on the observed fault motion were either insignificant (have been ignored) or excludable (have been removed from the long-term average rates), as analyzed in subsections 3.1 to 3.7 and in Supplementary Materials 2 and 3.
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The data given in Table 2 indicate that the average strike-creep rates estimated using two different methods, i.e., the simple average and the nonlinear average, respectively, are similar for the relatively long
ED
periods shown in bold digits for each site, but are either somewhat or markedly different for relatively short periods associated with earthquake effects. The latter include strike-slip creep estimates for the Laoqianning
PT
site for periods after November 2001 and the Gelou and Xuxu sites for periods after May 2008. In addition, strike-slip creep rates estimated using the nonlinear method are more steady and predictable than those estimated by the simple average method (Table 2; Figs 4, 5, and 7). This indicates that the effects of
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earthquakes need to be determined and minimized (as much as possible) during estimates of the long-term average rate of fault movement using fault-crossing geodetic observations. Furthermore, Figs 4, 5, and 7, as
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well as the corresponding analyses presented in subsections 3.1–3.7, indicate that both sinistral and transverse fault motion components are relatively sensitive to earthquake effects, whereas vertical components appear to be much less sensitive.
18
ACCEPTED MANUSCRIPT
Table 2. Fault-crossing short-baseline and short-leveling survey-based estimated creep rates along the NWXFZ for different periods at seven sites Strike-slip component Sites
Horizontal transverse component
Vertical component
Cumulative Simple Non-linear Cumulative Simple Cumulative Simple Periods Periods sinistral slip average rate average rate movement average rate movement average rate ymd―ymd ymd―ymd (mm) (mm/a) (mm/yr) (mm) (mm/yr) (mm) (mm/yr) 1980/06/14―1981/04/30 1.6±0.22 1.83±0.25 2.17±0.41 1980/06/26―1989/07/19 9.84±0.24 1.08±0.03 1980/06/14―1983/05/03 -3.43±0.18 -1.19±0.06 1982/06/18―2001/11/08 26.49±0.22 1.36±0.01 1.38±0.02 1989/07/19―2001/11/08 7.80±0.24 0.63±0.02 1983/05/03―2001/11/08 1.39±0.18 0.08±0.01 2001/12/17―2008/05/07 4.79±0.22 0.75±0.03 0.89±0.06 2001/12/18―2008/05/07 3.87±0.24 0.60±0.04 Zhuwo 2001/12/17―2008/05/07 2.78±0.18 0.43±0.03 2008/06/11―2014/06/10 3.45±0.22 0.57±0.04 0.75±0.06 2008/06/11―2014/06/10 2.64±0.24 0.44±0.04 2008/06/11―2014/06/10 3.72±0.18 0.62±0.03 1982/06/18―2008/05/07 1980/06/26―2014/06/10 31.95±0.22 1.23±0.01 1.26±0.01 24.05±0.24 0.71±0.01 1982//08/06—1987/03/12 2.3±0.13 0.5±0.03 0.34±0.07 1986/01/15—2008/05/07 1982/08/06—2008/05/07 0.49±0.06 0.02±0.00 -2.92±0.13 -0.13±0.01 1982/08/06—2008/05/07 5.33±0.13 0.21±0.01 0.15±0.01 Gelou 2008/06/11—2011/03/09 0.22±0.06 0.08±0.02 2008/05/07—2011/03/09 0.10±0.13 0.04±0.05 2008/06/11—2013/01/08 0.73±0.13 0.16±0.03 0.08±0.07 2011/03/09—2013/06/08 -2.81±0.06 -1.25±0.02 2011/03/09—2013/06/08 1.04±0.13 0.46±0.04 2013/01/08—2013/06/08 3.85±0.13 9.31±0.09 0.07±0.22 1980/06/13—2008/05/08 4.43±0.17 0.16±0.01 0.16±0.01 1980/06/13—2001/11/09 -3.35±0.42 -0.15±0.02 Xuxu 1980/05/07—2014/06/11 -2.99±0.22 -0.09±0.01 2008/06/11—2014/06/12 -1.51±0.17 -0.25±0.03 0.15±0.04 2001/12/18—2014/06/12 7.01±0.42 0.56±0.03 1976/06/18—1980/06/30 30.39±0.20 7.53±0.05 7.89±0.24 1976/06/18—1980/09/01 2.94±0.39 0.7±0.09 1976/06/20—1980/06/29 5.02±0.15 1.25±0.04 1980/06/30—1997/06/10 62.86±0.20 3.71±0.01 3.71±0.06 1981/04/24—1998/06/10 6.72±0.39 0.39±0.02 1980/06/29—1997/06/15 8.09±0.15 0.48±0.01 Xialatuo 1997/06/20—2008/05/10 25. 88±0.20 2.38±0.02 2.30±0.09 1998/07/20—2008/05/10 2.90±0.39 0.30±0.04 1997/06/15—2008/05/11 2.78±0.15 0.25±0.01 2008/05/20—2014/07/05 10.7±0.20 1.75±0.03 1.98±0.15 2008/05/20—2014/07/05 2.28±0.39 0.37±0.06 2008/05/12—2014/07/06 1.96±0.15 0.32±0.02 1976/06/18—2014/07/05 130.51±0.20 3.43±0.01 3.47±0.02 1976/06/18—2014/07/05 12. 05±0.39 0.32±0.01 1976/06/20—2014/07/06 17.79±0.15 0.47±0.00 1983/05/20—1986/06/14 10.21±0.12 3.32±0.04 2.38±0.35 1983/05/20—1986/06/14 -0.84±0.07 -0.27±0.02 1983/05/19—1986/06/14 -2.64±0.15 -0.86±0.05 1986/06/14—1991/05/18 5.88±0.12 1.19±0.03 1.39±0.22 1986/06/14—1991/05/18 -3.51±0.07 -0.71±0.01 1986/06/14—1991/05/18 -2.42±0.15 -0.49±0.03 Goupu 1991/05/18—2001/11/10 10.9±0.12 1.04±0.01 0.94±0.10 1991/05/18—2001/11/10 -5.73±0.07 -0.55±0.01 1991/05/18—2001/11/10 -4.63±0.15 -0.44±0.01 2001/11/10—2013/03/12 8.61±0.12 0.76±0.01 0.74±0.09 2001/11/10—2013/03/12 -0.32±0.07 -0.03±0.01 2001/11/10—2013/03/12 -2.12±0.15 -0.19±0.01 1983/05/20—2013/03/12 35.59±0.12 1.19±0.00 1.09±0.04 1983/05/20—2013/03/12 -10.4±0.07 -0.35±0.00 1983/05/19—2013/03/12 -11.81±0.15 -0.4±0.01 1985/05/31—1988/05/19 -0.32±0.10 -0.11±0.03 0.06±0.06 1985/05/31—1988/05/19 -0.54±0.10 -0.18±0.03 1985/07/12—1988/05/19 1.60±0.15 0.57±0.05 Longdengba 1986/04/22—2008/05/09 1.62±0.10 0.07±0.00 0.06±0.01 1985/05/31—2011/01/09 2.57±0.10 0.10±0.00 1986/01/30—2014/06/14 -0.78±0.15 -0.03±0.01 2011/01/09—2014/06/13 0.12±0.10 0.03±0.03 0.05±0.06 2011/01/09—2014/06/13 0.42±0.10 0.12±0.03 2011/01/09—2014/06/14 -0.13±0.15 -0.04±0.04 1985/01/28—2001/11/11 2.08±0.31 0.12±0.01 0.15±0.02 1979/09/08—2001/11/11 -1.39±0.13 -0.06±0.01 1979/09/07—2014/05/11 2.88±0.21 0.08±0.01 Laoqianning 2001/12/20—2008/05/09 -0.34±0.31 -0.05±0.05 0.15±0.07 2001/12/20—2008/05/09 1.4±0.13 0.22±0.02 2001/11/11—2008/05/10 1.26±0.21 0.19±0.03 2008/06/12—2014/06/13 -1.6±0.31 -0.27±0.05 0.15±0.08 2008/06/12—2014/06/13 1.9±0.13 0.32±0.02 2008/06/13—2014/06/14 0.47±0.21 0.08±0.03 Notes: Bold values represent selected and relatively long periods and the associated cumulative displacements and long-term average rates for all three components estimated for these periods, in which effects of earthquakes on the observed fault movement were either insignificant or have been excluded. Strike-slip rates are also estimated using a non-linear method based on Equations (4) and (5). The uncertainties of average rates of the three components were estimated by propagating errors associated with the short-baseline and short-leveling measurements and, for non-linear estimated rates, the RMS errors from the fitting to Equation (4) (see Table s1 in the online Supplementary Material 1 for measurement errors and Table 1 for RMS errors). Positive and negative strike-slip rates represent sinistral and dextral creep, respectively, and positive and negative transverse motions represent extension and shortening in a fault-normal direction, respectively, and positive and negative values of vertical motion represent creep associated with normal and reverse faulting components, respectively. The real accuracy of the estimated fault movement rates and their errors should be limited to one digit after the decimal point, rather than to two digits after the point as listed in this table for a result of the calculation. Periods ymd―ymd
T P
I R
C S U
N A
D E
M
T P E
C C
A
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ACCEPTED MANUSCRIPT 4. Creep depth estimates The surface-observed sinistral-creep series at the seven sites along the NWXFZ (especially at Zhuwo, Xialatuo, and Goupu) have a time-dependent characteristic that manifests as a logarithmic increase in cumulative creep displacement or an exponential decrease in creep rate over time. These series accurately fit curves based on Equation (4) (Figs 4, 6, and 7; Table 1) and therefore represent either earthquake afterslip or interseismic slow slip at shallow depths governed by a velocity-strengthening mechanism (Marone et al., 1991; Scholz, 1990). Similar time-dependent creep series have been reported for other active strike-slip fault systems, including the Ismetpasa segment of the North Anatolian fault, Turkey
SC RI PT
(Bilham et al., 2016; Cakir et al., 2005; Cetin et al., 2014; Kunako et al., 2013), and the central and southernmost segments of the San Andreas fault, California (e.g., Marone et al., 1991; Wei et al., 2015). In this section, we use the surface-observed sinistral-creep series to estimate the depths of fault creep at all sites along the NWXFZ from the approach of Marone et al (1991) based on a velocity- and statedependent friction law. At such depths the slip behavior of the studied fault zone theoretically transits from shallow velocity-strengthening friction to deep velocity-weakening friction.
Surface-observed sinistral-creep series data were fitted to Equation (4) using a nonlinear inversion
NU
approach based on the Levenberg–Marquardt algorithm, yielding the best-fitting values of parameters ,
, γ, and v0 for each site (Table 1). Equation (4) is derived from a velocity- and state-dependent friction
MA
law (Marone et al., 1991; Scholz, 1990), in which =n'(a–b)/G/h, is a function associated with the friction rate parameter (a–b), the effective normal stress n', and the thickness h (in km) of the shallow velocity-strengthening layer of the crust, assuming a constant shear modulus G (Marone et al., 1991). The thickness h is also the depth of shallow fault creep. As such, the relationship curves between (a–b) and h
ED
can be plotted using a rearranged definition of (Fig. 8): (a – b) = ( G)/(n' h)
(6)
PT
and by substituting the inverted values of parameter (from Table 1) into Equation (6) for each of the sites where n' = 1.0 + 13.0 h (in MPa) and the shear modulus G = 30 GPa (after Kaneko et al., 2013).
CE
Laboratory measurements of the friction rate parameter (a–b) range between 0.001 and 0.005 (Marone et al., 1990, 1991) and are shown as the vertical distance between the two horizontal dotted lines in Fig. 8. Chang et al. (2013) presented the results of numerical modeling of the depth-variable fault friction of
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aseismic afterslip associated with the 2004 Parkfield, California earthquake based on a multi-layer dynamic model. This research narrowed the range of possible (a–b) values down to ~0.004 to 0.007 for depths of 0–1 km, 0.002 to 0.004 for depths of 1–3 km, and 0.001 to 0.003 for depths of 3–7 km. These new (a–b) values from Chang et al. (2013), which are shown as gray-shadowed rectangles in Fig. 8, were used to estimate the ranges of creep depth h from Equation (6) for the individual sites in the study area, with the results shown in the table insets in Fig. 8. The maximum estimate of h is restricted to 7 km as a result of the distribution model of (a–b) values used during this study. The data shown in Fig. 8 suggest that the estimated depths h of shallow fault creep are highly variable from site to site along the NWXFZ, range between 0.3 and 7 km, and positively correlate with the average surface sinistral-creep rates determined at these sites. For instance, the Xialatuo, Goupu, and Longdengba sites have different creep depths (i.e., h values) of 5.8 to 7, 2.0 to 2.8, and 0.4 to 0.6 km, which
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ACCEPTED MANUSCRIPT correspond to average sinistral-creep rates of 3.47, 1.09, and 0.06 mm/yr, respectively. Experimental and modeling studies suggest that the maximum values of the depth of shallow creep associated with active faults are restricted by the thickness of the sediment cover (e.g., Marone et al., 1990, 1991; Scholz, 1998; Wei et al., 2013). A seismic profile within the study area suggests that the thickness of poorly consolidated sediments (i.e., the topmost layer of the crust with P-wave velocities of ≤5.6 km/s) in the NWXFZ region varies between 1 and 7 km (Wang et al., 2007), suggesting that the range of the estimated creep depth h in Fig.8 is reasonable and that the distribution model of (a–b) values we used during this study (from Chang et al., 2013) is applicable to the study area. The next section focuses on the probable relationship between
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along-strike variations in shallow-creep depth and surface-observation-based fault creep rates.
Fig. 8. Friction rate parameter (a–b) versus shallow creep depth h for all seven sites along the NWXFZ. Each estimate of h is a range of uncertainty represented by a distance shadowed by a gray rectangular box on a specific depth curve.
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The estimated h values for all sites are listed in the inset tables along with the best estimates of derived from Equation (4) and listed in Table 1, as well as long-term average sinistral-creep rates (Table 2).
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5. Spatio-temporal variations in surface creep and influencing factors The long-term average creep rates for all three component directions estimated in section 3 and given in Table 2 are now shown in Fig. 9a. These along-strike distributions of creep rates, combined with the creep time series given in Figs 4, 5, and 7, indicate that the geodetic-observed surface and near-field creep along the NWXFZ is highly spatially and temporally variable. Similar along-strike variations in creep rate have been reported for the Hayward fault in California (e.g., Galehouse and Lienkaemper, 2003; Simpson et al., 2001). Here, we first summarize these spatio-temporal variations in creep rate and their implication for the segmentation of surface slip behavior along the fault zone, and then discuss the probable factors that control these variations by combining additional data such as coseismic surface slip, the elapsed time since previous major earthquakes (Fig. 9b and c), historical ruptures since 1893 (Fig. 9c), relocated earthquake distribution (Fig. 9d and e), and fault geometry (Fig. 9e). The shallow-creep depths estimated in Fig. 8 are 21
ACCEPTED MANUSCRIPT also shown in Fig. 9d.
5.1 Surface creep distribution and segmentation of slip behavior Along the NWXFZ, the southeastern and northwestern parts (or the two peripheral parts) of the 1973 rupture section have been occurring time-decaying sinistral-creep with the largest and second largest average rates, 3.47mm/yr and 1.26mm/yr, respectively, accompanied with much smaller average transverse and vertical motion rates (indicating secondary normal faulting), as observed at the Xialatuo and Zhuwo sites (Fig 9a). The sinistral-creep series at these two sites have similar time-dependent decaying patterns (Figs 4 and 5), a characteristic that follows Equation (4), which is based on a velocity-strengthening friction
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law (Marone et al., 1991), indicating that these sites have been recording the afterslip following the 1973 rupture, which occur within the shallow velocity-strengthening layer of the fault at depths above 6.2–7 km and above 5.8–7 km, respectively (Fig. 8). However, the central part of the 1973 rupture section has recorded little post-1973 afterslip since the early 1980s, despite the fact that very slow time-decaying sinistral creep (at 0.15–0.16 mm/yr) has occurred on this fault part at very shallow depths (above 0.4–1.5 km) (Fig. 8 and Fig. 9d), as observed at the Geluo and Xuxu sites (Fig. 4). This suggests that the central part of the 1973 rupture section has re-coupled rapidly after the rupture, and is now almost relocked.
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Overall, the 1973 rupture section of the NWXFZ shows spatially and temporally variable surface slip behavior: long-lasting and time-decaying post-1973 afterslip on the southeastern and northwestern parts but little post-1973 afterslip and rapidly relocking on the central part.
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The third largest average rate of surface sinistral-creep along the NWXFZ (1.09 mm/yr at the Goupu site) is located within the central part of the 1923/1981 rupture section, which is accompanied with much smaller average transverse and vertical movement rates that are indicative of a reverse-creep component
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(Fig. 9a). The observed sinistral-creep at the Goupu site occurs at a depth above 2.0–2.8 km (Figs 8 and 9d) and has the time-dependent decaying characteristics before 2000 at least (Fig. 5), indicating that this site has recorded the post-1981 afterslip, at least up to the year 2000. The surface sinistral-creep rate also
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gradually decreases from northwest to southeast along the ~70km-long 1923/1981 rupture section, from 3.47 mm/yr at Xialatuo to 1.09 mm/yr at Goupu and perhaps even lower farther to the southeast (Fig. 9a).
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Length-averaged sinistral-creep rates along the 1973 and 1923/1981 rupture sections of the NWXFZ are <2 mm/yr (Fig. 9a), much smaller than typical average geological slip rates (812 mm/yr; Wen et al.,
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1996) and the average far-field sinistral displacement rate of ~10 mm/yr derived from fault-parallel GPS velocity differences across the same two fault sections (Du et al., 2010; Lü et al., 2003). This indicates that the surface slip behavior along these two NWXFZ sections is generally not completely creeping or completely locked, but instead is in locking state with time-decaying post-seismic creep/afterslip and subsequent likely interseismic creep. This inference is supported by the study of Wang et al. (2009), who made a joint inversion using InSAR and GPS data and yielded an average locking depth of 3–6 km for a portion of the NWXFZ that is roughly equivalent to the total extent of the two NWXFZ sections of the 1973 and 1923/1981 ruptures, outlined in our segmentation model (Fig. 9a). The 1893 rupture section is located within the ~40 km long southeastern-most portion of the NWXFZ and has very slow sinistral creep (0.05 and 0.15 mm/yr) along with very slow transverse and vertical motions at the Longdengba and Laoqianning sites (Figs 7 and 9a). Combining these observations with the modeling results shown in Fig. 8 suggests that this fault section is now completely locked up, at least at
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ACCEPTED MANUSCRIPT very shallow depths (0.3–0.6 km). This completely locked section of the fault formed at least as early as the early 1980s (Fig. 7), some 90 years after the 1893 rupture. This means that on this fault section the transfer of the slip behavior from healing to coupling and finally tight locking was relatively fast, given the size of the 1893 surface co-seismic slip (1.6–2.9 m; Li et al., 1998; Papadimitriou et al., 2004; Fig. 9b) and the farfield sinistral-displacement rate (~8 mm/yr from an average fault-parallel GPS velocity difference across the same NWXFZ section; Lü et al., 2003). The section below discusses possible reasons for the relatively
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rapid interseismic coupling along this fault section.
Fig. 9. Distributions of creep rate, slip behavior, coseismic surface slip, historical ruptures, seismicity, and fault geometry along the NWXFZ. (a) Average creep rates of all three components (from Table 2); the sinistral-creep 23
ACCEPTED MANUSCRIPT rate is shown with the estimates from the non-linear method. (b) The 1973 coseismic surface slips from Wen (1995; solid green lines) and from Papadimitriou et al. (2004) (and hence from field investigations by R. Weldon and X. Wen in 1995 and 1996) (dashed green lines). The 1923 surface slips (solid and dashed orange lines) are from Li et al. (1998), with dashed orange lines indicating uncertain surface slips. The 1981 and 1893 rupture data are from Li et al. (1998; solid lines) and Papadimitriou et al. (2004; dashed lines). (c) Pattern of historical ruptures and seismic gaps (modified from Wen et al., 2008) where horizontal lines represent the extent of historical ruptures and dashed lines indicate inferred rupture extents. (d) Depths and uncertainties of shallow creep at all sites (shown as purple boxes) are derived from Fig. 8. A 95% cutoff depth in seismicity is used to infer the bottom boundary of the seismogenic layer (after Smith-Konter et al., 2011). The relocated seismic data are for the period
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between Jan. 1981 and Dec. 2012, and are from a final research report by Jiang et al. (2014). Relocations were performed by Fang Lihua using a simplex algorithm and an updated velocity model, and have the following uncertainties of hypocentral locations: <1.5 km for 40% of the events and <3.0 km for 70% of the events in the horizontal direction, <2.0 km for 20% of the events and <4.0 km for 40% of the events in the depth direction. Areas A–D are delineated with blue dashed curves and indicate fault patches with seismicity distributions, whereas E–G indicate fault patches lacking seismicity. The 1981 aftershock area is shown after Yi et al. (2001) with aftershock hypocenters excluded. (e) Map view of the main fault trace and its geometry (Allen et al., 1991)
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with the relocated epicenters from Jiang et al. (2014).
5.2 Discussion: factors affecting spatio-temporal variations in surface creep
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This section focuses on the factors that probably influenced or controlled spatio-temporal variations (especially the marked along-strike variations) in surface-observed sinistral-creep rates along the NWXFZ (Fig. 9a) using the additional information in Fig. 9b–e as well as by combining velocity- and statedependent friction theories. The along-strike distribution of shallow creep (or velocity-strengthening) is
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shown in Fig. 9d as a purple dotted curve that connects estimated shallow-creep depths (purple boxes) for all of the sites. Figure 9d also shows several fault areas with various rates of seismicity since January 1981
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(labeled ‘A’ to ‘G’) within the deeper and velocity-weakening seismogenic layer based on the relocated earthquake catalog of Jiang et al. (2012). These areas (including the 1981 aftershock area) indicate fault patches with various degrees of interseismic coupling, including locked patches (e.g., areas E–G, all of
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which are free of seismicity), as the surface-observed sinistral-creep rates over these areas (03.47 mm/yr) are much lower than the far-field sinistral motion rates along the NWXFZ (810 mm/yr; Du et al., 2010; Lü
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et al., 2003). Geodetically observed locking segments of faults may also show microseismicity, as exemplified by the Anza section of the San Jacinto fault in California (e.g., Jiang and Fialko, 2016; Wang, et al., 2013). Theoretically, during interseismic periods a fault plane within the shallow-creep layer can produce stable sliding that is controlled by velocity-strengthening friction, whereas fault planes within the underlying seismogenic layer may create unstable sliding that is associated with microseismicity and governed by velocity-weakening friction (e.g., Marone et al., 1990, 1991; Scholz, 1998). This theoretical framework means that surface-observed creep rates are not simply translations of the sliding rates of faults within the underlying seismogenic layer. 5.2.1 Along-strike variations in shallow-creep depth The shallow creep depth, or the thickness of the shallow velocity-strengthening layer, is markedly variable along the strike of the NWXFZ, ranging from 0.4 to 7 km (including uncertainties; Fig. 9d). The analysis outlined in section 4 suggests the presence of a rough positive correlation between shallow-creep 24
ACCEPTED MANUSCRIPT depth and surface-observed average sinistral-creep rate at all of the sites (Fig. 8). This means that the thickness of the shallow velocity-strengthening (or creep) layer and its along-strike change probably control the spatio-temporal variations in surface-observed sinistral-creep rates (Fig. 9a and d). This is examined further below. A number of observations, including laboratory experiments focusing on fault friction, field investigations of fault zones, statistical analyses of the depth distribution of earthquakes along faults with and without well-developed gouge zones, and dynamic simulations, all suggest that the vertical thickness of the shallow velocity-strengthening layer (or the depth of the shallow-creep layer) within strike-slip fault
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zones is closely associated with the degree of development of shallow unconsolidated gouge and is restricted by thicknesses of poorly consolidated sediment (e.g., Kaneko et al., 2013; Marone and Scholz, 1988; Wei et al., 2013). A shallow velocity-strengthening layer with a well-developed gouge zone may act to arrest co-seismic slip from an underlying velocity-weakening layer during major earthquakes, yielding a slip deficit that upon relaxation produces afterslip during postseismic periods, and to produce slow stable sliding (shallow interseismic creep) under tectonic loading during interseismic periods (Marone and Scholz, 1988; Marone et al., 1991). Therefore, where thicker shallow velocity-strengthening layers exist, more co-
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seismic slip from underlying velocity-weakening layers are arrested and longer duration and larger cumulative amount (or higher rate) of afterslip yield. This theory matches very well the near-fault geodetic and seismo-geological observations on the NWXFZ.
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The northwestern and southeastern parts of the 1973 rupture section have relatively large thicknesses of the shallow velocity-strengthening layer and hence relatively high average sinistral-creep rates (longlasting and time-dependent afterslip), but both have only relatively small coseismic surface slip during the
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1973 rupture; however, the central part of the same rupture section has a very small thickness of the shallow velocity-strengthening layer and hence very low average sinistral-creep rates ( with little afterslip), but had relatively large coseismic surface slip during the rupture (Fig. 9a and b, also see Figs 4, 5, and 8).
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This complementary relation between the 1973 surface coseismic sinistral-slips and the average post-1973 sinistral-creep rates actually suggests a causal relationship between the long-term sinistral-creep rates and
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the thicknesses of the shallow velocity-strengthening layer along the NWXFZ. This in turn indicates that the marked spatial and temporal variations in sinistral-creep rate along the NWXFZ are likely related to variations in the thickness (and along-strike variations) of the shallow velocity-strengthening (or creep)
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layer. This probable correlation is summarized and illustrated in Fig. 10, which uses the data shown in Fig. 9 and data presented in this section. The analysis outlined above is based solely on the estimates of shallow creeping depths shown in Fig. 8, meaning that these analyses are based on the theoretical framework of rate-state friction. These estimates therefore depend on assuming the depth-varying friction properties of the fault within the brittle crust and hence on the validity of Equations (4) and (6) in the real NWXFZ as well as whether the simulation- and experiment-derived values of the friction rate parameters ab are applicable to the real fault. These dependencies would result in large uncertainties in the estimated creep depths and the physical meaning of these depths. As such, our above analysis of the relationship between surface-observed creep rates and shallow creep depths is only one scenario based on the assumed depth-varying frictional properties of faults. Another possible scenario would be fault creeping with no limit of depth (e.g., Avouac, 2015), or some
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intermediate model (Kelin Wang, 2017, personal communication, 13 Sept.), within the brittle crust.
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Fig. 10. Sketch 3D model of the NWXFZ that summarizes and illustrates the probable correlation between the spatial distribution of surface-observed sinistral-creep rates, estimated shallow-creep depths, inferred fault patches with various degrees of interseismic coupling, and the relative movement of blocks on either side of the fault zone. The
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distribution of sinistral-creep rate is from Fig. 9a, and the depth distribution of the shallow creep layer is from Figs 8 and 9d. The bottom boundary of the seismogenic layer is inferred from Fig. 9d, and the fault patches within the seismogenic layer are from Fig. 9d based on distinct seismicity distributions. The sinistral shear rates along the ductile
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shear zone are from the far-field fault-parallel GPS velocity differences across various sections of the NWXFZ, and the fault geometry is from Fig. 9e. Paired arrows on the surface fault trace indicate the locations of the survey sites as well
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as the average sinistral-creep rates at these sites.
5.2.2 Restraining bend geometries of the NWXFZ Subsection 5.1 indicates that the NWXFZ contains two surface-observed locked portions, namely the
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central part of the 1973 rupture section and the entirety of the 1893 rupture section. These fault portions have thin (0.41.5 km and 0.30.6 km thick, respectively) shallow creep (or velocity-strengthening) layers
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(Figs 9a and d, 10), but show rapid or relatively rapid post-seismic coupling of the fault. So, what is the key factor controlling the rapid coupling and the long-term locking behavior of these two fault portions? Allen et al. (1991) used geological mapping to identify two restraining bends along the NWXFZ; i.e., a clockwise change in strike of ~9° over ~12 km centered near Gelou along the central part of the 1973 rupture section and a clockwise change in strike of ~10° over ~20 km (a length updated during this study) centered near Longdengba along the 1893 rupture section (Figs 9e, 10). These authors suggested that both restraining bends may act as geometrical asperities during major earthquake cycles, a hypothesis that is now supported by new data given during this study. Firstly, a comparison of the coseismic surface slip shown in Fig. 9b and the fault geometry shown in Fig. 9e indicates that coseismic slip increased rather than decayed or terminated when passing through the restraining bends near Gelou or near Londengba during the 1973 or 1893 earthquakes. In addition, these restraining bends may not only be present within the thin and shallow velocity-strengthening layer but also within the underlying velocity-weakening seismogenic 26
ACCEPTED MANUSCRIPT layer. This is indicated by the fact that these restraining bends correspond to two distinct parts of the fault (patches ‘E’ and ‘G’) that are free of seismicity at depths above ~14 and 9–16 km, respectively (Figs 9d, 10). This complete lack of seismicity implies that almost no detectable unstable sliding has occurred within fault patches E and G since at least January 1981, an indicative that these two fault patches respectively along two restraining bends have been completely locked within the whole depth of the seismogenic layer. These suggest that the two restraining bends near Gelou and Londengba have the property of large-scale and persistent compressive asperities. The faults surrounding these types of asperities have a relatively large frictional resistance to sliding and undergo relatively fast stressing, both of which allow a more rapid
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increase in strain on the asperity-associated fault parts during interseismic periods, and hence allows more moment release and larger coseismic slip during earthquakes along these fault parts (e.g., Ruff, 1992; Scholz, 1990). Therefore, the two compressive asperities (in the form of the restraining bends near Gelou and near Londengba) along the NWXFZ are likely the key factors that controlled the rapid fault coupling along the central part of the 1973 rupture and the relatively rapid fault coupling along the 1893 rupture, respectively, and have resulted these parts/portions of the NWXFZ the in long-term locking behavior (as per the surface observations).
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Although it remains unclear what have caused the two surface-observed locked parts/portions of the NWXFZ to be associated with thin shallow creep (or velocity-strengthening) layers (Figs 9d and e, 10), a dynamic numerical simulation by Kaneko et al. (2013) suggested that the frictional properties of surface-
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observed locked segments of a strike-slip fault are dominated by velocity-weakening conditions at shallow depths (figure 11 in Kaneko et al., 2013). This is most likely the case for the two surface-observed locked parts/portions with restraining bend geometries located near Gelou and Londengba, as both of them have
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very thin to negligible velocity-strengthening layers at shallow depths. Therefore, distinct restraining bend
6. Conclusions
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geometries can be a significant control on the shallow slip behavior of strike-slip faults.
The ~200 km long northwestern Xianshuihe fault zone (NWXFZ) of southwestern China has been the
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subject of more than three decades of fault-crossing geodetic observations. These data indicate that the near-field surface movement of the fault zone is dominated by sinistral creep but that creep rates in all three component directions (i.e., strike-slip, transverse, and vertical) show marked spatial and temporal variations.
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Combining these data with additional information on historical ruptures, coseismic surface slip, relocated seismicity, fault geometry, and far-fault movement velocity, as well as using an approach derived from the velocity- and state-dependent friction theory, our analysis indicates that these variations reflect the alongstrike segmentation of surface slip behavior. The 1973 rupture section of the NWXFZ shows spatiotemporal variable surface-slip behavior, where long-lasting and time-dependent post-1973 afterslip has occurred along the northwestern and southeastern parts of the section at depths above 5.8–7.0 km with sinistral-creep rates of 1.26 and 3.47 mm/yr, respectively, and nearly locked with little post-1973 afterslip but very slow sinistral-creep (0.16 mm/yr) along the central part of the section at depths above 0.4–1.5 km. The 1923/1981 rupture section of the NWXFZ is generally in locking state with shallow sinistral creep at 1.09 mm/yr on its central part at depths above 2.0–2.8 km, which contains the post-1981 afterslip, and probably the subsequent interseismic creep. The 1893 rupture section has been completely locked since at
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ACCEPTED MANUSCRIPT least the early 1980s, indicating that this section underwent relatively rapid interseismic coupling. Our study also suggests that the thickness (and along-strike variations in thickness) of the shallow velocity-strengthening (or creep) layer is an important control on the temporal and spatial variations in surface-observed creep rates along the NWXFZ. This is not just because the shallow-creep depths and surface-observed sinistral-creep rates show a positive correlation at all of the sites in the study area, but also because a complementary relationship is present between average sinistral-creep/afterslip rates and the coseismic surface slip along the 1973 rupture section of the NWXFZ. The other factor affecting the spatial and temporal variations in surface-observed creep rates is the geometry of restraining bends in the fault.
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The two surface-observed locked fault portions, namely the central part of the 1973 rupture section and the entirety of the 1893 rupture section, are located along two distinct restraining bends within the NWXFZ associated with clockwise rotations in fault strike of ~910° over distances of ~1220 km near Gelou and Longdengba, respectively. These restraining fault bends would act as persistent and compressive asperities and have enabled these two fault portions to have long-term locking behavior.
At least nine earthquakes, including five M6.5–8.0 events at epicentral distances of 50–300 km, two M7.5–8.1 events at distances of 1000–1400 km, and two Mw9.0–9.2 megathrust events at distances of
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3000–4000 km, have affected, to various extents, the geodetically observed motion of the NWXFZ. It is necessary to exclude or deduce significant earthquake effects in the average rate estimation of surface fault
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movement.
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ACCEPTED MANUSCRIPT Acknowledgments The authors thank Kelin Wang, Xinglin Lei and an anonymous reviewer for their comments, suggestions, criticisms and advices that enable us to have significantly improved earlier versions of the manuscript. The authors also thank the Institute of Geodetic Engineering, Earthquake Administration of Sichuan Province, for providing the fault-crossing survey data, Fang Lihua for sharing relocated seismicity data, He Chang-rong for information related to friction theory, Liu Jing for introducing the authors to relevant publications, and Li Fei-fei for helping in the construction of two figures in Supplementary Material 1. This research was supported by the National Key Technology Research and Development
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Program of China (Grant No. 2012BAK19B01-02) and the State Key Laboratory of Earthquake Dynamics
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of China (Grant No. LED2015A02).
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ACCEPTED MANUSCRIPT References Allen, C.R., Luo, Z.L., Qian, H., Wen, X.Z, Zhou, H.W., Huang, W.S., 1991. Field study of a highly active fault zone: the Xianshuihe fault of southwestern China. Geol. Soc. Am. Bull. 103, 1178–1199. Allen, C.R., Smith, S.W., 1966. Parkfield earthquakes of June 27–29, 1966, Monterey and San Luis Obispo counties, California: preliminary report, pre-earthquake and post-earthquake surficial displacements. Bull. Seismol. Soc. Am. 56, 966–967. Allen, C.R., Wyss, M., Brune, J.N., Granz, A., Wallace, R., 1972. Displacements on the Imperial,
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Graphical Abstract
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Highlights Decades of fault-crossing geodesy show fault afterslip and slow creep and locking.
Along-strike variation in slip behavior is related to creep depth and fault geometry.
Time-decaying afterslip occurs on fault sections with relatively large creep depths.
Two surface-locked fault sections are located along restraining fault bends.
Observed fault motion response to M6.5–9.2 earthquakes at distances of 50–
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4000 km.
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