SCIENCE CHINA Earth Sciences

© Science China Press and Springer-Verlag Berlin Heidelberg 2014 earth.scichina.com link.springer.com

*Corresponding author (email: gaoyuan@seis.ac.cn)

• RESEARCH PAPER • September 2014 Vol.57 No.9: 2036–2044  doi: 10.1007/s11430-014-4827-2

A rupture blank zone in middle south part of Longmenshan Faults: Effect after Lushan Ms7.0 earthquake of 20 April 2013 in Sichuan,

China

GAO Yuan1*, WANG Qiong1,3, ZHAO Bo2,3 & SHI YuTao1,3

1 Institute of Earthquake Science, China Earthquake Administration, Beijing 100036, China; 2 China Earthquake Networks Center, China Earthquake Administration, Beijing 100045, China;

3 Institute of Geophysics, China Earthquake Administration, Beijing 100081, China

Received July 19, 2013; accepted November 14, 2013; published online May 20, 2014

On April 20, 2013, the Lushan Ms7.0 earthquake struck at the southern part of the Longmenshan fault in the eastern Tibetan Plateau, China. The shear-wave splitting in the crust indicates a connection between the direction of the principal crustal com-pressive stress and the fault orientation in the Longmenshan fault zone. Our relocation analysis of the aftershocks of the Lushan earthquake shows a gap between the location of the rupture zone of the Lushan Ms7.0 earthquake and that of the rup-ture zone of the Wenchuan Ms8.0 earthquake. We believe that stress levels in the crust at the rupture gap and its vicinity should be monitored in the immediate future. We suggest using controlled source borehole measurements for this purpose.

Lushan earthquake, Longmenshan Fault, rupture gap, crustal seismic anisotropy, double difference relocation, bore-hole measurements of stress change

Citation: Gao Y, Wang Q, Zhao B, et al. 2014. A rupture blank zone in middle south part of Longmenshan Faults: Effect after Lushan Ms7.0 earthquake of 20 April 2013 in Sichuan, China. Science China: Earth Sciences, 57: 2036–2044, doi: 10.1007/s11430-014-4827-2

1 The Lushan Ms7.0 earthquake and the back-ground of regional seismicity

The Ms7.0 earthquake that hit Lushan County, Ya’an, in the Sichuan province of China on April 20, 2013 at 08:02 (Bei-jing time) caused serious casualties in Lushan, Baoxing, and the surrounding area. According to the China Earthquake Network Center (CENC), the location of the epicenter was at 30.0° N, 103.0° E and the magnitude and depth of the hypocenter were Ms7.0 and 13 km, respectively. Here, we refer to this earthquake, called the “4·20” Ms7.0 Lushan earthquake in Sichuan, as the Lushan earthquake.

Since Cenozoic times, the Indian plate has been pushing

the Eurasian plate, causing the uplift of the Tibetan Plateau and a shortening and thickening of the crust in that region. The eastern part of the Tibetan Plateau, from north to south, extrudes northeastward, eastward, and southwestward, re-spectively. This large-scale continuous tectonic movement frequently brings about strong earthquakes in the interior of the Tibetan Plateau and its vicinity (Gao et al., 2000, 2001). The Lushan earthquake occurred in the southern part of the Longmenshan (LMS) fault which is located along the east-ern margin of the Qinghai-Tibetan block (QTB). Because of the eastward extrusion of the QTB and the resistance of the rigid crust of the Sichuan basin, crustal material beneath the eastern margin of the QTB flows eastward, leading to an upward thrust of the ductile material in the lower crust which forms the thrust-type LMS fault (Zhang et al., 2013). The tectonic features and velocity structure of the LMS

Gao Y, et al. Sci China Earth Sci September (2014) Vol.57 No.9 2037

zone display velocity variations that are strongly transverse, and heterogeneity in the crust (Wang et al., 2010; Zhang et al., 2009a, 2013; Lei et al., 2009; Zhang et al., 2011). Based on the crustal structure in the LMS zone (Lei et al., 2009), the hypocenter of the Lushan earthquake lies in the southern part of the LMS fault in the fast P-velocity variation zone; this is highly consistent with reports from recent studies of the southeast margin of the Qinghai-Tibetan Plateau (Gao et al., 2000).

According to the global earthquake catalog of the Pre-liminary Determination of Epicenters (PDE), the distribu-tion of earthquakes of M≥5 after the year 2000 in the east-ern margin of the QTB and its vicinity shows the following characteristics (Figure 1). (1) Before the Wenchuan Ms8.0 earthquake on May 12, 2008, earthquakes occurred mainly in the east and southeast margins of the QTB and extended into the Yunnan Province. Over a period of eight years and four months a total of 23 earthquakes of M≥5 were rec-orded in the region, about 2.8 earthquakes per year. (2) In the period after the Wenchuan Ms8.0 earthquake and before the Lushan Ms7.0 earthquake, the distribution of earth-quakes mainly in the east and southeast margin of the QTB was similar and extended to Yunnan. In the central and northern parts of the LMS fault, however, many strong af-tershocks of the Wenchuan earthquake were recorded, as well as one earthquake in the Sichuan basin. In less than five years, 19 earthquakes of M≥5 occurred in this region in addition to the Wenchuan earthquake and its 73 after-shocks of M≥5, an average of about 3.9 earthquakes per year. (3) After the Lushan earthquake, only one earthquake occurred apart from the six Lushan earthquake aftershocks of M>5; this was the Mb5.3 earthquake on April 25, 2013.

The preliminary report of the CENC shows that this earth-quake was located at the junction of three counties: Changning, Hongxian, and Wenxing in Sichuan province and its magnitude was Ms4.8. Thus, we see an increase in the earthquake frequency in the period between the Wen-chuan Ms8.0 earthquake and the Lushan Ms7.0 earthquake, compared with the period before the Wenchuan earthquake, apart from the Wenchuan earthquake and its aftershocks.

2 Direction of the principal compressive stress indicated by shear-wave splitting in the crust and its relationship to the LMS fault

To study the crustal anisotropy around the LMS fault, rec-ords of the aftershocks of the Wenchuan Ms8.0 earthquake were analyzed. These were recorded by portable seismic stations around the LMS fault set up after the Wenchuan earthquake, and by the permanent Sichuan Seismic Net-works (Zhang Y J et al., 2008). The shear-wave splitting results beneath the stations were obtained using the System-atic Analysis Method (SAM) of shear-wave splitting (Gao et al., 2008b). Shi et al. (2009) found that up to the bounda-ry of Anxian County, the predominant direction of the po-larizations of the fast shear-waves was NNE at stations lo-cated in the northeast part of the LMS fault (zone B). This is consistent with the strike of the fault. For stations located in the central and southern central part of the LMS fault (zone A), the predominant direction of the polarization of the fast shear-waves was NW, nearly perpendicular to the strike of the fault. From the area southwest of the epicenter of the Lushan earthquake in the southern part of the LMS fault,

Figure 1 Seismicity of the Longmenshan fault zone and vicinity. Distribution of M≥5 earthquakes (a) from January 1, 2000 to May 11, 2008; (b) from May 12, 2008 to April 19, 2013, yellow pentagram represents Wenchuan earthquake; (c) from April 20, 2013 to April 30, 2013, yellow pentagram represents the Lushan earthquake. Faults are depicted in blue lines. Earthquake data are from the PDE USA catalog. Red dots indicate Ms5.0–5.9 earthquakes and brown larger dots indicate Ms6.0–6.9 earthquakes. Yellow pentagrams represent the Wenchuan earthquake of May 12, 2008 and the Lushan eartqhake of April 20, 2013. The magnitudes of these two earthquakes are from China Earthquake Network Center (CENC).

2038 Gao Y, et al. Sci China Earth Sci September (2014) Vol.57 No.9

extending almost to the Xianshuihe (XSH) fault (zone C), the polarization directions of the fast shear-waves appear very scattered; however, the average direction is almost E-W (Figure 2).

Studies have shown that the predominant direction of fast shear-wave polarization is generally parallel to the direction of the in situ principal compressive stress (Gao et al., 2008c, 2011, 2012) whose characteristics are related to the regional tectonics and could indicate a hidden strike-slip fault (Gao et al., 2011). The varying polarization directions of the fast shear-waves of the Wenchuan earthquake aftershock se-quence indicate a thrust in the southwest section of the LMS fault and clear strike-slip motion in the northeast section of the fault (Figure 2). Geological studies after the Wenchuan earthquake verified the existence of local strike-slip faults in the northeast section of the LMS fault. Comprehensive re-sults at three stations (MDS, GZA, and L5503) near the intersection of the LMS fault, the XSH fault, and the An-

ninghe (ANH) fault show that the average direction of the fast shear-wave polarization was close to E-W, although that of station L5503 was very scattered (Shi et al., 2009). This is related to the complicated stress distribution induced by the complex local tectonics around station L5503.

After combining the records of the temporary and per-manent seismic stations, additional observational infor-mation of crustal shear-wave splitting in the LMS fault zone was obtained from longer near-field records of small local earthquakes including aftershock sequences of the Wen-chuan earthquake (Shi et al., 2013). Using records of small local earthquakes from January 2000 to April 2010, the central and western parts of the LMS fault were further di-vided into two sections, with their boundary at the location of the Lushan earthquake. Along the LMS fault we identi-fied three subsections of predominant polarizations of fast shear-waves. The aftershock sequence of the Wenchuan earthquake also showed that the boundary line between the

Figure 2 Equal-area projection rose diagram of polarizations of fast shear-waves in the crust from the Wenchuan aftershock sequence data.The Longmen-shan Fault is divided into three parts: zones A, B, and C. The three equal-area rose diagrams of fast shear-wave polarizations (white circles) are the results of all the available data in zones A, B, and C. Black lines indicate faults and black thick arrows indicate the direction of the compressive stress; arrow with circle represents the principal compressive strain induced from GPS data. Yellow pentagram in the north represents the 2008 Wenchuan earthquake and that in the south is the 2013 Lushan earthquake. Blue triangles are regional seismic stations and white triangles are temporary stations. Straight lines at the station show the average polarization direction per station. Red lines represent effective data from more than 11 records and brown lines represent effective data from only one or two records. Grey small dots are Wenchuan aftershocks after relocation. QTB: Qinghai-Tibetan Plateau; LMS: Longmenshan Fault; XSH: Xianshuihe Fault; ANH: Anninghe Fault.

Gao Y, et al. Sci China Earth Sci September (2014) Vol.57 No.9 2039

sections of different polarizations was in the same area, near Anxian County. Our analysis showed results similar to those of a previous study that was based only on the after-shock sequence of the Wenchuan earthquake (Shi et al., 2009): the predominant direction of the fast shear-wave polarizations in zone B is close to NNE, consistent with the strike of the fault. The predominant direction in zone A is close to NW, nearly perpendicular to the strike of the LMS faults but showing an obvious predominant direction con-sistent with the surface strike of the fault. In zone C, the polarizations of the fast shear-waves are scattered; however, the overall predominant polarization is nearly E-W (Figure 3).

In the southern part of the LMS fault and in zone C where several faults intersect, the shear-wave splitting is related to the regional stress field influenced by the faults and the deep tectonics. At stations MDS and SMI, along the Sichuan basin, the direction of the predominant fast shear-wave polarization was close to E-W. This is related to

the eastward push of the QTB that is obstructed by the Si-chuan basin (Figure 3). We used local records (11 records from MDS and 19 from GZA (Shi et al., 2009)) of the Wenchuan earthquake aftershock sequences and some small earthquakes before and after the earthquake to obtain effec-tive data of shear-wave splitting. The predominant fast shear-wave polarization at MDS had two directions; one was almost N-W and the other was close to ENE. The fast shear-wave polarization at GZA was scattered, without an obvious predominant direction, but the average direction was about E-W with a standard error of 32.8° (Figure 2).

From seismic activity recorded between January 2000 to April 2010 at MDS and GZA we obtained effective shear- wave splitting data (48 records from MDS and 52 from GZA) (Shi et al., 2013). The predominant fast shear-wave polarization direction at MDS is clearly ENE (63.5°). The polarizations of the fast shear-waves at GZA is scattered, with no obvious predominant polarization, but the average direction is about in ENE, close to E-W, with a standard

Figure 3 Equal-area projection rose diagram of polarizations of fast shear-waves in the crust in the study area. Data are from January 2000 to April 2010, and the temporary stations recorded only the Wenchuan aftershock sequences. The Longmenshan Fault is divided into three parts: zones A, B, and C. The three equal-area rose diagrams of fast shear-wave polarization (white circles) are the results of all the available data in zones A, B, and C. The diagram in zone C includes three permanent stations and a temporary station L5503 with only one record; red lines show the polarization. Blue and white triangles rep-resent the regional permanent and temporary seismic stations, respectively. Yellow pentagrams represent the 2008 Wenchuan earthquake and the 2013 Lushan earthquake. Other notations are the same as in Figure 2.

2040 Gao Y, et al. Sci China Earth Sci September (2014) Vol.57 No.9

error of 35.1° (Figure 3). Thus, we found that the predomi-nant polarization of the fast shear-waves at station MDS changed from two predominant polarizations to one pre-dominant polarization. This suggests a change of the stress field in the crust under station MDS. The predominant fast shear-wave polarization in the ENE direction indicates an in situ horizontal principal compressive stress in the ENE di-rection, which leads to a NE-striking rupture of the thrust fault. This inference needs further evidence. In addition, the predominant polarization of the fast shear-waves at SMI is clearly in the WNW direction, close to E-W (95.8°). The horizontal principal compressive stress in this direction is almost perpendicular to the ANH fault nearby, which results in some locking of the ANH fault.

3 Relocation analysis of the Lushan earthquake and its aftershocks

To monitor the aftershocks of the Lushan earthquake, tem-porary seismic networks were deployed immediately after the earthquake by the research institutes of the China Earthquake Administration and the Earthquake Administra-tion of Sichuan Province. Combining the data recorded by the permanent and temporary seismic networks, we per-formed relocation analysis of the aftershock sequence of the Lushan earthquake.

The double difference relocation algorithm (Waldhauser et al., 2000), as a relative location technique, has been ap-plied most widely to relocation problems of earthquakes in recent years. It utilizes travel-time differences for pairs of earthquakes at each seismic station to reduce the depend-ence of the velocity model. The algorithm does not need to define the main shock and does not rely on the initial loca-tion of the earthquake, thus avoiding inaccuracy due to lo-cation error of the main shock (Zhao et al., 2013). It was used for the relocation of the aftershock sequence of the 2010 Yushu earthquake and the results showed that the af-tershocks were well distributed along the strike of the Yu-shu fault, confirming the reliability of this method (Zhao et al., 2012). In this study we applied the double difference relocation algorithm to the aftershock data of the Lushan earthquake.

Using observations from the temporary seismic network set up after the Lushan earthquake by the earthquake emer-gency management system of the China Earthquake Ad-ministration, the aftershock sequence of the Lushan earth-quake from April 20 to May 2, 2013 (Figure 4) was used for the relocation. We collected records from a total of 3813 earthquakes and obtained relocation estimates of 3708 of these earthquakes using the double difference relocation algorithm. We used records from 75 seismic stations, which included 60 stations of the regional permanent seismic net-works and 15 stations of the temporary seismic networks

Figure 4 Relocation of the April 20, 2013 Lushan earthquake, Sichuan and its aftershock sequences. Aftershock sequence was recorded from April 20 to May 2, 2013. Yellow circle is the mainshock of the Lushan Ms7.0 earthquake and small red circles are the aftershocks after relocation. Triangles are seismic stations (blue: permanent stations, white: temporary stations). Other notations are the same as in Figure 2.

Gao Y, et al. Sci China Earth Sci September (2014) Vol.57 No.9 2041

(Figure 5). Comparing the histograms before and after the relocation, the aftershocks of the Lushan earthquake oc- curred mainly at a depth range of 10–20 km.

The initial rupture depths of the hypocenters of the Lushan earthquake and the five strong aftershocks of mag-nitude M≥5 range from 15 to 20 km. The initial rupture depth of the hypocenter of the main Ms7.0 shock is about 18 km (Table 1). In this depth at the southern part of the LMS fault, the hypocenter of the Lushan earthquake is at the transition zone between the high velocity and low velocity zones and is almost within the high velocity zone, which is the structure that promotes generation of large earthquakes (Gao et al., 2000).

The parameters of the focal locations of the Lushan earthquake and its strong aftershocks of M≥5 (Table 1) show a significant change in the focal depth parameters. It should be noted that the focal depth is a seismic parameter that is difficult to determine since it involves measurements and definitions which vary. For a strong earthquake with a large range of rupture, different depth definitions will result in different depth values (Gao et al., 1997).

4 The “no rupture zone” in the central Long-menshan fault

Based on the relocation of the Wenchuan earthquake after-shock sequence (Zhao et al., 2011), the Lushan earthquake and its aftershock sequence occurred in the southern part of the LMS fault, southwest of the Wenchuan earthquake. From the earthquake location map (Figure 6) we see no ac-tivity in the middle and southern regions of the central part of the LMS fault, between the location of the Lushan earth-quake including its aftershocks and that of the Wenchuan earthquake including its aftershocks; this region is called the “no rupture zone”.

Based on the results of the emergency analysis of the fo-cal source of the Lushan earthquake by the Institute of Earthquake Science 1) , China Earthquake Administration (Gao et al., 2001), and the rapid results from other research institutions around the world, the focal mechanism of the main shock of the Lushan earthquake shows a reverse-type motion of high dip angle. Combining this with the distribu-tion of faults and aftershocks, we conclude that the fault

Figure 5 Histograms before and after relocation of aftershock sequences of the April 20, 2013 Lushan Ms7.0 earthquake, Sichuan. (a) Before relocation; (b) After relocation. Aftershock sequence is recorded from April 20 to May 2, 2013.

Table 1 Relocation results of M≥5 aftershocks of the Lushan Ms7.0 earthquake a)

Time Before relocation

After relocation

Magnitude Ms Latitude (N) Longitude (E) Depth (km) Latitude (N) Longitude (E) Depth (km)

2013-04-20 08:02:46 30.3° 103.0° 13 30.29° 102.97° 17.8 7.0

2013-04-20 08:07:29 30.3° 102.9° 10 30.35° 102.95° 18.5 5.1

2013-04-20 11:34:14 30.1° 102.9° 11 30.18° 102.88° 15.1 5.3

2013-04-21 04:53:44 30.3° 103.0° 17 30.35° 103.04° 19.8 5.0

2013-04-21 17:05:22 30.3° 103.0° 17 30.33° 103.03° 16.4 5.0

a) Hypocenter parameters before relocation; magnitudes are from the rapid official report of CENC

1) Institute of Earthquake Science, China Earthquake Administration. 2013. Working information, Nos. 31, 33, 34.

2042 Gao Y, et al. Sci China Earth Sci September (2014) Vol.57 No.9

 Figure 6 Aftershock sequences of the 2013 Lushan Ms7.0 earthquake and 2008 Wenchuan Ms8.0 earthquake. Large and small yellow circles represent the 2008 Wenchuan earthquake and Lushan earthquake, respectively. White circles represent strong aftershocks of 5.0≤M

Gao Y, et al. Sci China Earth Sci September (2014) Vol.57 No.9 2043

Several faults intersect in the southern end of the LMS fault. Unlike the thrust motion in the central part of the LMS fault, the tectonics in the southern end of the LMS fault varies because of the intersection of several faults; therefore, the stress is also dissimilar. However, the Lushan earthquake itself was a thrust event, as was the Wenchuan earthquake. This highlights the existence of an unbroken zone between the location of the Lushan earthquake and that of the Wenchuan earthquake.

The energy released by the Wenchuan earthquake origi-nated from the slow accumulation of long-term large-scale crustal strain in the LMS fault in the western margin of Si-chuan Basin. This strain is caused by the Bayan Har block pushing against the LMS fault from the west (i.e. the Songpan-Garze block in Figure 6) (Jiang et al., 2009). The seismic profile across the LMS fault reveals a discontinuity in the Moho and a sharp increase in the velocity ratio Vp/Vs (Zhang et al., 2009b, 2010). The velocity structure also shows a velocity variation under the LMS fault (Lü et al., 2013), indicating deep structure. Based on the tectonic characteristics of the region and on past and current earth-quake activity, we note that for at least 1100 years before 2008 there were no earthquakes of magnitude ≥Ms7.0 in the LMS fault. Along the LMS fault, a zone formed with very low seismicity compared with the seismic activity of the surrounding faults, and in 2008 the Wenchuan Ms8.0 earthquake occurred in this “blank zone” (Wen et al., 2009). After the Wenchuan earthquake, because of the continuous squeezing eastward of the Songpan-Garze block in the east-ern Tibetan Plateau, studies suggested that a risk of rupture existed in the unbroken south segment of the LMS faults. The Lushan earthquake released some of this accumulated strain, thus reducing the possibility of an earthquake larger than Ms7.0 occurring in the near future in the southern seg-ment of the LMS faults. However, we still need to keep a watchful eye on this “no rupture zone” to monitor the risk of it rupturing again.

5 Discussion and conclusions

The focal locations (Zhao et al., 2011; Zhang R Q et al., 2008) and stress background (Zhang Y J et al., 2008; Shi et al., 2009, 2013; Ding et al., 2008) of the Wenchuan and Lushan earthquakes on the LMS fault were obtained by analyzing the activity of aftershocks and studying shear- wave splitting data. In this study we examined the orienta-tion of the regional principal compressive stress in the zone that extends southwest from the epicenter of the Lushan earthquake along the LMS fault to the intersection of the LMS, XSH, and ANH faults and the adjacent region. We found that the orientation changes from NW in the northern part of this zone to almost E-W further south. The spatial variation of the stress indicates deep tectonic movement which will require further investigation.

The Lushan earthquake occurred in the southern part of the LMS fault. A “no rupture zone” appears after the Lushan earthquake, which is of about 60 km long between the rupture zone of Lushan earthquake and the rupture zone of Wenchuan earthquake. Within this no-rupture zone, an earthquake of magnitude Ms6.2 occurred in 1970; however, more than 40 years have passed since then. Our relocation study of the aftershocks of the Lushan earthquake indicates that the Lushan earthquake did not cut through itself and the rupture zone of the Wenchuan earthquake. It is important for scientific research and for the safety of the population in the region to pay close attention to any changes in the “no rupture zone” between the locations of the Lushan and Wenchuan earthquakes, by enhancing seismic monitoring in the southern part of the LMS faults.

Close monitoring of stress variation in the crust can de-tect signs of tectonic activity. Studies have shown that crus-tal shear-wave splitting is one of the valid methods for monitoring stress change caused by earthquake activity (Gao et al., 2004, 2008a; Crampin et al., 2008). However, this technique requires a large number of small earthquake events in the vicinity of the seismic stations since the data sources are seismic waves of small earthquakes. More ef-fective methods are the cross-well techniques using artifi-cial borehole sources and borehole records, e.g. stress- monitoring sites (Gao et al., 2008a; Crampin et al., 2008). A study of the San Andreas Fault, California continuously monitored artificial borehole signals and measured velocity changes of seismic waves (Niu et al., 2008). This is another proven effective borehole method to measure the change of stress by a controllable source. Both borehole measurement methods require significant expenditure; therefore, their use is limited to measuring changes of stress and wave velocity in key zones only.

Earthquakes occur frequently in the eastern and south-eastern areas of the Tibetan Plateau. Monitoring very small changes in stress and velocity throughout this region will deepen our understanding of the seismic structure and seis-mic sources affecting the tectonic activity in the region.

This work was supported by the National Natural Science Foundation of China (Grant Nos. 41174042, 41040034) and the China National Special Fund for Earthquake Scientific Research in Public Interest (Grant No. 201008001). The relocation data of the aftershocks of the Lushan earth-quake were obtained from the China Earthquake Network Center (CENC) and several research institutions of the China Earthquake Administration. We would like to thanks the data sharing program of earthquake emergen-cy. We thank reviewers for their comments and suggestions to improve this manuscript.

Crampin S, Gao Y, Peacock S. 2008. Stress-forecasting (not predicting) earthquakes: A paradigm shift? Geology, 36: 427–430

Ding Z F, Wu Y, Wang H, et al. 2008. Variations of shear wave splitting in the 2008 Wenchuan earthquake region. Sci China Ser D-Earth Sci, 51: 1712–1716

Gao Y, Crampin S. 2004. Observations of stress relaxation before earth-quakes. Geophys J Int, 157: 578–582

2044 Gao Y, et al. Sci China Earth Sci September (2014) Vol.57 No.9

Gao Y, Crampin S. 2008a. Shear-wave splitting and earthquake forecasting. Terra Nova, 20: 440–448

Gao Y, Shi Y T, Liang W, et al. 2008b. Systematic analysis method of shear-wave splitting SAM (2007): Software system (in Chinese). Earthquake Res China, 24: 345–353

Gao Y, Wu J. 2008c. Compressive stress field in the crust deduced from shear-wave anisotropy: An example in capital area of China. Chin Sci Bull, 53: 2933–2939

Gao Y, Shi Y T, Wu J, et al. 2012. Shear-wave splitting in the crust: Re-gional compressive stress from polarizations of fast shear-waves. Earthq Sci, 25: 35–45

Gao Y, van der Lee S, Giardini D, et al. 2001. A study on source mecha-nism of large mani earthquake of 8 november 1997 in China Xizang by mode summation (in Chinese). Chin J Geophys, 44(Suppl): 98–106

Gao Y, Wu J, Fukao Y, et al. 2011. Shear-wave splitting in the crust in North China: Stress, faults and tectonic implications. Geophys J Int, 187: 642–654

Gao Y, Wu Z L, Liu Z, et al. 2000. Seismic source characteristics of nine strong earthquakes from 1988 to 1990 and earthquake activity since 1970 in the Sichuan-Qinghai-Xizang (Tibet) zone of China. Pure Appl Geophy, 157: 1423–1443

Gao Y, Zhou H L, Zheng S H, et al. 1997. Preliminary discussion on im-plication of determination on source depth of earthquake (in Chinese). Earthq Res China, 13: 321–329

Jiang Z S, Fang Y, Wu Y Q, et al. 2009. The dynamic process of regional crustal movement and deformation before Wenchuan Ms8.0 earthquake, China (in Chinese). Chin J Geophys, 52: 505–518

Lei J S, Zhao D P, Su J R, et al. 2009. Fine seismic structure under the Longmenshan fault zone and the mechanism of the large Wenchuan earthquake (in Chinese). Chin J Geophys, 52: 339–345

Lü Y, Zhang Z, Pei S, et al. 2013. 2.5-Dimensional tomography of upper-most mantle beneath Sichuan-Yunnan and surrounding regions. Tecto-nophysics, doi: 10.1016/j.tecto.2013.03.008

Niu F, Silver P, Daley T, et al. 2008. Preseismic velocity changes observed from active source monitoring at the Parkfield SAFOD drill site. Nature, 454: 204–208, doi: 10.1038/nature07111

Shi Y T, Gao Y, Zhang Y J, et al. 2013. Shear-wave splitting in the crust in Eastern Songpan-Garze block, Sichuan-Yunnan block and Western Si-chuan Basin (in Chinese). Chin J Geophys, 56: 481– 494

Shi Y T, Gao Y, Zhao C P, et al. 2009. A study of seismic anisotropy of Wenchuan earthquake sequence (in Chinese). Chin J Geophys, 52: 398–407

Waldhauser F, Ellsworth L W. 2000. A double-difference earthquake loca-tion algorithm: Method and application to the northern Hayward Fault, California. Bull Seism Soc Amer, 90: 1353–1368

Wang Q, Gao Y. 2014. Rayleigh wave phase velocity tomography and strong earthquake activity on the southeastern front of the Tibetan Platea. Sci China Earth Sci, 57, doi: 10.007/s11430-014-4908-2

Wang W M, Zhao L F, Li J, et al. 2008. Rupture process of the Ms8.0 Wenchuan earthquake of Sichuan, China (in Chinese). Chin J Geophys, 51: 1403–1410

Wang Z, Zhao D P, Wang J. 2010. Deep structure and seismogenesis of the north-south seismic zone in southwest China. J Geophys Res, 115: B12334, doi: 10.1029/2010JB007797

Wen X Z, Zhang P Z, Du F, et al. 2009. The background of historical and modern seismic activities of the occurrence of the 2008 Ms8.0 Wen-chuan earthquake, Sichuan (in Chinese). Chin J Geophys, 52: 444–454

Zhang R Q, Wu Q J, Li Y H, et al. 2008. Focal depths for moderate-sized aftershocks of the Wenchuan Ms8.0 earthquake and their implications. Sci China Ser D-Earth Sci, 51: 1694–1702

Zhang Y J, Gao Y, Shi Y T, et al. 2008. A study on S wave splitting using waveform data from Sichuan Regional Seismic Network. Acta Seismol Sin, 30: 123–134

Zhang Y, Feng W P, Xu L S, et al. 2009. Spatio-temporal rupture process of the 2008 great Wenchuan earthquake. Sci China Ser D-Earth Sci, 52: 145–154

Zhang Z J, Chen Y, Tian X B. 2009a. Crust-upper mantle structure on the eastern margin of Tibetan Plateau and its geodynamic implications (in Chinese). Chin J Geol, 44: 1136–1150

Zhang Z J, Wang Y, Chen Y, et al. 2009b. Crustal structure across Long-menshan fault belt from passive source seismic profiling. Geophys Res Lett, 36: L17310, doi: 10.1029/2009GL039580

Zhang Z J, Deng Y F, Liu C, et al. 2013. Seismic structure and rheology of the crust under mainland China. Gondwana Res, 23: 1455–1483

Zhang Z J, Deng Y, Teng J, et al. 2011. An overview of the crustal struc-ture of the Tibetan plateau after 35 years of deep seismic soundings. J Asian Earth Sci, 40: 977–989

Zhang Z J, Yuan X, Chen Y, et al. 2010. Seismic signature of the collision between the east Tibetan escape flow and the Sichuan Basin. Earth Planet Sci Lett, 292: 254–264

Zhao B, Gao Y, Shi Y T. 2013. Precise relocation of small earthquakes occurred in North China and its tectonic implication (in Chinese). Earthquake, 33: 12–21

Zhao B, Shi Y T, Gao Y. 2011. Relocation of aftershock of the Wenchuan Ms8.0 earthquake and its implication to seismotectonics. Earthq Sci, 24: 107–113

Zhao B, Shi Y T, Gao Y. 2012. Seismic relocation, focal mechanism and crustal seismic anisotropy associated with the Ms7.1 Yushu earthquake and its aftershocks. Earthq Sci, 25: 111–119