Spatial and temporal variations of stress field in the Longmenshan Fault Zone after the 2008 Wenchuan, China earthquake

Tectonophysics 767 (2019) 228172

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Spatial and temporal variations of stress field in the Longmenshan Fault Zone after the 2008 Wenchuan, China earthquake

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Yan Luoa, Li Zhaob, , Jianhui Tiana a b

Key Laboratory of Earthquake Prediction, Institute of Earthquake Forecasting, China Earthquake Administration, Beijing 100036, China School of Earth and Space Sciences, Peking University, Beijing 100871, China

A R T I C LE I N FO

S UM M A R Y

Keywords: Longmenshan Fault Zone Earthquake focal mechanism Stress field Wenchuan earthquake

We investigate the temporal and spatial patterns of the regional stress field along the Longmenshan Fault Zone (LFZ) following the 2008 Wenchuan Mw7.9 earthquake. Regional broadband seismic records are used to determine the focal mechanisms and centroid source depths of > 400 aftershocks of magnitudes 3.3 ≤ Mw ≤ 5.9. The focal mechanisms are then used to invert for the temporal and spatial variations of the regional stress field. Results show clear spatial changes in the directions of principle stress axes as well as the faulting types along the LFZ. The regional stress field is in an overall thrust regime with nearly horizontal maximum compressional axis. In the southern section of the LFZ, in addition to the primarily thrust background with maximum compressional axis perpendicular to the main fault zone, strike-slip component is also present. In the middle section, the maximum compressional axis varies spatially, changing abruptly from perpendicular to parallel to the LFZ. In the northernmost section, the stress environment becomes primarily strike-slip. Our result suggests that the collision of Indian and Eurasian plates led to the first-order feature of the stress field in the study area. The geometry of blocks and faults, the lower crustal flow as well as the shallow crustal property difference are the major factors to controlling the local stress field. Temporal variations of stress field suggest that the mainshock caused a large stress disturbance at early stage in certain areas of LFZ, and the stress field variations with depth imply a different deformation pattern and different stress background in the shallow and lower crust.

1. Introduction The Longmenshan Fault Zone (LFZ) is located in Sichuan Province in southwestern China. Collision between India and Eurasia leads to a large-scale fault zone with dominant thrust and minor strike-slip movement. From west to east, three principle active faults comprise the northeast-trending LFZ: the back range fault zone including the Qingchuan, Wenchuan-Maowen, and Gengda-Longdong faults, the central main fault zone including the Chaba-Linansi, Beichuan-Yingxiu, and Yanjing-Wulong faults, and the front range fault zone involving the Jiangyou-Guangyuan, Guanxian-Jiangyou, and Shuanshi-Dachuan faults. To the west of the LFZ locate the relatively active Minjiang and Huya faults, with the N-S-trending Minshan uplift (Tang and Han, 1993) in between. GPS measurements show a slow slip rate of < 2–3 mm/a across the LFZ (Shen et al., 2005). Due to the eastward movement of the Tibetan Plateau and geometry of the fault system, the relative motion across the LFZ is largely thrust with a right-lateral component on a number of reverse faults. As the middle section of the eastern boundary of the Tibetan Plateau, the LFZ is the location of

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drastic changes in various properties such as gravity and crustal thickness. The average crustal thickness changes from 60–70 km in the west to < 50 km in the east (An et al., 2004). Geodetic observations have shown that the LFZ is experiencing a significant overall uplift as well as a variable horizontal shortening in different segments (Zhang et al., 2010). Before the 2008 Wenchuan earthquake, the LFZ was considered to be seismically locked for thousands of years with relatively low tectonic activity (Densmore et al., 2007). Fig. 1 shows the tectonic background and the stress distribution retrieved from in-situ stress (Xie et al., 2007) and shear-wave splitting (Shi et al., 2009, 2013), the surface horizontal velocity inferred from GPS velocity dataset (Zhao et al., 2015), as well as the seismicity of magnitudes larger than M5.0 since 1970 around the LFZ. The 12 May 2008 earthquake in Wenchuan, China occurred along the central section of the LFZ. The earthquake, with a moment magnitude of Mw = 7.9, caused tremendous human and property losses. Studies based on analysis of geodetic, seismological and geological data demonstrated that the mainshock initiated near Yingxiu and propagated northeastward along multiple fault traces, until Qingchuan,

Corresponding author. E-mail address: [email protected] (L. Zhao).

https://doi.org/10.1016/j.tecto.2019.228172 Received 1 May 2019; Received in revised form 2 August 2019; Accepted 4 August 2019 Available online 05 August 2019 0040-1951/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Tectonic faults and historical earthquakes around the Longmenshan Fault Zone. Circles are epicenters of M ≥ 5.0 earthquakes from January 1970 to December 2017. Triangles are stations used in waveform inversions for focal mechanisms in this study. Thin black lines represent the tectonic faults, while thick red lines indicate boundaries between active blocks. GPS velocity observations with respect to the Eurasian frame are shown in blue arrows (Zhao et al., 2015). Green, yellow, and black bar denote the in-situ stress measurement from hydraulic fracturing, well-hole breaking and de-stressing, respectively (Xie et al., 2007). Blue bars depict the directions of fast shear-wave polarization (Shi et al., 2013).

2011). The tectonic stress field inversion based on earthquake focal mechanism solutions in LFZ also was conducted (e.g. Luo et al., 2015; Zhang et al., 2015; Lin et al., 2018; Yang et al., 2018; Li et al., 2019), obtaining different scale stress variation pattern in LFZ. In addition, there also are stress distribution from in-situ stress measurements (Xie et al., 2007; Wu et al., 2010; Du et al., 2013; Meng et al., 2013; Qin et al., 2014; Meng et al., 2015) and shear wave splitting analysis (Shi et al., 2009, 2013), which are sparsely distributed. Lin et al. (2018) show the complex stress field of the LFZ retrieved from focal mechanisms of the Wenchuan earthquake aftershocks that occurred before September 2010 and analyzed their tectonic implications. Yang et al. (2018) inverted regional-scale (0.1° spacing) pattern of stress orientation based on focal mechanisms and the widespread lateral variations are observed in the LFZ. They also analyzed the stress field in two time periods, before and after September 2008, and found the differences between two stress fields is pretty small. But they didn't carry out the stress inversion with depth. Li et al. (2019) inverted the stress field across the LFZ based on 391 focal mechanisms of earthquakes (M ≥ 3.5) that occurred between 2009 and 2016. They gridded the study area with a note spacing of 0.2° × 0.2° in longitude and latitude with a constant radius of 0.2°. It is usually believed that the early postseismic processes are more rapid and early aftershocks are more abundant, thus the stress inversion only using the early aftershocks can provide new information about temporal variation immediately after the mainshock. Furthermore, the attempt to study stress field for every depth of 5 km might suffer from too few earthquakes in some depth ranges. These studies help us better understand the rupture distribution

generating ~300 km-long surface rupture zone along the LFZ (Xu et al., 2009). Following the mainshock, abundant aftershocks with different faulting types, with hundreds of Mw ≥ 4.0, > 70 Mw ≥ 5.0 and 9 Mw ≥ 6.0 as of 30 October 2010. The aftershocks of the great Wenchuan earthquake are distributed along a 240-km long northeast trending rupture zone and a northwest-trending belt extending for 60 km from Xiaoyudong (Fig. 2). Earthquakes are attributed to the buildup of regional stress in the crust over a long period of time (Zhang et al., 2009). Therefore, focal mechanisms of the earthquakes are closely linked to the stress field in the source region. Besides, the formation and evolution of geological structures are the result of the tectonic stress field accumulation and changing temporally and spatially. Therefore, stress fields control the activities and properties of seismologic structures, and on the other hand, seismicities and tectonic deformation models can also reflect the characteristics of stress fields. Depiction of the detailed temporal and spatial pattern of stress field in LFZ is important for better understanding the seismogenic processes of intraplate earthquakes, the stress environment, the tectonic deformation and evolution model and improving the seismic hazard assessments. Many previous efforts have been made in studying the focal mechanisms using the Wenchuan earthquake aftershocks, obtaining the similar results with obviously segmented feature that the southern part of the rupture zone was dominated by thrust faulting, while the northern part is under the reverse faulting stress regime with some strike-slip components (e.g. Hu et al., 2008; Wang et al., 2009; Zheng et al., 2009; Yi et al., 2012; Lv et al., 2013; Yi et al., 2016; Cui et al., 2

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Fig. 2. Distribution of the aftershocks of 2008 Wenchuan earthquake up to 30 October 2010 used in this study. Circles are epicenters with sizes representing magnitudes and colors indicating source depths (red: depth above 10 km; green: depth below 10 km). Other map features are the same as Fig. 1.

several regional provincial seismic networks. Most of the records were from Sichuan (SC) provincial network, and the rest came from Shanxi (SN), Gansu (GS) and Chongqing (CQ) regional broadband networks. Fig. 1 shows the distribution of the stations. We use the cut-and-paste (CAP) method (Zhao and Helmberger, 1994; Zhu and Helmberger, 1996) to determine the focal mechanisms of the earthquakes. The CAP method solves for the best double-couple fault-plane solution, the depth and the moment magnitude of an earthquake by fitting the three-component broadband waveforms. It cuts a three-component record into five waveform windows: two for the Pnl wave trains on the vertical and radial components and three for S/ surface waves on the vertical, radial and transverse components. Synthetic seismograms are computed using one-dimensional (1D) Earth models by the frequency-wavenumber integration method (Zhu and Rivera, 2002). The windowed Pnl and S/surface waveforms are bandpass filtered in different frequency bands with the Pnl having a slightly shorter period, and appropriate weights are assigned to Pnl and S/ surface waves to prevent the solution from being dominated by the higher-amplitude S/surface waves. The enhanced contribution by the Pnl waves is helpful to better constrain the focal depth because the depth phases such as sPmP (or sPg) and sPn within the Pnl wave train are more sensitive to the source depth (Stein and Wiens, 1986; Langston, 1987). The fitness between recorded and synthetic waveforms are measured by cross-correlation, and a certain amount of time shift is allowed between each pair of recorded and synthetic traces to account for the possible travel time uncertainty caused by errors in the event location and origin time and the difference between the actual Earth structure and the 1D model used in computing the synthetics. The CAP method adopts a grid search process to find the optimal

and process as well as the state of stress during the earthquake. However, many questions about Wenchuan earthquakes still remain unclear. The purpose of this study is to conduct a systematical determination of the early focal mechanisms of the Wenchuan earthquake sequence and then invert reliable stress variation pattern based on these focal mechanisms to investigate the detailed spatial and temporal variations of the regional stress field in a small-scale after the Wenchuan earthquake using early aftershocks, to investigate the relation between the stress fields and the seismologic structure responsible for large earthquake, to explore the early stress disturbance caused by mainshock and the differences of the stress fields at different depths associated with tectonic deformation model issue, to better understand the seismogenic process and physical mechanism of large earthquakes. A large number of aftershocks following the Wenchuan mainshock provide us with sufficient data to study more detailed spatial and temporal variations of the stress field. We first use the cut-and-paste (CAP) method (Zhao and Helmberger, 1994; Zhu and Helmberger, 1996) to solve for the focal mechanisms of > 400 aftershocks, and then invert the focal mechanisms for the spatial variations with the grid spacing of 0.1° x 0.1° in latitude and longitude as well as with two depths intervals and temporal variations with three time intervals of the responsible stress field around the LFZ.

2. Focal mechanism solutions of Wenchuan aftershocks The earthquakes used in this study are events occurred after the 12 May 2008 Wenchuan earthquake until 30 October 2010 (Fig. 2). We collected broadband waveform records from permanent stations of 3

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the radial and transverse directions. The 1D model used in this study (Fig. 3a) is an average layered model (Zheng et al., 2009) for the LFZ region, which combines results from previous active-source studies and Crust2.0. It incorporates features in the crustal structures of the Sichuan Basin in the east and the Tibetan Plateau in the west. Each focal mechanism is determined manually through an iterative process. After each iteration, traces with low cross-correlation coefficients (< 60% for Pnl and 70% for S/Surface waveforms) between records and synthetics are discarded. Fig. 3 shows an example of the CAP solution for an aftershock on 16 May 2008. We determined the focal mechanisms for a total of 404 Wenchuan aftershocks using the CAP method. The magnitudes of these events range in 3.3 ≤ Mw ≤ 5.9, and all except two events have centroid depth above 20 km. To examine the reliability of our results, we searched the catalog of the global centroid-moment tensor (gCMT) solution for Wenchuan aftershocks to compare with our solutions. We found 29 events in the gCMT catalog for which we also have CAP solutions. We quantify the difference between our CAP solutions and the gCMT results by the Kagan angles (Kagan, 1991). The Kagan angle between two faultplane solutions is the minimum angle of rotation from the P, B and T axes of one fault-plane to those of the other. The larger the Kagan angle,

solution of the focal depth and fault-plane solution. Based on the depth range of the seismogenic zone, a series of trial focal depths are selected. For each trial focal depth, a grid search is conducted for the strike, dip and rake angles to find the optimal fault-plane solution which provides the best overall fit between the synthetic and recorded waveform traces. The moment magnitude is then determined by a least-squares fit between the amplitudes of the records and synthetics. For the case of sparse seismic network, the CAP method has been improved for better constrained focal depth via including both teleseismic and local waveform data (Chen et al., 2015), as well as spectral amplitude of Rayleigh waves (Jia et al., 2017). But, for the case of relatively dense seismic network in our study region, we just adopt the classic CAP method which is more straightforward in inversion of earthquake source parameters for hundreds of events. In practice, we select the phases used in the CAP solution based on the earthquake magnitude. For earthquakes of magnitudes below 5, we use P waves within 100 km and S/surface waves within 200 km; whereas for earthquakes of magnitudes above 5, we use P waves within 150 km and S/surface waves within 300 km. We discard traces with low signal-to-noise ratio, remove the instrument responses as well as the means and linear trends, and then rotate the horizontal components to

Fig. 3. Example of focal mechanism inversion using the CAP method for the Mw5.5 aftershock that occurred at 13:25 on 16 May 2008. (a) 1D layered model used for computing the synthetic seismograms. (b) Locations of stations used in the inversion. (c) Normalized waveform misfits for all the trial centroid depths. The best trial depth is 12 km. (d) Comparison of observed (black) and synthetic (red) waveforms at 10 stations for the best trial centroid depth. The waveform fit is evaluated independently for a maximum of five traces at each station including P waves on the vertical and radial components (PZ, PR) and S/surface waves on the vertical, radial, and transverse components (SZ, SR and ST). The left column lists station codes with epicentral distances in km. The beachball at the top-left corner (lowerhemisphere stereographic projection) is the optimal fault-plane solution for the best trial centroid depth of 12 km. The text at the top gives the root-mean-square (rms) waveform misfit value (in cm/s) and the errors (err) in strike, dip and rake angles (in degrees). 4

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Fig. 4. Plot of the source centroid depth differences vs. Kagan angles between the CAP results in this study and gCMT solutions for the 29 common earthquakes. The beachballs represent the focal mechanisms from the CAP results. The sizes and colors of the beachballs indicate the magnitudes and centroid depths, respectively, in the CAP results.

events (above 10 km) have dipping angles of 50°–70°, larger than deeper events (20°–30°). The P axes of the events (Fig. 5c) are oriented northwest-southeast (NW-SE), perpendicular to the LFZ, and are nearly horizontal (plunge < 35°); whereas the T axes (Fig. 5d) are nearly vertical. The events in the middle section (from Anxian to Nanba) are also mostly thrust, with a few strike-slip and normal ones. From Anxian to Beichuan, the P axes display a complicated pattern. Although largely nearly horizontal, some of the P axes are oriented northeast-southwest (NE-SW), sub-parallel to the LFZ, while others are oriented NW-SE, nearly perpendicular to the LFZ. Similarly, the T axes are also highly variable with some nearly vertical and others nearly horizontal. North of Beichuan, the P axes become largely oriented NE-SW, sub-parallel to the LFZ, with only a few P axes in NW-SE direction perpendicular to the LFZ. In the northeast section, although still primarily thrust, there are more strike-slip events but no normal ones. North end of Qingchuan, strike-slip becomes the predominant mechanism, with a few thrust events. P axes are mostly horizontal, with both NW-SE and NE-SW orientations. In the NW-trending Xiaoyudong section, the events are primarily left-lateral strike-slip with fault planes dipping 60°-70°. The dip angle of the Xiaoyudong Fault (XF) is quite large in the range of 60–70°. This fault seems to be aligned with geologically mapped Miyaluo Fault, and a ~6 km long ground break is observed (Xu et al., 2009). But a 60 km long band of aftershocks occurred near the Miyaluo Fault, which could have been reactivated by the Wenchuan mainshock. The mainshock might have triggered this fault along both horizontal and vertical dimension, thus explaining scattered distribution of focal depth. The XF also cut through the Penguan complex and Xuelongbao complex (Chen et al., 2009), which are mechanically strong and may store substantial strain energy in the shallow section, thus making possible multiple M5+ aftershocks. Previous studies (Deng et al., 2011) demonstrated that the XF is a transpressure fault which accommodate different slip rate of LFZ at this location. As the slip rate of the southwestern segment is much bigger than the northeastern segment of LFZ, thus left-lateral motion is expected for XF, consistent with focal solutions of aftershocks. There are a few events with normal faulting mechanisms in this band of aftershocks along XF, which could be due to strong stress release of the mainshock. Both P and T axes of Xiaoyudong section are nearly horizontal, with P axes oriented east-west (E-W) and NE-SW. The nearly EW compression leads to the strike-slip motion. Events near the main rupture zone are clearly influenced by the compression across the LFZ and have increasing thrust component. Our results are consistent with other studies (Li et al., 2019; Yang et al.,

the more different are the two fault-plane solutions. Ideally, for the same earthquake, different methods should yield similar fault-plane solutions, i.e., the Kagan angle between the two fault-plane solutions should be small. Fig. 4 shows the Kagan angles for the 29 common earthquakes in our CAP solutions and in the gCMT catalog. Most of the events in Fig. 4 have Kagan angles < 30°, indicating a good agreement between the two sets of focal mechanism solutions. The discrepancy between focal mechanism solutions obtained by different methods can be caused by several factors. One possible factor is the difference in the source depths determined by different methods due to the differences in the structural models as well as the types and frequencies of seismic waves used. The gCMT solutions are obtained using a global Earth model and relatively long-period teleseismic surface waves, which may lead to larger uncertainties in source depths, especially for smaller events. On the other hand, in our study we use a regional 1D model and intermediate-period P (3–20 s) and S (10–50 s) waves, which provides a better resolution for source depth. From the plot in Fig. 4 we can see that the source depths in the gCMT solutions are systematically greater than our CAP solutions. However, Fig. 4 seems to show no clear correlation between the discrepancies in source depths and Kagan angles. Another factor that may influence the Kagan angle is the difference in the point source parameterization. The CAP method only solves for the double-couple fault-plane solutions, whereas gCMT solutions contain non-double-couple (CLVD) components. Therefore, we may expect that events with a stronger CLVD component would yield larger Kagan angles between the two solutions. From Fig. 4 we can see that this is indeed the case. However, the events with stronger CLVD and larger Kagan angles are mostly events of smaller magnitude, for which the gCMT solutions may be less robust given the possible lack of observations as well as the tradeoff between the CLVD strength and other source parameters. Therefore, for events with larger Kagan angles in Fig. 4, we conclude that our CAP results are more robust than the gCMT solutions. The 404 focal mechanisms are displayed in Fig. 5 in both map and cross-sectional views. Also plotted in Fig. 5 are the horizontal projections of the P and T axes of these focal mechanisms. The results show that the focal mechanisms of Wenchuan aftershocks are segmented along the rupture zone of the mainshock and display similar characteristics as its surface rupture (Xu et al., 2009). In general, the aftershocks are primarily thrust events that are distributed throughout the rupture zone. In the southern section (south of Anxian), thrust events dominate the aftershocks, with a few strike-slip events. Shallow 5

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Fig. 5. Beachball plots of the 404 focal mechanisms determined in this study and the horizontal projections of their P and T axes. (a) Focal mechanism distribution with size and color of each beachball indicating the moment magnitude and faulting type an earthquake, respectively. Blue, green and red indicate thrust (45° ≤ rake ≤ 135°), strike-slip (|rake| ≤ 45° or |rake| ≥ 135°) and normal (−135° ≤ rake ≤ −45°) faulting types, respectively. Black lines represent faults. The cross sections along the profiles A, B, C, D, E, F, and G in (a) are shown in top right panels. (b) Focal mechanisms in cross-sectional view along the KK′ profile shown in (a). Red star indicates the location of the mainshock with a focal depth of 19 km (USGS). (c) and (d) are the horizontal projections of the P and T axes, respectively, of the focal mechanisms in (a). In all plots, the epicenter of the Wenchuan mainshock is depicted by the red star.

However, we have not been able to determine the causative fault planes of the M6+ aftershocks of Wenchuan earthquake, because CAP inversion only resolve point source parameters. Recently, a wavefield decomposition and reconstruction method has been proposed to avoid the ambiguity of focal planes of point source inversion and thus capable of resolving the ruptured fault plane (He and Ni, 2017). In the future,

2018; Yi et al., 2012; Wang et al., 2009; Zheng et al., 2009). From the cross sections of the focal mechanism solutions along the profiles A, B, C, D, E, F, and G, shown in Fig. 5(a), we can see clearly that the geometries of the seismogenic faults are quite different with the lower dip angles in the southwest and nearly vertical dip angles in the northeast along the LFZ. 6

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Michael (1984, 1987) was later extended by Hardebeck and Michael (2006) into the spatial and temporal stress inversion algorithm, called damped regional-scale stress inversion (DRSSI), so that spatially and/or temporally varying stress field can be inverted. The linear stress inversion using focal mechanisms is based on the assumption that the slip vector s of an earthquake coincides with the direction of the tangential traction t on the fault plane (e.g. Michael, 1984, 1987). Therefore, we can write

we will adopt this method to study the rupture details of these strong aftershocks and their connection with nearby weaker aftershocks. The depth distribution of the events in Fig. 5(b) shows that most of the aftershocks occurred in a depth range of 10–20 km, except for a few shallow events towards the two ends of the main rupture zone. In the northeastern section of the rupture zone, the source depths are more scattered, with both the deepest and shallowest aftershocks happening in this region. The deepest event in this region, with a focal depth of 20 ± 2 km, was also the strongest Wenchuan aftershock, a Ms6.4 event occurring on 25 May 2008 in Qingchuan. There are also a few very shallow events with depths of 1–3 km. Luo et al. (2010) also observed moderate aftershocks occurring at very shallow depth, which might be related to the creation of new faults. In the central section from Mianzhu to Pingwu, the aftershocks are all below magnitude 6 with a depth range of 8–20 km, which might be related to the shallow structure in this region, which is characterized by relatively soft layers of Siluric unconsolidated slate, phyllite, and tuff (Deng et al., 2011). The events in the southern section also have a variety of focal depths with deeper events (~19 km) in the southern end and shallow ones (~4 km) in the northwestern tip of the Xiaoyudong rupture zone. This result is consistent with the source depth distribution of 22 km and above observed for earthquakes that occurred before 1995 (Zhao et al., 1997). From a seismogenic point of view, the depth of the deepest earthquake can be considered as a transition from the brittle regime to ductile. The depth distribution of the focal depths determined in this study indicates that the brittle-ductile transition in the LFZ is located at least below the depth of 20 km, close to the transition depth in the destroyed North China Craton (Dong et al., 2018).

t ̂=

 , σ) t (n = s ,̂  , σ) ∣ ∣t (n

(1)

 is the unit normal where σ is the deviatoric part of the stress tensor, n vector of the fault plane, and s ̂ and t ̂ are the unit vectors in the directions of s and t, respectively. The tangential traction on the fault plane can thus be expressed as  − (n  ⋅σ ⋅n ) n . t = σ ⋅n

(2)

The relation in Eq. (1) between the components of the stress tensor and the slip vector is linearized by assuming a constant |t|, which enables the establishment of a linear inverse problem to invert for a spatially uniform deviatoric stress tensor using the slip vectors of a set of earthquakes. Hardebeck and Michael (2006) extended this problem to spatially and temporally variable stress tensors and introduced a damping matrix to control the spatial-temporal smoothness of the stress field, resulting in the damped linear inverse problem

(GT⋅G + e 2DT ⋅D)⋅m = GT⋅d ,

(3)

where m is the vector containing the elements of the stress tensor σ on all the spatial-temporal grid points, d is the vector containing the elements of slip vectors of all earthquakes on the corresponding grid points, and G is the coefficient matrix formed by the unit normal vectors of the fault planes. The spatial-temporal smoothness of the solution is controlled by the damping factor e and the damping matrix D. The damping factor is chosen so as to achieve an optimal compromise between the data error and the model length. Once the stress tensor elements are obtained on each grid point, we can solve for the eigenvectors to derive the orientations of the principle stress axes. In this study, we use the 404 focal mechanisms of Wenchuan aftershocks to invert for the regional stress field around the LFZ by the DRSSI method. We first inverted all the mechanisms using a two-dimensional (2D) horizontal grid, resulting in a map for the orientations of the principle stress axes. After trial inversions using different grid sizes, we settled on a grid size of 0.1° in both latitude and longitude. Taken into account the earthquake location errors and the fact that each earthquake is influenced by the stress field in a certain area, we assign to each grid point the earthquakes within a distance of 15 km. We define a constant radius of 15 km and require a minimum of 5 events within this radius. Namely, the stress tensors are calculated only when the number of events within the radius of 15 km around the node

3. Inversion for regional stress field Earthquakes are caused by the buildup of regional stress in the crust over a period of time. Therefore, focal mechanisms, i.e. the orientations of the P, B and T axes, are closely linked to the stress field. However, the focal mechanism of an individual earthquake is also dependent on the geometry of the fault plane. Therefore, there is usually not an immediate relation between the focal mechanism of a particular earthquake and the stress field (McKenzie, 1969). From the collective behavior of many earthquakes in a region, we will be able to deduce the geometrical properties of the stress field. Angelier (1979) proposed an approach to the inversion of stress field from a cluster of earthquakes by minimizing the difference in the directions of earthquake dislocation and the tangential traction on the fault plane. Since then other inversion methods have also been proposed for spatially uniform and temporally static stress field based on the same assumption, including grid search based focal mechanism stress inversion (FMSI) method of Gephart and Forsyth (1984) and the linear stress inversion with bootstrapping (LSIB) method of Michael (1984, 1987). The LSIB approach of

Fig. 6. Determining the optimal damping factor in the inversion of focal mechanisms in Fig. 5 for regional stress field. (a) Trade-off curve for the damped regionalscale stress inversion using the 404 fault-plane solutions in Fig. 5. Crosses show the trade-off between the data misfit and model length for different values of the damping parameter e. From left to right along the trade-off curve, the damping parameter e runs from 0 to 50. (b) Curvature of the trade-off curve in (a) as a function of damping parameter e. The optimal damping parameter e = 0.6 is chosen where the trade-off curve has the maximum curvature. 7

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Fig. 7. Results of stress field inversion from the 404 fault-plane solutions on a 2D grid with uniform grid spacing of 0.1° in both longitude and latitude. (a) Horizontal projections of the maximum principal stress axis σ1 (maximum compression). The orange circles represent the earthquakes used in stress field inversion. Black, green, and blue line segments indicate thrust, strike-slip, and mixed faulting environments, respectively. Yellow ellipses denote that the 2D stress field display segmented characteristics along the LFZ. Red lines indicate the surface rupture observed (Xu et al., 2009). (b) Horizontal projections of the minimum principal stress axis σ3 (minimum compression).

reaches 5 or more. Following the same approach as in Zhao et al. (2013), we determine the optimal damping factor e by first calculating the trade-off curve between data error and model length (Fig. 6a), and then finding the maximum curvature of the trade-off curve (Fig. 6b). The value of the optimal damping factor is found to be e = 0.6. The 2D map of the stress field is shown in Fig. 7. Shown in the map are horizontal projections of the maximum and minimum compressional axes σ1 and σ3. The stress environment can be considered to be favorable to strike-slip faulting if both σ1 and σ3 are nearly horizontal (plunge < 35°), green line segments in Fig. 7(a), normal if σ1 is nearly vertical (plunge > 55°), thrust if σ3 is nearly vertical (black line segments in Fig. 7a), or mixed-type for the rest of cases (blue line segments in Fig. 7a). The stress field inversion result in Fig. 7 shows that around the LFZ the tectonic stress field is predominantly in a thrust faulting environment with nearly horizontal maximum compression and nearly vertical minimum compression. The stress field also has spatial variation with clear segmentation. Based on the orientation of the principle axes, we can roughly recognize three different sections along the LFZ with different characteristics: the southern section south of Anxian, the middle section between Anxian and Pingwu, and the northern section north of Pingwu. In the southernmost part of the southern section (south of the mainshock epicenter), the stress field is purely thrust, with vertical minimum compressional axes and nearly horizontal maximum compressional axes oriented in WNW-ESE. However, near the HongkouYingxiu and Beichuan regions the orientation of the maximum compressional axes displays a highly variable pattern, and the favorable faulting styles include thrust, strike-slip and mixed type. Studies on the mainshock have suggested that most of the Wenchuan earthquake energy was released in two subevents with the peak coseismic slip near Hongkou-Yingxiu and a secondary peak near Beichuan (e.g. Wan et al., 2017). The relatively complex pattern of the stress axes in these two regions may be resulted from the large spatial variation in the tectonic stress change during the mainshock. Between Wenchuan and Anxian, the maximum compressional axes are horizontal and oriented consistently in NW-SE direction, perpendicular to the LFZ. The minimum compressional axes are mainly vertical, suggesting a primary thrust faulting environment. From Anxian to Beichuan, the stress faulting type is mainly still reverse faulting stress regime, but the orientations of σ1

axes vary abruptly. From Beichuan to Pingwu, the stress environment is still thrust faulting with vertical minimum compressional axes. However, the mostly horizontal maximum compressional axes are largely oriented parallel to the LFZ. Around the areas of Nanba and Donghekou, the orientations of the maximum compressional axes change abruptly. Although still a thrust faulting environment with a small component of strike-slip, the orientation of the maximum compressional axes changes suddenly from NE-SW south of Pingwu to NNW-SSE in the north; and then swings to nearly EW and NW-SE near Donghekou. North of Qingchuan, the stress field becomes mixed and strikeslip faulting types, but the orientation of the maximum compressional axis changes to nearly E-W and NEE-SWW. In the NW-trending Xiaoyudong section, the stress environment is also mainly strike-slip, with the maximum compressional axes in NE-SW direction and minimum compressional axes nearly NNW-SSE, coherent with the Xiaoyudong fault property of transpressure fault accommodating different slip rate of LFZ and expecting left lateral strike-slip motion (Deng et al., 2011). There are also some thrust faulting types near the main rupture zone. The 2D stress variation pattern we obtained for the maximum compressional axes is consistent with the results of Li et al. (2019), Yang et al. (2018) and Lin et al. (2018). Our result also roughly coherent with the surface horizontal velocity field from Global Positioning System (GPS) observations (blue arrows in Fig. 1) in southwest segment of LFZ (Zhao et al., 2015). Shi et al. (2009) used S-wave splitting to study the tectonic stress field and the anisotropy in the crust around the LFZ after the Wenchuan earthquake. They estimated the pattern of the maximum compressional axes beneath several stations and suggested that north of Anxian the fast axis of shear waves, which represents the direction of the maximum compressional axis, is oriented NE-SW, parallel to the LFZ; whereas south of Anxian the fast axis is NW-SE, perpendicular to the LFZ. This agrees with our results, indicating the crustal anisotropy in these area is controlled by regional tectonic stress field, although their measurements reflect the stress field in the crust beneath the stations whose coverage does not coincide with our study region. We then examine the variation of stress field with depth. As the error in earthquake focal depth is usually larger than in the horizontal location, we only choose two intervals of focal depths: 0–14 km in the 8

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Fig. 8. Stress field patterns in different depth ranges. (a) Focal mechanisms distribution in the depth range 0–14 km. (b) Focal mechanisms in cross-sectional view along the KK′ profile shown in (a) in the depth range 0–14 km. (c) Stress field pattern in the depth range of 0–14 km. (d) Focal mechanisms distribution in the depth range 14–25 km. (e) Focal mechanisms in cross-sectional view along the KK′ profile shown in (a) in the depth range 14–25 km. (f) Stress field pattern in the depth range of 14–25 km.

the north end of LFZ in shallow crust, while the thrust stress regime dominates with the maximum principal compression stress axes striking NW-SE directions in lower crust. Therefore, we infer that the stress field varies with depth in LFZ. Previous studies have found that large earthquakes can cause changes in regional stress field (e.g., Zhao et al., 1997; Yi et al., 2012). Beside the spatial variation of stress field, we also analyze its temporal variation. We divide the time span for the 404 aftershocks into three intervals according to the seismicity characteristics: Interval I from May 12 to May 18, 2008; Interval II from May 19 to July 30, 2008; and Interval III from July 31, 2008 to October 30, 2010. obvious stress changes can be observed over these three time intervals (see Fig. 9b, d

shallow crust and 14–25 km in the lower crust. In Fig. 8, we observe that the stress field varies with depth. In the area from Hongkou to Beichuan, although the maximum principal compression stress has similar directions of NW-SE in both shallow and lower crust, the stress field contains strike-slip, thrust as well as mixed type in the shallow crust, but in the lower crust the stress is mostly thrust with slight mixture components. And the area from Beichuan to Nanba is another remarkable contrast region, where the mixture stress regime is dominant in shallow crust but overall reverse stress regime in lower crust. In the north of Nanba, there are obviously rotations of the maximum principal compression stress axes from NNW, nearly E-W to NEE from south to north segment with the strike-slip stress regime dominating in 9

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Fig. 9. Stress field patterns in three time intervals. (a) The focal mechanisms in Interval I from May 12 to May 18, 2008. (b) The stress patterns in Interval I. (c) The focal mechanisms in Interval II from May 19 to July 30, 2008. (d) The stress patterns in Interval II. (e) The stress patterns in Interval III from July 31, 2008 to October 30, 2010. (f) The stress patterns in Interval III.

and f). As displayed in Fig. 9, the stress field pattern changed over time in certain areas of LFZ. The orientations of maximum principal compression stress axes or the type of stress field are obviously changed through these time intervals. The remarkable contrast is the Qingchuan area where the principal stress axes rotate from NE to nearly E-W, finally NW direction over time, meanwhile the faulting stress types vary from overall thrust, to thrust with increasing strike-slip component and to mixed as well as strike-slip and thrust stress regime over these three time intervals. In Beichuan area the stress nature varies from thrust to mixed and strikeslip and changes back to thrust through the time intervals. Similarly, the south of the Yingxiu and Hongkou area displays the similar temporal variation of stress field both in the stress nature and σ1 directions over time. Wan et al. (2017) indicates that the largest peak slip is near Yingxiu and Hongkou, respectively. The second largest peak slip is near Beichuan. The 3rd, 4th, and 5th largest peak slips are near Qingping, Qingchuan and Nanba, respectively, corresponding to these areas where the temporal variation of stress field. With the assumption that the stress disturbance heals the fastest at early stage (Yang et al., 2018), differences between the stress fields retrieved from different time intervals suggest that the mainshock caused the large stress disturbance in some areas of LFZ.

in the east. Near the eastern margin of the Tibetan Plateau, LFZ deforms in a complex way. The lower crust to the west of LFZ is weak and the upper crust deforms actively (Burchfiel et al., 2008). Upper crust rocks near LFZ are mechanically strong and the geometry of the fault zone does not facilitate aseismic slip. The rigid Sichuan Basin prevents the eastern motion of the Tibetan Plateau. These conditions make it possible to store huge strain energy, and lead to the strong Wenchuan earthquake with complex rupture. Zoback (1992) defined the first-order stress field as the large scale stress field associated with local plate motions. LFZ is the boundary of the first-stress field in China. Tectonic stress field in the Tibetan Plateau is under NS or ENE compression, in agreement with the relative motion between the Indian and Eurasian plates. But near its eastern margin, the directions of the maximum principal compression stress changes from northeast in the northeastern segment of LFZ to southeast in the southwestern segment (Xu, 2001; Xie et al., 2004). The stress field near LFZ is mostly generated by collision between the Indian and Eurasian plates as well as the rising of the Tibetan Plateau due to the collision. The 2D spatial variation of the stress field is complicated along the LFZ and obviously segmented under mainly thrust faulting stress regime, which is consistent with previous results (Li et al., 2019; Yang et al., 2018; Meng et al., 2015; Wang et al., 2009). However, our result shows a more detailed and smaller scale stress distribution. There is substantial localized variation of stresses in study area. Li et al. (2019) divided the LFZ into two segments by stress distribution, differing from three segments by our inversion result. Several factors are involved that affect the stress field distribution. The collision between the Indian and Eurasian plates generates the first-order stress field as well as the rising of the Tibetan Plateau due to the collision. In southwest segment of LFZ, the thrust stress regime predominates and the σ1 axes are roughly perpendicular to the fault, trending NW-SE direction. Located to the west of the LFZ, the Bayankala block moves southeastward, leading to compression almost

4. Discussion The Longmenshan Fault Zone (LFZ) is the eastern margin of the Tibetan Plateau and lies along the middle segment of North-South Seismic Bel. The Indian Plate is moving northward at a rate of 50 mm/ yr (Chen et al., 2015), and its collision with Eurasia is the major factor in generating present-day stress field in mainland of China. The Tibetan Plateau rises rapidly due the collision, and lower crustal material flows (Bai et al., 2010; Royden et al., 1997) eastward and southeastward due to the blockage of the Bayankala block in the north and Sichuan Basin 10

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Beichuan), where the orientation of the lower crust flow is vertical to the LFZ too, which mainly result in a compressional stress effect on the LFZ. Meanwhile, the orientations of σ1 axes is approximately parallel to the LFZ with largely thrust faulting type with some strike-slip components in the northeast segment side of LFZ (from Anxian to Nanba), where the orientation of the lower crust flow is inclined to the fault zone, which would result in both compressional and strike-slip like stress effect on the fault zone. The characteristic of the lower crustal flow is consistent with the stress state both in faulting types and stress orientations obtained by our study in LFZ, which may represent the upper crust response to lower crustal flow and the stress state in upper crust are quite affected by lower crustal flow. Our results also show that the stress field varies with depth, which may suggest that the crustal properties and crustal deformation model is different in shallow and lower crust in LFZ. Wan et al. (2017) also proposed that the crustal deformation is decoupled by differential motion across a decollement in the mid crust. The deformation above the decollement is dominated by brittle thrust and reverse faulting and below which viscous horizontal shortening and vertical thickening occur. Besides, Jia et al. (2010) identified two detachment levels in LFZ from the seismic reflection data, a shallow detachment at about 7 km depth in the northeast segment and a deep detachment at about 15–17 km depth in the southwest segment. All these evidence is coherent with our result, confirming that the stress state in shallow and lower depth is influenced by different crustal deformation and crustal properties in different depths. Our result indicates that a large stress disturbance was caused by the mainshock at an early stage, not only the σ1 orientations but also the faulting stress types were disturbed. Yang et al. (2018) also studied temporal stress variations, but they used later aftershocks (from May 2008 to December 2015). It is usually believed that the early postseismic processes vary more rapidly and thus our study provides new information about temporal variation immediately after the mainshock. In summary, we propose that the stress field near Longmenshan region is influenced not only by the collision between the Indian and Eurasian plates, but also by the interactions between smaller tectonic blocks as well as interactions between blocks and fault systems. The collision of Indian Plate with Eurasian Plate determines the first-order characteristic of stress field in the study area. The geometry of blocks and faults, together with the shallow crustal property difference and lower crustal flow are the major factors controlling the local stress field with intense variation. The stress state in this region can be explained in the framework of active tectonics, geological background and local tectonic stress field.

perpendicular to the LFZ. However, the effects of the compression differ at different segment of the fault. The southwestern segment of the LFZ is under very strong compression, which might be the significant factor to generate the stress distribution in this segment. The σ1 axes trending NW-SE direction is consistent with the feature of the compression, and it seems that the spatial variation pattern of stress field in the southwest segment of LFZ is mainly controlled by the strong compression on LFZ and consistent with the first-order characteristic of the stress field. According to the geological research (Xu et al., 2009), the BeichuanYingxiu rupture zone can be divided into two segments with different kinetic features, one is Hongkou-Qingping segment, having the feature of mainly reverse mechanism with certain dextral strike-slip components, and the other is Beichuan-Nanba segment, holding the kinetic feature of mainly dextral strike-slip mechanism with certain reverse components, indicating that the thrust faulting rupture is dominant in compression area, meanwhile the strike-slip mechanisms are reasonable in some regional areas. The structure property is in agreement with the stress distribution obtained from our study in these areas. Additionally, for the middle segment of the LFZ, the Minshan uplift acting as a strongholds impedes the strong compression on it, which is the reason why the large earthquakes (M ≥ 7.0) are predominantly located along the Minshan uplift and in the southwestern segment of the LFZ and the seismicity in northeast segment is relatively weak, which suggests that the southwestern segment of the LFZ and the Minshan uplift together constitute a new very active boundary of the Bayankala block (Tan et al., 2010; Chen et al., 2007). The geometry of the new active boundary may also influence the stress distribution pattern, controlling the present-day topography, fault structure and seismicity. Moving southward under the SEE trending compression along two reverse boundary faults (Huya and Minjiang faults in Fig. 1) with left-lateral strike-slip components (Tan et al., 2010), Minshan uplift might penetrate into the LFZ, and the junction zone of the structures displays a complex stress distribution with the σ1 direction varying rapidly. The maximum principal stress direction is approximately parallel to the fault in the middle segment. This is probably attributed to the change of the movement direction of Bayankala block and lead to SEE-trending compression on Minshan uplift, together with the interactions of the rising of north-south-striking Minshan uplift, the Minjiang and Huya faults and the Longmenshan Fault. Besides, the geometry of the faults or blocks is another significant factor that influence the stress field. The seismogenic fault geometry along the LFZ is obviously segmented with varying fault dip angle along strike. The dip angle near the surface is ~36° at the south end of the LFZ, changes to 43° and 51° at Hongkou and Beichuan, jumps to 70° north of the Nanba junction, and reaches ~83° on the Qingchuan segment of the fault (Wan et al., 2017), which coincide with the stress distribution segmentation along the LFZ. The stress distribution is extremely complicated around the faults junctions, such as Hongkou, Beichuan and Nanba areas, where both the stress orientations and faulting types vary intensely, implying the tectonic stress field in these areas are also affected by the fault structures. In the Southeast end of the Xiaoyudong fault, penetrating to the LFZ, the stress pattern is more complicated than its surrounding area. In Nanba area, on the northeast side of the fault junction and fault dip jumping to 70°, the σ1 directions change abruptly, that may related to the stepping geometry of the fault. The locations of complicated stress distribution seem to correlate well with complexity of fault geometry. In addition to the fault structures, the lower crustal flow is also another significant factor that cause this complicated stress field. The orientations of the lower crustal flow from southwest segment to northeast segment of LFZ varies from nearly perpendicular to inclined relative to fault (Bai et al., 2010; Royden et al., 1997). This lateral changing pattern of the lower crust flow may influence the stress distribution in the upper crust along different segments of the LFZ. The orientations of σ1 axes is perpendicular to the LFZ with predominant thrust faulting stress regime in southwest segment of LFZ (south of

5. Conclusion We determine the focal mechanisms and centroid source depths of 404 earlier aftershocks (12 May 2008 to October 2010) in LFZ using waveform fitting and invert the temporal and spatial variations of the regional stress field utilizing the focal mechanisms. The study results show that: (1) Thrust mechanisms dominate the aftershocks with a few strike-slip events in the southern section (southwest of Anxian), mostly thrust events with a few strike-slip and normal ones in the middle section (from Anxian to Nanba), and still primarily thrust with increasing strike-slip events in the northeast segment (northeast of Nanba). (2) Focal depth distribution also shows segmentation pattern. In the northeastern segment, there are abundant extremely shallow earthquakes (1–3 km), along with deep events up to 25 km, thus spanning a wide range of depth. In the central segment, there are no very shallow earthquakes (shallower than 5 km). In the southwestern segment, there are some aftershocks shallower than 5 km near the Xiaoyudong area. (3) The 2D stress field along the LFZ is predominantly under a thrust 11

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faulting stress regime with nearly horizontal maximum compression and nearly vertical minimum compression. The spatial variation of the stress field is clear segmentation. In the southwest segment, thrust faulting stress regime is dominant and the maximum principal compression stress axes strike NW-SE direction, perpendicular to LFZ. Primary thrust faulting type with slightly mixed faulting stress regime is featured and σ1 axes trend approximately NE-SW, largely parallel to the LFZ in the middle segment. In the northeast segment, still a thrust faulting environment with strikeslip and mixtured components is observed and the σ1 axes change abruptly, rotating from NE, NW to NNW from north end of the LFZ to Nanba area. (4) Several factors are involved to affect the stress field distribution in LFZ, including the collision of Indian and Eurasian plates, the geometry of blocks and faults, lower crustal flow as well as the shallow crustal property difference. (5) Temporal variation of stress field is observed, suggesting that the mainshock caused a large stress disturbance at an early stage, and the stress field varies with depths implying a different deformation pattern between the shallow crust and lower crust in some area of LFZ.

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Acknowledgement Seismic data used in this study were obtained from the Sichuan Earthquake Administration. This work has been supported by the Basic Research Fund of the Institute of Earthquake forecasting, China Earthquake Administration (2015IES010302), State Key Laboratory of Geodesy and Earth's Dynamics, Institute of Geodesy and Geophysics, Chinese Academy of Sciences (SKLGED2018-4-3-E), and China Earthquake Science Experiment project, China Earthquake Administration (2019CSES0102). All figures have been plotted by the Generic Mapping Tools (GMT) of Wessel and Smith (1998). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.tecto.2019.228172. References An, M.Q., Ding, L., Wang, H., 2004. Research of property and activity of Longmen Mountain fault zone. J. Geodesy Geodyn. 24 (2), 115–119 (in Chinese). Angelier, J., 1979. Determination of the mean principal directions of stresses for a given fault population. Tectonophysics 56, T17–T26. Bai, D., Unsworth, M., Meju, M., Ma, X., Teng, J., Kong, X., Sun, Y., Sun, J., Wang, L., Jiang, C., Zhao, C., Xiao, P., Liu, M., 2010. Crustal deformation of the eastern Tibetan plateau revealed by magnetotelluric imaging. Nat. Geosci. 3 (5), 358–362. Burchfiel, B.C., Royden, L.H., Van der Hilst, R.D., 2008. A geological and geophysical context for the Wenchuan earthquake of 12 May 2008. Sichuan, People's Republic of China. GSA Today 18, 4–11. Chen, G., Ji, F., Zhou, R., Xu, J., Zhou, B., Li, X., Ye, Y., 2007. Primary research of activity segmentation of Longmenshan fault zone since Late-Quaternary. Seismol. Geol. 29, 657–673. Chen, J.H., Liu, Q.Y., Li, S.C., Guo, B., Li, Y., Wang, J., Qi, S., 2009. Seismotectonic study by relocation of the Wenchuan Ms8.0 earthquake sequence. Chin. J. Geophys. 52, 390–397. Chen, W., Ni, S., Kanamori, H., Wei, S., Jia, Z., Zhu, L., 2015. CAPjoint, a computer software package for joint inversion of moderate earthquake source parameters with local and teleseismic waveforms. Seismol. Res. Lett. 86 (2A), 432–441. https://doi. org/10.1785/0220140167. Cui, X., Hu, X., Yu, C., Tao, K., 2011. Research on focal mechanism solutions of Wenchuan earthquake sequence. Acta Sci. Nat. Univ. Pekin. 6, 1063–1072. https://doi.org/10. 13209/j.0479-8023.2011.148. Deng, Q., Chen, G., Zhu, A., 2011. Discussion of rupture mechanisms on the seismogenic fault of the 2008 M s 8.0 Wenchuan earthquake. Sci. China Earth Sci. 54, 1360. https://doi.org/10.1007/s11430-011-4230-1. Densmore, A.L., Ellis, M.A., Li, Y., Zhou, R.J., Hancock, G.S., Richardson, N., 2007. Active tectonics of the Beichuan and Pengguan faults at the eastern margin of the Tibetan Plateau. Tectonics 26, TC4005. https://doi.org/10.1029/2006TC001987. Dong, Y., Ni, S., Yuen, D.A., Li, Z., 2018. Crustal rheology from focal depths in the North China Basin. Earth Planet. Sci. Lett. 497, 123–138. Du, J., Chen, Q., Ma, Y., An, Q., Wu, M., Meng, W., Li, G., 2013. Faults activity and stress

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