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Shallow structure of the Tangshan fault zone unveiled by dense seismic array and horizontal-to-vertical spectral ratio method
Accepted Manuscript Shallow structure of the Tangshan fault zone unveiled by dense seismic array and horizontal-to-vertical spectral ratio method Feng Bao, Zhiwei Li, David A. Yuen, Jianzhong Zhao, Jia Ren, Baofeng Tian, Qingjun Meng PII: DOI: Reference:
S0031-9201(18)30089-X https://doi.org/10.1016/j.pepi.2018.05.004 PEPI 6159
To appear in:
Physics of the Earth and Planetary Interiors
Received Date: Revised Date: Accepted Date:
25 March 2018 14 May 2018 20 May 2018
Please cite this article as: Bao, F., Li, Z., Yuen, D.A., Zhao, J., Ren, J., Tian, B., Meng, Q., Shallow structure of the Tangshan fault zone unveiled by dense seismic array and horizontal-to-vertical spectral ratio method, Physics of the Earth and Planetary Interiors (2018), doi: https://doi.org/10.1016/j.pepi.2018.05.004
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Shallow structure of the Tangshan fault zone unveiled by dense seismic array and horizontal-to-vertical spectral ratio method
Feng Bao1, Zhiwei Li1, David A. Yuen2, 3, Jianzhong Zhao1, 6, Jia Ren4, Baofeng Tian5, Qingjun Meng1
1
State Key Laboratory of Geodesy and Earth’s Dynamics, Institute of Geodesy and
Geophysics, Chinese Academy of Sciences, Wuhan 430077, China 2
Department of Earth Sciences, University of Minnesota, Minneapolis 55455, USA
3
Ultimate Vision Technology, Beijing 100190, China
4
Earthquake Administration of Hebei Province, Shijiazhuang 050021, China
5
Institute of Geophysics, China Earthquake Administration, Beijing 100081, China
6
Marine Science and Technology College, Zhejiang Ocean University, Zhoushan
316022, China Correspondence to: Zhiwei Li, [email protected]
Abstract The Tangshan fault zone has been considered to be a potential seismogenic fault from the devastating 1976 Ms 7.8 Tangshan earthquake. Based on the dense seismic array with interstation distance of ~ 1 km and length of ~ 40 km, HVSR (horizontal-to-vertical spectral ratio) curves at 37 stations are obtained with the micro-tremor seismic waveforms, and the two-dimensional Quaternary sedimentary 1
structures cross the Tangshan fault zone are imaged by frequency-to-depth conversion. Results suggest that two significant seismic impedance interfaces at ~ 100 m and 300 - 800 m depth are clearly revealed, respectively. The interface at ~ 100 m depth can be attributed to the strong contrasts between the unconsolidated sand-clay and semi-consolidated silt, which suggests very small variations and is well consistent with the seismic reflection interface from shallow seismic reflection exploration. While the deeper interface at 300 - 800 m depth can be attributed to the Quaternary sedimentary basement, which increases from 300 to 800 m from the west to the east, and is also consistent with the results in previous studies. Rapid variations (~ 200 m) of the Quaternary sedimentary basement depth just beneath the fault zone well agrees with spatial characteristics of the fault revealed by deep seismic reflection profiling. Our work suggests that the Tangshan fault zone has been significantly ruptured and modified by strong earthquake activities since Quaternary. This study also suggests that the HVSR method with dense seismic array is an efficient, effective, non-invasive and low-cost method for investigating active faults in shallow sediments. It is useful for probing the active faults buried by thick sediments in the densely populated urban settings where seismic investigation with explosive sources are too hazardous to carry out.
Keywords: Dense seismic array; Micro-tremors; HVSR; Tangshan fault zone; Quaternary sedimentary basement
2
1. Introduction Spatial characteristics of active faults in the shallow and deep crust can provide important information to understand the seismogenic structure and mechanisms of the devastating earthquakes (Dolan et al,. 1995; Shaw and Suppe, 1996; Liu et al, 2011). Moreover, high-resolution detection of active faults around urban areas is also essential for evaluating earthquake hazard and improving disaster prevention capabilities (Pratt et al., 2002; Davis et al., 1989). But many cities are built in sedimentary basins. The unconsolidated and consolidated low-velocity sediments make it very difficult to explore the detailed structures of active faults beneath the basement, especially for the areas covered with thick Quaternary sediments (e.g., Beijing, Tianjin and Tangshan which are cities in the North China basin). Moreover, the sediments are usually characterized as extremely low velocity, which can amplify the amplitude of seismic wave and enhance the damage caused by large earthquakes (Seed et al., 1988; Bard et al., 1988; Seale and Archuleta, 1989; Dong, et al., 2004; Jemberie and Langston, 2005; Chen et al., 2009; Li et al., 2014; Ni et al., 2014). Therefore, it is very urgent to study the sedimentary structures for investigating the seismogenic environment of the active faults covered with thick sediments, which can also provide essential information for seismic hazard assessment, earthquake emergency response and urban planning. Popular methods for detecting the active faults include both invasive and non-invasive methods (Aki 1957; Lindvall and Rockwell, 1995; Palmer et al., 1997; Pratt et al., 2002; Sakurai and Masuda, 2013; Suzuki et al., 2000; Picozzi et al., 2009). 3
Drilling method can obtain detailed stratum, lithology and velocity structures in the shallow crust (Carpenter et al., 2011; Sakurai and Masuda, 2013). On the other hand, seismic exploration methods with active source can detect the stratigraphic interfaces and velocity structure at different depths in shallow crust. These methods are usually based on the reflection and refraction of the body wave or dispersive surface waves, and can obtain high resolution imaging results (Palmer et al., 1997; Shtivelman et al., 1998; Itoh et al., 2005; Duffy et al., 2014; Wang et al., 2016). These methods have been successfully applied in active fault detection and shallow crustal structure imaging. However, the drilling depth and spatial distributions of boreholes are usually limited by the high cost and populous urban areas. The active source detection method is also expensive, and cannot be deployed in populous urban areas. In the last decade, seismic imaging methods with ambient seismic noise data have been widely used to investigate the detailed structures of shallow and deep crust, partially benefit from their low-cost, high-efficiency, and effectiveness. These methods have been applied to crustal velocity structure for imaging of active faults, carrying out basin surveys, exploring for geothermal resources. Their effectiveness and efficiency are convincing (Betting et al., 2001; Walling et al., 2009; Chen et al., 2009; Langston et al., 2009; Nunziata et al., 2009; Picozzi et al., 2009; Wang et al., 2009, 2011; Agostini, et al., 2015; Zhang et al., 2016; Li et al., 2016, 2017; Civico et al., 2017; Yan et al., 2018). The ambient seismic noise imaging methods can be classified as seismic array method and single-station method. Seismic array method is mainly based on the 4
waveform cross-correlation and spatial autocorrelation to extract the surface wave dispersion and then image the shear-wave velocity structure (Nunziata et al., 2009; Picozzi et al., 2009; Betting et al., 2001; Zhang et al., 2016; Li et al., 2016, 2017). The Horizontal-to-Vertical Spectral Ratio (HVSR) method is one of the single-station method (Parolai et al., 2002; Fäh et al., 2003; Arai and Tokimatsu, 2004) and can extract the frequency responses for the sharp seismic impedance between the sediment and basement. This method can also be used to invert the shallow shear-wave velocity structures. Moreover, with the frequency-to-depth conversion, the HVSR curves can be imaging the continuous seismic impedance depth directly. Because of its efficiency, the HVSR method has been widely used in the field of the shallow velocity structure investigation, seismic field response assessment and seismic micro-zonation (Vella et al., 2013; Gosar 2007; Chen et al., 2009; Langston et al., 2009; Wang et al., 2009, 2011; Leyton et al., 2013; Agostini et al., 2015; (Tarabusi and Caputo, 2017; Civico et al., 2017). The HVSR method has also been employed to investigate reliably the Antarctic ice sheet thickness, thus demonstrating its effectiveness for detecting the buried seismic impedance in the shallow crust (Yan et al., 2018). In this study, we deployed the dense seismic profiling array perpendicular to the Tangshan fault zone, composed of 37 three-component seismometers with interstation distance of ~ 1 km and total length of ~ 40 km (Fig. 1b). The Tangshan fault zone is regarded to be the seismogenic fault responsible for the 1976 Ms7.8 Tangshan earthquake, North China, which caused more than 240,000 deaths and became one of 5
the most devastating earthquakes in the last 100 years in the world (Butler et al., 1979; Nabelek et al., 1987; Liu et al., 2013). Due to the thick (up to a few hundred of meters) Quaternary sediments near the Tangshan fault zone, this causes great difficulties for investigating the detailed structures of the Tangshan fault zone with geological field survey. We carry out a micro-tremor survey using HVSR method. The reliability of the result is extensively tested and verified by seismic data analysis and the comparison with the shallow features speculated by the nearby borehole, as well as the shallow and deep seismic reflection profiling with active sources. This study could provide an efficient and non-invasive method to detect the active fault in the sedimentary basin.
2. Study area Tangshan is a populous and industrialized city near Beijing and Tianjin in North China. In 1976, the MS 7.8 Tangshan earthquake occurred beneath the city, and the maximum seismic intensity is up to XI degree (Liu et al., 2002). It is one of the most devastating earthquakes in the last 100 years (Butler et al., 1979; Nabelek et al., 1987; Liu et al., 2013). The main ruptured fault of the Tangshan earthquake is deemed as the Tangshan fault zone. Tangshan earthquake region is located in the junction of the North China Basin depression and the Yanshan uplift, within a geologic diamond block cut by some active faults (Fig.1a) (Guo et al., 1977). A field geological survey indicates that the surface rupture length of the Tangshan fault zone is greater than 47 km and is associated with some fold structures, consisting of several NE-direction 6
faults (Guo et al., 2011). Moreover, most of the crustal deformation in the Tangshan area is associated with right-lateral slip on the NNE-striking fault rupture plane of the main shock (Huang and Yeh, 1997). A late Quaternary unconsolidated sediment covers extensively over the Tangshan fault zone (Guo et al., 2011). Due to the resonance effect from the loose sediments, site amplification together with ground deformation, subsidence, fissure and liquefaction can impact a large area during the enormous earthquake, and can destroy many buildings (Moss et al., 2011). Although about 40 years have passed since the Tangshan Great earthquake, Tangshan fault zone and adjacent areas still has strong seismic activities (Fig. 1). The alluvial and diluvial coverage since the Quaternary covered on the Tangshan fault zone, which buried the seismogenic environment in the deep crust. Some researchers have done studies in this area for the Quaternary sedimentary characteristics, buried faults and crustal structure (Li et al., 1998; Hao et al., 2001; You et al., 2002; Qiu et al., 2005; Jiang, 2007; Wang et al., 2016). The overall tendency of the Tangshan fault zone is with northwest dip from the shallow seismic prospecting, and the vertical offset reaches 15 meters since late Pleistocene (Li et al., 1998; Hao et al., 2001; You et al., 2002). The drilling section and stratigraphic dislocation information obtained by trench digging show that multiple fracture activities indeed occurred on the Tangshan fault zone since the late Quaternary. This geometrical distribution of the surface rupture zone can be divided into the northern and southern segments. Vertical displacement of the northern segment is increasing 7
westward but decreasing in the east. While the trend on the southern part is on the contrary. The profile obtained by a combined drilling and trenching shows the geological structural evidence of the multistage activities of the Tangshan buried faults covered by sedimentary deposition (Guo et al., 2011).
3. Data and Method 3.1 Data In order to verify the reliability of the HVSR results, we deploy the dense seismic array, which coincides with the location of previous deep seismic reflection survey profiling perpendicular to the Tangshan fault zone (Liu et al., 2011). Thirty-seven short-period (frequency bandwidth of 0.2 - 100 Hz), three-component seismometers (type QS-5) were deployed in January 2017 for two months recordings of ambient noise (Fig. 1). The two-month time observations can provide abundant data to analyze the length of seismic ambient noise waveforms for extracting reliable HVSR curves. The stations are located far from the high-speed railway and other traffic arteries to avoid obvious vibrations. The three-component recordings were calibrated with continuous GPS clock for time synchronization. The seismic stations were placed in free field for reducing low-frequency noise caused by winds impacting on high buildings and tall trees. In order to reduce the influences of wind flow and temperature variations on the ambient seismic noise observation, seismometers were buried underground. Similar observations with the dense seismic array in the Wudalianchi volcano field in the Northeast China and in the Gonghe-Guide region of 8
Qinghai province and they displayed good performance in seismic imaging with ambient noise for shallow magma chamber and investigations on hot-dry rock resources (Zhang et al., 2016; Li et al., 2016, 2017). The short-period seismometers used in the study have been calibrated before deployment for the dense seismic array. We deployed a pair of short-period (QS-5) and broadband (CMG-3ESPCDE) seismometers at the same place to conduct the consistency tests. HVSR curves are calculated from 2 to 4 o’clock in Beijing time with the same controlling parameters (such as time window length, smoothing, et al.). Examples for HVSR curve calculation and verification are shown in Fig. 2. The results suggest that the ambient noise recorded by the two seismometers are all relatively stable, and the HVSR curves with peak frequency and corresponding amplitude are also consistent with each other (Fig. 2c and 2d). These results imply that the HVSR curves can be derived from the short-period seismometers and are as reliable as those obtained from broadband seismometers. In order to study further the influence of the time-window length on the HVSR curves, we have calculated 24 groups of HVSR curves by incrementing a window length from 1- to 24-hour at interval of 1 hour. We found that the HVSR curves have two significant resonant frequencies at ~ 0.24 Hz and ~ 0.13 Hz (Fig. 3a). The resonant frequency and amplitude of the HVSR curves still remain stable when increasing the window length. The test also shows that one hour to several hours’ calculating length is good enough for gaining a robust HVSR curve. The calculating window length will be select 2-hour in later analysis. The effects of time periods in 9
the day- and night-time on the HVSR curves are also studied. Micro-tremor recordings for a whole day is divided into 12 groups with 2-hour window length for the HVSR curves (Fig. 3b). These results show that the variations of the resonant frequencies are pretty small, and the amplitudes slightly changed, especially at the low frequency peak near 0.24 Hz. We can attribute this finding to the noise level induced by human activities (such as transportation, factory et al.). Therefore, stable resonant frequencies in the HVSR curves can obtained at both day- and night-time. However, in order to minimize the noise from human activities in day time, we only consider only the micro-tremor data in the night time for each station to reduce the interference from non-random sources.
3.2 Method We calculate the HVSR curves from the three-component measurements of ambient noise waveforms in frequency domain as shown below (Nakamura, 1989; Lermo and Chavez-Garcia, 1993) （1） For the horizontal spectrum, the squared-average of amplitude spectrum of both N-S and E-W components are used. In order to improve the reliability of the measured HVSR curve, the average amplitude ratio algorithm of short-time to long-time window (STA/LTA) is applied to eliminate the non-random transient interference signals. In order to extract low frequency HVSR curves, seismic records are cut with 60 s windows and used to calculate the HVSR curve for each window. 10
We used the
Konno-Ohmachi smoothing algorithm to strengthen the stability of the HVSR curves in the low frequency (Konno and Ohmachi, 1998). The smoothing coefficient is set at 20 after a few trials to obtain reasonable smoothed HVSR curves. With different smoothing coefficients, the peak frequency of the HVSR curves remain stable with slight amplitudes changes. All HVSR curves from all time windows are stacked to derive the averaged HVSR curve for each station. The horizontal and vertical components consist of the information of ambient noise source (S), propagation path (P), site effect (T), instrument response (I). Thus, the horizontal component H(t) and the vertical component V(t) of the signal can be expressed by （2） （3） Equaitons (2) and (3) can be transformed to the frequency domain and expressed by （4） （5） The ratio of H(f) to V(f) can be shown as below （6） Thus, by dividing horizontal spectral with vertical spectral in the frequency domain, we can eliminate the influences of propagation paths and instrument responses, and thus obtain the frequency characteristics of site effect (Nakamura, 1989; Lermo and Chavez-Garcia, 1993; Chen et al., 2009). The resonant frequency (or peak frequency) of the HVSR curve has good 11
stability and corresponds to the main interface of the sedimentary layer (Nakamura, 1989; Seht and Wohlenberg, 1999; Mucciarelli et al., 2003; Panou et al., 2005). The relationship between the resonant frequency (
) and the thickness (H) of the
sedimentary layer can be determined by the formula （7） is the resonant frequency,
is the average shear-wave velocity, H is the thickness
of the sediment layer. Equation (7) can be utilized to calculate the thickness of sediments with HVSR curves (Yamanaka et al., 1994; Chandler and Lively, 2016; Parolai et al., 2002; Wang et al., 2011). Given the average velocity of shear-wave
,
the thickness of sediments can be calculated by （8）
4. Results The frequency-depth profile derived from HVSR curves is shown in Fig. 4. Each station has two significant peak frequencies, 0.1 - 0.5 Hz (marked as f1) and ~ 1 Hz (marked as f2). The first peak frequencies of f1 decreases gradually from west to the east, and the second peak frequencies of f2 are relatively stable. We set the average shear-wave velocity for the shallow sediments to be 500 m/s based on the studies at the 480 m deep borehole seismic station with local earthquake waveforms (Liu et al., 2011; Wang & Li, 2018). According to equation (8), the HVSR curves are converted into the pseudo-depth image along the dense seismic array after amplitude normalization on peak frequency f1 (Fig. 4c). Theoretical HVSR curves are also 12
calculated with the two-layer sedimentary models. The main features of the theoretical and observed HVSR pseudo-depth profiles are consistent with each other (Fig. 4b and 4c), suggesting that the inferred sedimentary models can fit the observations pretty well. The spatial characteristics of the main seismic impedance interfaces suggest that the first interface is at ~ 100 m depth without significant variations along the profile. The second interface varies from 300 to 800 m depth with gradually deepening trend from the west to the east of the profile. The depth of the second interface rapidly changes just beneath the Tangshan fault zone.
5. Discussions In order to ascertain the reliability of the pseudo-depth profile, we have carried out theoretical calculations and compared
with the observed data. The
one-dimensional, double-layer sedimentary model is constructed to investigate the depth variations of seismic impedance interfaces. We set the average shear-wave velocities of the two layers to be 350 m/s and 520 m/s, respectively. The shear-wave velocity of the half-space is set to be 800 m/s. We refer this shear-wave velocity model to the results at borehole seismometers and drilling data in the nearby North China basin (Chen et al., 2009; Ding et al., 2004; Liu et al., 2011). Based on the elasto-dynamic Green’s functions and the ambient noise interferometry theory (Sánchez-Sesma et al., 2011; García-Jerez et al., 2016; Piña-Flores et al., 2016), we calculate theoretical HVSR curves for all stations and then established the pseudo-depth profile based on the diffused field assumption (Fig. 5a) (). The resonant 13
frequencies are used to calculate the thickness of the two layers by equation (8). (f1 corresponding to the average shear-wave velocity of 500 m/s, and f2 corresponding to the average shear-wave velocity of 350 m/s). The two resonant frequencies are within the frequency bands of 0.1 - 0.7 Hz and 0.7 - 2 Hz, respectively. The main features of the theoretical and observed HVSR pseudo-depth profile are well consistent with each other (Fig. 5a). Two seismic impedance interfaces are then extracted from theoretical and observed predominant frequencies. They show very similar characteristics (Fig. 5b), thus suggesting the reliability of the inferred sedimentary structures under our dense seismic array. Previous borehole data, shallow and deep seismic reflection profiling in the Tangshan fault zone can provide robust constraints for HVSR profiling. In the shallow seismic reflection survey in the study area, the boreholes found one clear interface at 94.7 to 96.9 m depth, which shows the interface between the unconsolidated sand-clay and semi-consolidated silt (Cai et al., 2016). Therefore, we infer that the interface at ~ 100 m depth in the HVSR pseudo-depth profile could be attributed to the strong velocity contrast between unconsolidated sand-clay and semi-consolidated silt. Compared with the interface at 300 - 800 m depth in the HVSR pseudo-depth profile and the Quaternary sedimentary basement derived from seismic reflection (Guo et al., 2011), the deeper interface in the HVSR studies can be attributed to the Quaternary sedimentary basement. We also compared the deep seismic reflection profiling and the HVSR pseudo-depth imaging (Fig. 4). The Quaternary sedimentary basement beneath the Tangshan fault zone shows 100 - 200 m variations in depth. As 14
revealed by the deep seismic reflection profiling at the same location as the HVSR profile, the locations of the rapid changings also show a good correspondence to the distribution of the fault system. It suggests that the Tangshan fault zone could have experienced significant earthquake activities and strongly modified since the Quaternary. In the deep seismic reflection profilings (Fig. 4d), we found the high-angle reverse strike-slip fault system composed of several fault branches for the Tangshan fault zone (Zeng et al., 1988; Wang et al., 1981; You et al. 2002). These faults are characterized with cliffy slope at deeper depth and gentle slope in shallower depth, showing a flower-like tectonic system (Liu et al., 2011). The Tangshan fault zone is a combination of a near-vertical main fault and secondary faults distributed along the controlled tectonic with echelon fractures (Guo et al., 1977). In the HVSR and deep seismic reflection profiles, the normal fault F2 makes the depositional thickness of the upper wall slightly greater than that of the footwall. The F3 and F4 faults are secondary fractures of F2. The thickness of the upper wall is also slightly thicker due to that the F3 could be a west dipping normal fault. The center area of the Tangshan fault zone is located from F4 to F6, corresponding to south segment of the NNE striking Douhe fault (Fig.1b). The Douhe fault is a normal fault extend to 5 km depth or deeper (Zeng et al., 1988; Lu et al., 1988), leading to strong fluctuation with ~ 100 m in sediments. F6 is close to the transition zone of the rapid changes of the Quaternary sedimentary basement. The west area to F6 could be the depression area of basement, while the east area could be the uplift area of basement, where the 15
thickness of the Cenozoic strata is significantly thickened. Both F7 and F8 are in good agreement with the thickness changes with ~ 100 m of the Quaternary sediments. We can infer the rapid depression of F8 to be associated with the Ninghe-Changli deep fault (Jia et al., 2009). The deep seismic reflection profile provides clear deep structural features of the Tangshan fault zone, but few characteristics are provided within a few hundred meters deep. The two-dimensional HVSR pseudo-depth profile provides complementary information for the active faults in the Quaternary sediments. It is important to understand the shallow characteristics of the active fault system in the sediments. Furthermore, there are still some controversies related to the micro-tremor wavefield composition (Lunedei and Malischewsky, 2015; Piña-Flores et al., 2016). The question for the micro-tremor mainly composed of the Rayleigh surface-wave (Yamanaka et al., 1994; Konno and Ohmachi, 1998; Bonnefoy-Claudet et al., 2006) or the body-wave (Nakamura, 1989, 2010) still debates. However, a more consistent view is that the peak frequency obtained by the HVSR method is close to the resonant frequency of the shallow sediments (Lunedei and Malischewsky, 2015), although its accuracy still needs to be verified (Langston et al., 2009). In the future, we need to carry out more works on extracting high-frequency surface wave or body wave from ambient seismic noise will be conducted to investigate the shallow sedimentary structure in the Tangshan fault zone (Pasten et al., 2016; Bao, et al., 2014; Li et al., 2016; Zhan et al., 2010). In addition, HVSR method with dense array provides the sedimentary response beneath single station. Smaller interstation distance could 16
improve the lateral resolution for sedimentary structures. 1 km interstation distance adopted in this study could be proper for imaging the main characteristics of the active faults deep to a few hundred meters. For imaging active faults at a few ten meters depth, dense array with smaller interstation distances are still needed to improve the spatial resolution.
6. Concluding Remarks In this study, we deployed the dense seismic array with 37 seismometers with a nearly 1 km interstation distance across the Tangshan fault zone for micro-tremor survey with HVSR method. A two-dimensional HVSR imaging of the shallow sediments are obtained. And its reliability has been verified by carrying out theoretical tests and comparisons with previous seismic reflection profiles. Two distinct seismic impedance interfaces are found in the shallow sediments. The shallower one at ~ 100 m
depth
represents
the
interface
between
unconsolidated
sand-clay
and
semi-consolidated silt due to strong velocity contrast, which changes slightly along the profile. The deeper one at 300 - 800 m depth represents the Quaternary sedimentary basement with relatively large variations from west to east along the profile. Rapid variations of the Quaternary sedimentary basement show good spatial consistency with the main fault structure of the Tangshan fault zone revealed by the deep seismic reflection profiling. We inferred that the strong undulation of the Quaternary sedimentary basement could be strongly modified by the earthquake activity of the Tangshan fault zone with a variation of 100 - 200 m since Quaternary. 17
This study also suggests that the micro-tremor survey with the HVSR method and the dense seismic array could be potentially serve as an effective, efficient, environment-friendly and low-cost passive source method. It is suitable for active fault detection in populous cities, where it is difficult to conduct active source investigations. The spatial resolution of HVSR profile can be further improved by smaller interstation distance of seismometers, which may reveal more spatial characteristics of the fault system. However, further studies are still needed on demonstrating the prowess of HVSR method. This can be done by including how to improve the accuracy of the frequency-to-depth conversion and minimize the effects of the average shear-velocity. With the constraints from borehole well logging, drill core specimen and ambient noise tomography, the micro-tremor HVSR method can provide more reliable results in active fault investigations.
Acknowledgments We would like to acknowledge the Earthquake Administration of Hebei Province of China for assistance of the dense seismic array deployment. We also thank Professor Sidao Ni for helpful comments. This work was funded by NSFC41674065, NSFC41404052, XDB06030203 and the China-ASEAN Marine Geosciences Research and Disaster Reduction Initiative Project (1062391). We appreciate the constructive comments by the editor and reviewers.
References 18
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Figure Captions Figure 1. (a) Tectonic setting and seismic station distribution. The seismic profile is perpendicular to the Tangshan fault zone (red line). (b) Red triangle represents short period seismometer, while the blue triangle represents comparison stations between short period and broadband seismometer. The circle represents the seismicity from 2010 to today. It shows that the aftershock seismicity still persists with the linear distribution, and consistent with the Tangshan fault zone.
Figure 2. Comparisons and verifications of the HVSR curves from short-period and broadband seismometers. (a) Two-hour micro-tremor waveform recorded by short-period seismometer (the instrument type: QS-5); (b) Two-hour micro-tremor waveform
recorded
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Figure 3. The parameter selection for time length and time period. (a) The effects of time length of micro-tremor waveforms on the HVSR curve. The time length is 31
gradually increasing from 1-hour to 24-hour with 1-hour interval; (b) The stability of the HVSR curves at different time periods in each day with 2-hour time window on the UTC time 18:00-20:00 on January 20, 2017 (corresponding to 2:00-4:00 at night in Beijing).
Figure 4. (a) The calculated HVSR curve profile; (b) The theoretical HVSR pseudo-depth profile; (c, d) Comparing calculated pseudo-depth profile with deep seismic reflection profiling (modified from Liu et al., 2011), and the distribution in Fig.4c corresponds roughly to the red dotted box in Fig.4d. F1-F8 represent the conjectural faults. TQ is Quaternary sedimentary basement, TN is the substratum of Neogene. TMz is Mesozoic crystalline basement, TO and TC-P are the interfaces of Paleozoic Ordovician and Carboniferous-Permian strata. TC is the interface between upper and lower crust, and TM represents crust-mantle transitional zone.
Figure 5. (a) Theoretical HVSR pseudo-depth profile; (b) Comparison between the theoretical pseudo-depths and the calculated results. Red crosses show the thickness calculated from observed HVSR curves. Blue crosses show the thickness based on the model obtained from inversion.
32
Highlights A dense seismic array with 37 seismometers are deployed at Tangshan fault zone. Profile derived from HVSR method reveal the basement of Quaternary sediments. Dense seismic array with HVSR method is effective for investigating active faults.
33
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S0031-9201(18)30089-X https://doi.org/10.1016/j.pepi.2018.05.004 PEPI 6159
To appear in:
Physics of the Earth and Planetary Interiors
Received Date: Revised Date: Accepted Date:
25 March 2018 14 May 2018 20 May 2018
Please cite this article as: Bao, F., Li, Z., Yuen, D.A., Zhao, J., Ren, J., Tian, B., Meng, Q., Shallow structure of the Tangshan fault zone unveiled by dense seismic array and horizontal-to-vertical spectral ratio method, Physics of the Earth and Planetary Interiors (2018), doi: https://doi.org/10.1016/j.pepi.2018.05.004
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Shallow structure of the Tangshan fault zone unveiled by dense seismic array and horizontal-to-vertical spectral ratio method
Feng Bao1, Zhiwei Li1, David A. Yuen2, 3, Jianzhong Zhao1, 6, Jia Ren4, Baofeng Tian5, Qingjun Meng1
1
State Key Laboratory of Geodesy and Earth’s Dynamics, Institute of Geodesy and
Geophysics, Chinese Academy of Sciences, Wuhan 430077, China 2
Department of Earth Sciences, University of Minnesota, Minneapolis 55455, USA
3
Ultimate Vision Technology, Beijing 100190, China
4
Earthquake Administration of Hebei Province, Shijiazhuang 050021, China
5
Institute of Geophysics, China Earthquake Administration, Beijing 100081, China
6
Marine Science and Technology College, Zhejiang Ocean University, Zhoushan
316022, China Correspondence to: Zhiwei Li, [email protected]
Abstract The Tangshan fault zone has been considered to be a potential seismogenic fault from the devastating 1976 Ms 7.8 Tangshan earthquake. Based on the dense seismic array with interstation distance of ~ 1 km and length of ~ 40 km, HVSR (horizontal-to-vertical spectral ratio) curves at 37 stations are obtained with the micro-tremor seismic waveforms, and the two-dimensional Quaternary sedimentary 1
structures cross the Tangshan fault zone are imaged by frequency-to-depth conversion. Results suggest that two significant seismic impedance interfaces at ~ 100 m and 300 - 800 m depth are clearly revealed, respectively. The interface at ~ 100 m depth can be attributed to the strong contrasts between the unconsolidated sand-clay and semi-consolidated silt, which suggests very small variations and is well consistent with the seismic reflection interface from shallow seismic reflection exploration. While the deeper interface at 300 - 800 m depth can be attributed to the Quaternary sedimentary basement, which increases from 300 to 800 m from the west to the east, and is also consistent with the results in previous studies. Rapid variations (~ 200 m) of the Quaternary sedimentary basement depth just beneath the fault zone well agrees with spatial characteristics of the fault revealed by deep seismic reflection profiling. Our work suggests that the Tangshan fault zone has been significantly ruptured and modified by strong earthquake activities since Quaternary. This study also suggests that the HVSR method with dense seismic array is an efficient, effective, non-invasive and low-cost method for investigating active faults in shallow sediments. It is useful for probing the active faults buried by thick sediments in the densely populated urban settings where seismic investigation with explosive sources are too hazardous to carry out.
Keywords: Dense seismic array; Micro-tremors; HVSR; Tangshan fault zone; Quaternary sedimentary basement
2
1. Introduction Spatial characteristics of active faults in the shallow and deep crust can provide important information to understand the seismogenic structure and mechanisms of the devastating earthquakes (Dolan et al,. 1995; Shaw and Suppe, 1996; Liu et al, 2011). Moreover, high-resolution detection of active faults around urban areas is also essential for evaluating earthquake hazard and improving disaster prevention capabilities (Pratt et al., 2002; Davis et al., 1989). But many cities are built in sedimentary basins. The unconsolidated and consolidated low-velocity sediments make it very difficult to explore the detailed structures of active faults beneath the basement, especially for the areas covered with thick Quaternary sediments (e.g., Beijing, Tianjin and Tangshan which are cities in the North China basin). Moreover, the sediments are usually characterized as extremely low velocity, which can amplify the amplitude of seismic wave and enhance the damage caused by large earthquakes (Seed et al., 1988; Bard et al., 1988; Seale and Archuleta, 1989; Dong, et al., 2004; Jemberie and Langston, 2005; Chen et al., 2009; Li et al., 2014; Ni et al., 2014). Therefore, it is very urgent to study the sedimentary structures for investigating the seismogenic environment of the active faults covered with thick sediments, which can also provide essential information for seismic hazard assessment, earthquake emergency response and urban planning. Popular methods for detecting the active faults include both invasive and non-invasive methods (Aki 1957; Lindvall and Rockwell, 1995; Palmer et al., 1997; Pratt et al., 2002; Sakurai and Masuda, 2013; Suzuki et al., 2000; Picozzi et al., 2009). 3
Drilling method can obtain detailed stratum, lithology and velocity structures in the shallow crust (Carpenter et al., 2011; Sakurai and Masuda, 2013). On the other hand, seismic exploration methods with active source can detect the stratigraphic interfaces and velocity structure at different depths in shallow crust. These methods are usually based on the reflection and refraction of the body wave or dispersive surface waves, and can obtain high resolution imaging results (Palmer et al., 1997; Shtivelman et al., 1998; Itoh et al., 2005; Duffy et al., 2014; Wang et al., 2016). These methods have been successfully applied in active fault detection and shallow crustal structure imaging. However, the drilling depth and spatial distributions of boreholes are usually limited by the high cost and populous urban areas. The active source detection method is also expensive, and cannot be deployed in populous urban areas. In the last decade, seismic imaging methods with ambient seismic noise data have been widely used to investigate the detailed structures of shallow and deep crust, partially benefit from their low-cost, high-efficiency, and effectiveness. These methods have been applied to crustal velocity structure for imaging of active faults, carrying out basin surveys, exploring for geothermal resources. Their effectiveness and efficiency are convincing (Betting et al., 2001; Walling et al., 2009; Chen et al., 2009; Langston et al., 2009; Nunziata et al., 2009; Picozzi et al., 2009; Wang et al., 2009, 2011; Agostini, et al., 2015; Zhang et al., 2016; Li et al., 2016, 2017; Civico et al., 2017; Yan et al., 2018). The ambient seismic noise imaging methods can be classified as seismic array method and single-station method. Seismic array method is mainly based on the 4
waveform cross-correlation and spatial autocorrelation to extract the surface wave dispersion and then image the shear-wave velocity structure (Nunziata et al., 2009; Picozzi et al., 2009; Betting et al., 2001; Zhang et al., 2016; Li et al., 2016, 2017). The Horizontal-to-Vertical Spectral Ratio (HVSR) method is one of the single-station method (Parolai et al., 2002; Fäh et al., 2003; Arai and Tokimatsu, 2004) and can extract the frequency responses for the sharp seismic impedance between the sediment and basement. This method can also be used to invert the shallow shear-wave velocity structures. Moreover, with the frequency-to-depth conversion, the HVSR curves can be imaging the continuous seismic impedance depth directly. Because of its efficiency, the HVSR method has been widely used in the field of the shallow velocity structure investigation, seismic field response assessment and seismic micro-zonation (Vella et al., 2013; Gosar 2007; Chen et al., 2009; Langston et al., 2009; Wang et al., 2009, 2011; Leyton et al., 2013; Agostini et al., 2015; (Tarabusi and Caputo, 2017; Civico et al., 2017). The HVSR method has also been employed to investigate reliably the Antarctic ice sheet thickness, thus demonstrating its effectiveness for detecting the buried seismic impedance in the shallow crust (Yan et al., 2018). In this study, we deployed the dense seismic profiling array perpendicular to the Tangshan fault zone, composed of 37 three-component seismometers with interstation distance of ~ 1 km and total length of ~ 40 km (Fig. 1b). The Tangshan fault zone is regarded to be the seismogenic fault responsible for the 1976 Ms7.8 Tangshan earthquake, North China, which caused more than 240,000 deaths and became one of 5
the most devastating earthquakes in the last 100 years in the world (Butler et al., 1979; Nabelek et al., 1987; Liu et al., 2013). Due to the thick (up to a few hundred of meters) Quaternary sediments near the Tangshan fault zone, this causes great difficulties for investigating the detailed structures of the Tangshan fault zone with geological field survey. We carry out a micro-tremor survey using HVSR method. The reliability of the result is extensively tested and verified by seismic data analysis and the comparison with the shallow features speculated by the nearby borehole, as well as the shallow and deep seismic reflection profiling with active sources. This study could provide an efficient and non-invasive method to detect the active fault in the sedimentary basin.
2. Study area Tangshan is a populous and industrialized city near Beijing and Tianjin in North China. In 1976, the MS 7.8 Tangshan earthquake occurred beneath the city, and the maximum seismic intensity is up to XI degree (Liu et al., 2002). It is one of the most devastating earthquakes in the last 100 years (Butler et al., 1979; Nabelek et al., 1987; Liu et al., 2013). The main ruptured fault of the Tangshan earthquake is deemed as the Tangshan fault zone. Tangshan earthquake region is located in the junction of the North China Basin depression and the Yanshan uplift, within a geologic diamond block cut by some active faults (Fig.1a) (Guo et al., 1977). A field geological survey indicates that the surface rupture length of the Tangshan fault zone is greater than 47 km and is associated with some fold structures, consisting of several NE-direction 6
faults (Guo et al., 2011). Moreover, most of the crustal deformation in the Tangshan area is associated with right-lateral slip on the NNE-striking fault rupture plane of the main shock (Huang and Yeh, 1997). A late Quaternary unconsolidated sediment covers extensively over the Tangshan fault zone (Guo et al., 2011). Due to the resonance effect from the loose sediments, site amplification together with ground deformation, subsidence, fissure and liquefaction can impact a large area during the enormous earthquake, and can destroy many buildings (Moss et al., 2011). Although about 40 years have passed since the Tangshan Great earthquake, Tangshan fault zone and adjacent areas still has strong seismic activities (Fig. 1). The alluvial and diluvial coverage since the Quaternary covered on the Tangshan fault zone, which buried the seismogenic environment in the deep crust. Some researchers have done studies in this area for the Quaternary sedimentary characteristics, buried faults and crustal structure (Li et al., 1998; Hao et al., 2001; You et al., 2002; Qiu et al., 2005; Jiang, 2007; Wang et al., 2016). The overall tendency of the Tangshan fault zone is with northwest dip from the shallow seismic prospecting, and the vertical offset reaches 15 meters since late Pleistocene (Li et al., 1998; Hao et al., 2001; You et al., 2002). The drilling section and stratigraphic dislocation information obtained by trench digging show that multiple fracture activities indeed occurred on the Tangshan fault zone since the late Quaternary. This geometrical distribution of the surface rupture zone can be divided into the northern and southern segments. Vertical displacement of the northern segment is increasing 7
westward but decreasing in the east. While the trend on the southern part is on the contrary. The profile obtained by a combined drilling and trenching shows the geological structural evidence of the multistage activities of the Tangshan buried faults covered by sedimentary deposition (Guo et al., 2011).
3. Data and Method 3.1 Data In order to verify the reliability of the HVSR results, we deploy the dense seismic array, which coincides with the location of previous deep seismic reflection survey profiling perpendicular to the Tangshan fault zone (Liu et al., 2011). Thirty-seven short-period (frequency bandwidth of 0.2 - 100 Hz), three-component seismometers (type QS-5) were deployed in January 2017 for two months recordings of ambient noise (Fig. 1). The two-month time observations can provide abundant data to analyze the length of seismic ambient noise waveforms for extracting reliable HVSR curves. The stations are located far from the high-speed railway and other traffic arteries to avoid obvious vibrations. The three-component recordings were calibrated with continuous GPS clock for time synchronization. The seismic stations were placed in free field for reducing low-frequency noise caused by winds impacting on high buildings and tall trees. In order to reduce the influences of wind flow and temperature variations on the ambient seismic noise observation, seismometers were buried underground. Similar observations with the dense seismic array in the Wudalianchi volcano field in the Northeast China and in the Gonghe-Guide region of 8
Qinghai province and they displayed good performance in seismic imaging with ambient noise for shallow magma chamber and investigations on hot-dry rock resources (Zhang et al., 2016; Li et al., 2016, 2017). The short-period seismometers used in the study have been calibrated before deployment for the dense seismic array. We deployed a pair of short-period (QS-5) and broadband (CMG-3ESPCDE) seismometers at the same place to conduct the consistency tests. HVSR curves are calculated from 2 to 4 o’clock in Beijing time with the same controlling parameters (such as time window length, smoothing, et al.). Examples for HVSR curve calculation and verification are shown in Fig. 2. The results suggest that the ambient noise recorded by the two seismometers are all relatively stable, and the HVSR curves with peak frequency and corresponding amplitude are also consistent with each other (Fig. 2c and 2d). These results imply that the HVSR curves can be derived from the short-period seismometers and are as reliable as those obtained from broadband seismometers. In order to study further the influence of the time-window length on the HVSR curves, we have calculated 24 groups of HVSR curves by incrementing a window length from 1- to 24-hour at interval of 1 hour. We found that the HVSR curves have two significant resonant frequencies at ~ 0.24 Hz and ~ 0.13 Hz (Fig. 3a). The resonant frequency and amplitude of the HVSR curves still remain stable when increasing the window length. The test also shows that one hour to several hours’ calculating length is good enough for gaining a robust HVSR curve. The calculating window length will be select 2-hour in later analysis. The effects of time periods in 9
the day- and night-time on the HVSR curves are also studied. Micro-tremor recordings for a whole day is divided into 12 groups with 2-hour window length for the HVSR curves (Fig. 3b). These results show that the variations of the resonant frequencies are pretty small, and the amplitudes slightly changed, especially at the low frequency peak near 0.24 Hz. We can attribute this finding to the noise level induced by human activities (such as transportation, factory et al.). Therefore, stable resonant frequencies in the HVSR curves can obtained at both day- and night-time. However, in order to minimize the noise from human activities in day time, we only consider only the micro-tremor data in the night time for each station to reduce the interference from non-random sources.
3.2 Method We calculate the HVSR curves from the three-component measurements of ambient noise waveforms in frequency domain as shown below (Nakamura, 1989; Lermo and Chavez-Garcia, 1993) （1） For the horizontal spectrum, the squared-average of amplitude spectrum of both N-S and E-W components are used. In order to improve the reliability of the measured HVSR curve, the average amplitude ratio algorithm of short-time to long-time window (STA/LTA) is applied to eliminate the non-random transient interference signals. In order to extract low frequency HVSR curves, seismic records are cut with 60 s windows and used to calculate the HVSR curve for each window. 10
We used the
Konno-Ohmachi smoothing algorithm to strengthen the stability of the HVSR curves in the low frequency (Konno and Ohmachi, 1998). The smoothing coefficient is set at 20 after a few trials to obtain reasonable smoothed HVSR curves. With different smoothing coefficients, the peak frequency of the HVSR curves remain stable with slight amplitudes changes. All HVSR curves from all time windows are stacked to derive the averaged HVSR curve for each station. The horizontal and vertical components consist of the information of ambient noise source (S), propagation path (P), site effect (T), instrument response (I). Thus, the horizontal component H(t) and the vertical component V(t) of the signal can be expressed by （2） （3） Equaitons (2) and (3) can be transformed to the frequency domain and expressed by （4） （5） The ratio of H(f) to V(f) can be shown as below （6） Thus, by dividing horizontal spectral with vertical spectral in the frequency domain, we can eliminate the influences of propagation paths and instrument responses, and thus obtain the frequency characteristics of site effect (Nakamura, 1989; Lermo and Chavez-Garcia, 1993; Chen et al., 2009). The resonant frequency (or peak frequency) of the HVSR curve has good 11
stability and corresponds to the main interface of the sedimentary layer (Nakamura, 1989; Seht and Wohlenberg, 1999; Mucciarelli et al., 2003; Panou et al., 2005). The relationship between the resonant frequency (
) and the thickness (H) of the
sedimentary layer can be determined by the formula （7） is the resonant frequency,
is the average shear-wave velocity, H is the thickness
of the sediment layer. Equation (7) can be utilized to calculate the thickness of sediments with HVSR curves (Yamanaka et al., 1994; Chandler and Lively, 2016; Parolai et al., 2002; Wang et al., 2011). Given the average velocity of shear-wave
,
the thickness of sediments can be calculated by （8）
4. Results The frequency-depth profile derived from HVSR curves is shown in Fig. 4. Each station has two significant peak frequencies, 0.1 - 0.5 Hz (marked as f1) and ~ 1 Hz (marked as f2). The first peak frequencies of f1 decreases gradually from west to the east, and the second peak frequencies of f2 are relatively stable. We set the average shear-wave velocity for the shallow sediments to be 500 m/s based on the studies at the 480 m deep borehole seismic station with local earthquake waveforms (Liu et al., 2011; Wang & Li, 2018). According to equation (8), the HVSR curves are converted into the pseudo-depth image along the dense seismic array after amplitude normalization on peak frequency f1 (Fig. 4c). Theoretical HVSR curves are also 12
calculated with the two-layer sedimentary models. The main features of the theoretical and observed HVSR pseudo-depth profiles are consistent with each other (Fig. 4b and 4c), suggesting that the inferred sedimentary models can fit the observations pretty well. The spatial characteristics of the main seismic impedance interfaces suggest that the first interface is at ~ 100 m depth without significant variations along the profile. The second interface varies from 300 to 800 m depth with gradually deepening trend from the west to the east of the profile. The depth of the second interface rapidly changes just beneath the Tangshan fault zone.
5. Discussions In order to ascertain the reliability of the pseudo-depth profile, we have carried out theoretical calculations and compared
with the observed data. The
one-dimensional, double-layer sedimentary model is constructed to investigate the depth variations of seismic impedance interfaces. We set the average shear-wave velocities of the two layers to be 350 m/s and 520 m/s, respectively. The shear-wave velocity of the half-space is set to be 800 m/s. We refer this shear-wave velocity model to the results at borehole seismometers and drilling data in the nearby North China basin (Chen et al., 2009; Ding et al., 2004; Liu et al., 2011). Based on the elasto-dynamic Green’s functions and the ambient noise interferometry theory (Sánchez-Sesma et al., 2011; García-Jerez et al., 2016; Piña-Flores et al., 2016), we calculate theoretical HVSR curves for all stations and then established the pseudo-depth profile based on the diffused field assumption (Fig. 5a) (). The resonant 13
frequencies are used to calculate the thickness of the two layers by equation (8). (f1 corresponding to the average shear-wave velocity of 500 m/s, and f2 corresponding to the average shear-wave velocity of 350 m/s). The two resonant frequencies are within the frequency bands of 0.1 - 0.7 Hz and 0.7 - 2 Hz, respectively. The main features of the theoretical and observed HVSR pseudo-depth profile are well consistent with each other (Fig. 5a). Two seismic impedance interfaces are then extracted from theoretical and observed predominant frequencies. They show very similar characteristics (Fig. 5b), thus suggesting the reliability of the inferred sedimentary structures under our dense seismic array. Previous borehole data, shallow and deep seismic reflection profiling in the Tangshan fault zone can provide robust constraints for HVSR profiling. In the shallow seismic reflection survey in the study area, the boreholes found one clear interface at 94.7 to 96.9 m depth, which shows the interface between the unconsolidated sand-clay and semi-consolidated silt (Cai et al., 2016). Therefore, we infer that the interface at ~ 100 m depth in the HVSR pseudo-depth profile could be attributed to the strong velocity contrast between unconsolidated sand-clay and semi-consolidated silt. Compared with the interface at 300 - 800 m depth in the HVSR pseudo-depth profile and the Quaternary sedimentary basement derived from seismic reflection (Guo et al., 2011), the deeper interface in the HVSR studies can be attributed to the Quaternary sedimentary basement. We also compared the deep seismic reflection profiling and the HVSR pseudo-depth imaging (Fig. 4). The Quaternary sedimentary basement beneath the Tangshan fault zone shows 100 - 200 m variations in depth. As 14
revealed by the deep seismic reflection profiling at the same location as the HVSR profile, the locations of the rapid changings also show a good correspondence to the distribution of the fault system. It suggests that the Tangshan fault zone could have experienced significant earthquake activities and strongly modified since the Quaternary. In the deep seismic reflection profilings (Fig. 4d), we found the high-angle reverse strike-slip fault system composed of several fault branches for the Tangshan fault zone (Zeng et al., 1988; Wang et al., 1981; You et al. 2002). These faults are characterized with cliffy slope at deeper depth and gentle slope in shallower depth, showing a flower-like tectonic system (Liu et al., 2011). The Tangshan fault zone is a combination of a near-vertical main fault and secondary faults distributed along the controlled tectonic with echelon fractures (Guo et al., 1977). In the HVSR and deep seismic reflection profiles, the normal fault F2 makes the depositional thickness of the upper wall slightly greater than that of the footwall. The F3 and F4 faults are secondary fractures of F2. The thickness of the upper wall is also slightly thicker due to that the F3 could be a west dipping normal fault. The center area of the Tangshan fault zone is located from F4 to F6, corresponding to south segment of the NNE striking Douhe fault (Fig.1b). The Douhe fault is a normal fault extend to 5 km depth or deeper (Zeng et al., 1988; Lu et al., 1988), leading to strong fluctuation with ~ 100 m in sediments. F6 is close to the transition zone of the rapid changes of the Quaternary sedimentary basement. The west area to F6 could be the depression area of basement, while the east area could be the uplift area of basement, where the 15
thickness of the Cenozoic strata is significantly thickened. Both F7 and F8 are in good agreement with the thickness changes with ~ 100 m of the Quaternary sediments. We can infer the rapid depression of F8 to be associated with the Ninghe-Changli deep fault (Jia et al., 2009). The deep seismic reflection profile provides clear deep structural features of the Tangshan fault zone, but few characteristics are provided within a few hundred meters deep. The two-dimensional HVSR pseudo-depth profile provides complementary information for the active faults in the Quaternary sediments. It is important to understand the shallow characteristics of the active fault system in the sediments. Furthermore, there are still some controversies related to the micro-tremor wavefield composition (Lunedei and Malischewsky, 2015; Piña-Flores et al., 2016). The question for the micro-tremor mainly composed of the Rayleigh surface-wave (Yamanaka et al., 1994; Konno and Ohmachi, 1998; Bonnefoy-Claudet et al., 2006) or the body-wave (Nakamura, 1989, 2010) still debates. However, a more consistent view is that the peak frequency obtained by the HVSR method is close to the resonant frequency of the shallow sediments (Lunedei and Malischewsky, 2015), although its accuracy still needs to be verified (Langston et al., 2009). In the future, we need to carry out more works on extracting high-frequency surface wave or body wave from ambient seismic noise will be conducted to investigate the shallow sedimentary structure in the Tangshan fault zone (Pasten et al., 2016; Bao, et al., 2014; Li et al., 2016; Zhan et al., 2010). In addition, HVSR method with dense array provides the sedimentary response beneath single station. Smaller interstation distance could 16
improve the lateral resolution for sedimentary structures. 1 km interstation distance adopted in this study could be proper for imaging the main characteristics of the active faults deep to a few hundred meters. For imaging active faults at a few ten meters depth, dense array with smaller interstation distances are still needed to improve the spatial resolution.
6. Concluding Remarks In this study, we deployed the dense seismic array with 37 seismometers with a nearly 1 km interstation distance across the Tangshan fault zone for micro-tremor survey with HVSR method. A two-dimensional HVSR imaging of the shallow sediments are obtained. And its reliability has been verified by carrying out theoretical tests and comparisons with previous seismic reflection profiles. Two distinct seismic impedance interfaces are found in the shallow sediments. The shallower one at ~ 100 m
depth
represents
the
interface
between
unconsolidated
sand-clay
and
semi-consolidated silt due to strong velocity contrast, which changes slightly along the profile. The deeper one at 300 - 800 m depth represents the Quaternary sedimentary basement with relatively large variations from west to east along the profile. Rapid variations of the Quaternary sedimentary basement show good spatial consistency with the main fault structure of the Tangshan fault zone revealed by the deep seismic reflection profiling. We inferred that the strong undulation of the Quaternary sedimentary basement could be strongly modified by the earthquake activity of the Tangshan fault zone with a variation of 100 - 200 m since Quaternary. 17
This study also suggests that the micro-tremor survey with the HVSR method and the dense seismic array could be potentially serve as an effective, efficient, environment-friendly and low-cost passive source method. It is suitable for active fault detection in populous cities, where it is difficult to conduct active source investigations. The spatial resolution of HVSR profile can be further improved by smaller interstation distance of seismometers, which may reveal more spatial characteristics of the fault system. However, further studies are still needed on demonstrating the prowess of HVSR method. This can be done by including how to improve the accuracy of the frequency-to-depth conversion and minimize the effects of the average shear-velocity. With the constraints from borehole well logging, drill core specimen and ambient noise tomography, the micro-tremor HVSR method can provide more reliable results in active fault investigations.
Acknowledgments We would like to acknowledge the Earthquake Administration of Hebei Province of China for assistance of the dense seismic array deployment. We also thank Professor Sidao Ni for helpful comments. This work was funded by NSFC41674065, NSFC41404052, XDB06030203 and the China-ASEAN Marine Geosciences Research and Disaster Reduction Initiative Project (1062391). We appreciate the constructive comments by the editor and reviewers.
References 18
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Figure Captions Figure 1. (a) Tectonic setting and seismic station distribution. The seismic profile is perpendicular to the Tangshan fault zone (red line). (b) Red triangle represents short period seismometer, while the blue triangle represents comparison stations between short period and broadband seismometer. The circle represents the seismicity from 2010 to today. It shows that the aftershock seismicity still persists with the linear distribution, and consistent with the Tangshan fault zone.
Figure 2. Comparisons and verifications of the HVSR curves from short-period and broadband seismometers. (a) Two-hour micro-tremor waveform recorded by short-period seismometer (the instrument type: QS-5); (b) Two-hour micro-tremor waveform
recorded
by
a
broadband
seismometer
(the
instrument
type:
CMG-3ESPCDE); (c) Average HVSR curve measured by short-period seismometer (dashed lines represent the HVSR standard deviation, same for (d)); (d) Average HVSR curve measured by a broadband seismometer, and comparison of average HVSR curves measured by short-period (black line) and broadband seismometer (red line).
Figure 3. The parameter selection for time length and time period. (a) The effects of time length of micro-tremor waveforms on the HVSR curve. The time length is 31
gradually increasing from 1-hour to 24-hour with 1-hour interval; (b) The stability of the HVSR curves at different time periods in each day with 2-hour time window on the UTC time 18:00-20:00 on January 20, 2017 (corresponding to 2:00-4:00 at night in Beijing).
Figure 4. (a) The calculated HVSR curve profile; (b) The theoretical HVSR pseudo-depth profile; (c, d) Comparing calculated pseudo-depth profile with deep seismic reflection profiling (modified from Liu et al., 2011), and the distribution in Fig.4c corresponds roughly to the red dotted box in Fig.4d. F1-F8 represent the conjectural faults. TQ is Quaternary sedimentary basement, TN is the substratum of Neogene. TMz is Mesozoic crystalline basement, TO and TC-P are the interfaces of Paleozoic Ordovician and Carboniferous-Permian strata. TC is the interface between upper and lower crust, and TM represents crust-mantle transitional zone.
Figure 5. (a) Theoretical HVSR pseudo-depth profile; (b) Comparison between the theoretical pseudo-depths and the calculated results. Red crosses show the thickness calculated from observed HVSR curves. Blue crosses show the thickness based on the model obtained from inversion.
32
Highlights A dense seismic array with 37 seismometers are deployed at Tangshan fault zone. Profile derived from HVSR method reveal the basement of Quaternary sediments. Dense seismic array with HVSR method is effective for investigating active faults.
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