Shear-wave splitting in the crust beneath the southeast Capital area of North China

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1 J Seismol (2009) 13: DOI /s x ORIGINAL ARTICLE Shear-wave splitting in the crust beneath the southeast Capital area of North China Jing Wu Yuan Gao Yun-Tai Chen Received: 9 June 2007 / Accepted: 17 January 2008 / Published online: 4 September 2008 Springer Science + Business Media B.V Abstract This study focuses on the southeast Capital area of North China ( N, E). Shear-wave splitting parameters at 20 seismic stations are obtained by a systematic analysis method applied to data recorded by the Capital Area Seismograph Network (CASN) between the years 2002 and Although some differences in the results are observed, the average fast-wave polarization is N88.2 W ± 40.7 and the average normalized slow wave time delay is 3.55 ± 2.93 ms/km. The average polarization is consistent with the regional maximum horizontal compressive stress and also with the maximum principal strain derived from global positioning system measurements in North China. In spite of the uneven distribution of faults around the array stations that likely introduce some amount of scatter in the shear-wave splitting measurements, site-dependent polarizations of fast shear wave are clearly observed: in the northern half of the study area, the polarizations at CASN stations show E W direction, whereas in the southern half the polarizations exhibit a variety of possible azimuths, thus suggesting dissimilar stress field and tectonic frame in both areas. Comparing the splitting results with those previously obtained in the northwest part of the region, we find a difference in polarization of about 20 between the southeast and northwest parts of the Capital area; also, in the southeast Capital area the average time delay is smaller than in the northwest Capital area, thus making clear that the magnitude of crustal seismic anisotropy is not the same in the two zones. Being the shear-wave splitting polarizations in the southeast Capital area, which lies on the basin, clearly different from the observed polarizations in the northwest Capital area, where uplifts and basin converge, it is quite evident that the shear-wave splitting results are consequence of the tectonics and stress field affecting the two regions. Keywords Shear-wave splitting Seismic anisotropy in the crust Southeast Capital area of China J. Wu Y. Gao (B) Institute of Earthquake Science, China Earthquake Administration, Beijing , China gaoyuan@seis.ac.cn Y.-T. Chen Institute of Geophysics, China Earthquake Administration, Beijing , China 1 Introduction Most researches show that seismic anisotropy exists widely in both the crust and the upper mantle (Crampin 1978; Gao et al. 1995, 1999; Zhang et al. 2000; Müller 2001; Liu et al. 2001).

2 278 J Seismol (2009) 13: Extensive-dilatancy anisotropy microcracks are typical sources of seismic anisotropy in the crust, whose dynamic characteristics can be modeled by anisotropy poroelasticity (Zatsepin and Crampin 1997). The spatial distribution of the predominant polarization of fast shear-waves is consistent with the maximum horizontal compressive stress in the region. However, the polarizations of fast shearwave at stations near to active faults are commonly parallel to the strikes of faults (Peng and Ben-Zion 2004; Shi et al. 2006; Wuetal.2007; Gao et al. 2008). Furthermore, the polarizations of fast shear-wave are much scattered in complex tectonic areas, such as crossing fault zones. These anisotropy features can provide detailed information about the regional tectonics and lead to a better understanding of the real stress field in situ (Gao and Crampin 2006; Gao et al. 2008). The main tectonic units around the Capital area of North China are Yanshan Uplift, Taihang Uplift, and North China Basin (Fig. 1). It is a region with a relatively complex tectonic and strong seismic activity. It is found from global positioning system observations that the stress in the east part is quite different from the stress in the west part (Jiang et al. 2000). The crustal seismic velocity structure in Taihang Uplift and Yanshan Uplift is that of a relatively simple crust of stable paleoland, very different from the Neozoic crust beneath the North China Basin (Huang and Zhao 2005; Jia et al. 2005). 3D tomography of the upper crust in the Beijing area (Wang et al. 2005) based on deep seismic sounding data reveals that seismic velocity structure and fault movements are quite related to each other. Due to the requirements of the shear-wave splitting analysis and the limitations of the seismic network, the research about shear-wave splitting in the Capital area is still at present an interesting study matter. Lai et al. (2006) have studied by shear-wave splitting analysis the seismic anisotropy and stress field of the crust in the Capital area from events occurring between May 2002 and March 2003 recorded by permanent stations and events occurring between March 2002 and November 2002 recorded by temporary stations. More recently, Wu et al. (2007), from data recorded during 2 years, have obtained preliminary results about the seismic anisotropy of the crust in the northwest Capital area. In this paper, we analyze the seismic anisotropy of the crust beneath the southeast Capital area based on data recorded during a period of more than 3 years and a half and waveforms selected with an extended shear-wave window of 55.Wefurther Fig. 1 Faults and seismic stations in the southeast Capital area, North China. Short lines represent faults; black triangles are stations; star marks the city, and arrows indicate the direction of horizontal principal compressive stress in the area. Key to symbols: F1, Tangshan-Dacheng fault; F2, Baodi fault; F3, Tongxian-Nanyuan fault; F4, Cangdong fault

3 J Seismol (2009) 13: discuss the relation between shear-wave splitting and regional faulting. 2 Data The Capital Area Seismograph Network (CASN) array, which spans about 500 km from east to west and some 400 km from north to south, was installed in 1999, but it is in operation since the first of October The array is formed by 107 stations which keep an average interdistance between stations of about 40 km: 53 stations equipped with short-period seismometers are placed in North China Basin, while other 54 shortperiod stations are mostly distributed in Taihang Uplift and Yanshan Uplift. CASN is a very large regional seismic network and one of the denser seismic networks in China (Zhuang 1999). Figure 1 shows a map with the regional faults, the principal stress field direction, and the locations of 20 CASN stations in the southeast Capital area (except for DOH, the other 19 stations are put in boreholes). The data used in this study are waveforms generated by local events of M L -magnitude from 0.5 to 4.3 and depth from 5 to 30 km (average depth of 15 km), which occurred during the period January 2002 August 2005 in the southeast Capital area of North China ( N, E) and recorded by the 20 control stations belonging to the CASN array. Two main requirements for data acquisition were taken into account: high signal-to-noise ratio and small incidence angle within the shearwave window. This last means that the incidence angle is less than critical angle, i.e., less than θ = sin 1 (Vs/Vp), where Vp and Vs are the P- and S-wave velocities, respectively. Because of the low-velocity sediment layer, this effective shear-wave window is typically as much as 45 or 50 (Crampin and Peacock 2005). Thus, those records of quality and with incidence angle less than 45 were selected for shear-wave splitting analysis. As an example, Fig. 2 shows the filtered seismic waveforms generated by the 13 October 2004 earthquake of M L -magnitude 2.0 and depth 22 km recorded at station LUT at an epicentral distance of km from the source. The signal Fig. 2 Filtered waveforms generated by the 13 October 2004 earthquake of M L -magnitude 2.0 and depth 22 km recorded at station LUT. From top to bottom, vertical (UD), east west (EW), and north south (NS) ground motion components. A 2 20-Hz Butterworth band pass filter was used can be clearly picked from the seismograms. Figure 3 (left column) shows the east west and north south horizontal components of the ground motion including shear waves and the nonlinear particle motion. 3Method In order to obtain the seismic anisotropy parameters in the crust, a systematic analysis method (Gao et al. 2004) of split waveforms was used in this study. The method mainly includes time-delay correction, computation of the particle motion, and polarization analysis of split shear waves. As is well known, when a shear wave travels through an anisotropic medium, it will split into a fast shear wave and a slow shear wave, so that the polarization of fast shear wave is parallel to the vertically aligned microcracks, while the polarization of slow shear wave is nearly perpendicular to the aligned microcracks. The key parameters in shear-wave splitting analysis are the polarization of fast shear wave and the time delay of slow shear wave that gives the magnitude of seismic anisotropy of the medium. Theoretically, both the fast shear wave and the slow shear wave originate from the same source and therefore they should

4 280 J Seismol (2009) 13: Fig. 3 Shear-wave splitting analysis. Left and from top to bottom: east west (E) and north south (N) shear-wave components and particle motion as extracted from the original seismic signal (Fig. 2). Right and from top to bottom: fast shear wave (F, S 1 ) and slow shear wave (S, S 2 ) and particle motion as derived from polarization analysis. The fast-wave polarization and the slow-wave time delay are 135 and 0.08 s, respectively. The normalized time delay is 3.11 ms/km be similar as to the form. Based on this consideration, the correlation analysis is used in order to estimate the splitting parameters. The north south and east west shear-wave components are rotated and shifted in time. Then, the correlation coefficients are calculated for possible values of time shift of the split shear waves. Lastly, the splitting parameters are determined from the maximum of the correlation coefficients (Gao et al. 1995, 1999). Many factors may however influence the results, such as crustal structure, surface topography, geological tectonic conditions around the station, waveform data, applied method, etc. (Crampin and Peacock 2005; Gao and Crampin 2006). In some cases, the shear-wave splitting parameters cannot be obtained properly by crosscorrelation only, and polarization analysis is then necessary (Gao et al. 1995, 1999). Let us suppose that the polarization angle of fast shear wave is α; then, after rotating the north south and east west components by angle α, these horizontal components become the fast and slow shear-wave components (Fig. 3, right column). If the time delay of slow shear wave is t, the slow shear wave can be moved forward just this time t to eliminate the time delay, and the particle motion becomes more linear as can be seen in the polarization diagram (Fig. 3, bottom). Therefore, by rotation of waveforms, time-delay correction, computation of the particle motion, and polarization analysis of split shear waves, it is possible to control the process and to estimate reliable shear-wave splitting parameters. Here, these splitting parameters are always systematically adjusted by these operations (that we identify by its acronym SAM in a previous work by Gao et al. 2004) since the direct calculation often results in some mistakes (Crampin and Gao 2006). A very efficient semiautomatic method based on Expert System has been developed to analyze shear-wave splitting from small earthquakes; by now, it is ready for only specific seismic data from Iceland (Gao et al. 2006). The shear-wave splitting parameters are so obtained at 20 CASN stations (Fig. 1). Table 1 contains the station parameters and the shear-wave splitting results from measurements made at these 20 control stations.

5 J Seismol (2009) 13: Table 1 Station parameters and shear-wave splitting parameters in the southeast Capital area Station Station East North Number of Polarization Standard Time delay Standard error name code longitude latitude records (in degrees error (ms/km) of time delay East of North) (±degrees) (±ms/km) Ankang Hospital ANK Baodi BAD Beitang BET Caodian CAD Changhong Park CHH Chitu CHT Douhe DOH Erwangzhuang EWZ Fengtai Town FTZ Hangu HAG Jinghai JIH Lutai LUT Nanhe Town NHZ Qingguang QIG Tang23 T Wangkuang WAK Wenan WEA Xinan Town XAZ Yufa YUF Zhutangzhuang ZTZ Results From the statistical analysis of the shear-wave splitting results obtained for the southeast Capital area, the average polarization of fast shear waves is N88.2 W ± 40.7 (Fig. 4), whereas the average normalized time delay of slow shear waves is 3.55 ± 2.93 ms/km. The average slow wave time delay in the northwest Capital area is 4.44 ± 2.93 ms/km (Wu et al. 2007), which is comparatively higher than the average time delay in the southeast Capital area. The average polarization estimated for the southeast Capital area is close to the maximum horizontal principal compressive stress in North China (Xu 2001) and consistent with the maximum compressive strain in North China (Zhang et al. 2004). However, the average polarization of fast shear waves in the northwest Capital area is N69.9 W ± 44.5 (Wu et al. 2007) and consequently there is a difference in average polarization between the southeast and northwest parts of the Capital area of about 20. In addition, the standard error affecting the fast-shear-wave polarization in the southeast part is much smaller than the scatter observed in the northwest Capital area. Figure 5 shows the equal-area rose diagram (lower hemisphere projection) which describes the polarizations of the faster shear waves recorded at the Fig. 4 Rose diagram of fast-shear-wave polarizations observed in the southeast Capital area. The results correspond to the waveforms recorded at 20 CASN stations during the period January 2002 to August 2005

6 282 J Seismol (2009) 13: Fig. 5 Equal-area rose diagram (lower hemisphere projection) of fast-shear-wave polarizations at the 20 CASN stations installed in the southeast Capital area. In this diagram, the center of a circle represents the position of the respective station and within each circle the midpoint and direction of a short segment mark the position of an event and the fast-shear-wave polarization, respectively 20 CASN stations. In this diagram, the center of any circle represents the position of the respective station and within each circle the midpoint and direction of a short segment mark the position of an event and the fast-shear-wave polarization, respectively. It is obvious that the polarizations estimated here show different patterns at CASN stations. 5 Spatial distribution of polarizations The spatial distribution of fast-shear-wave polarizations at CASN stations is displayed in Fig. 6, wherein the directions of the short segments indicate the average fast-shear-wave polarization at the respective station, while the lengths of the segments are proportional to the average time

7 J Seismol (2009) 13: Fig. 6 Average fast-shear-wave polarization at CASN stations (Fig. 1). The directions of the short segments show the average fast-shear-wave polarization at the respective station, while the lengths of the segments are proportional to the average slow-wave time delay at each station. Black segments correspond to stations with two or more records. Four stations (YUF, QIG, CHT, NHZ, T23) show polarizations with high standard errors. A time-delay scale is inserted on the bottom left corner. Local faults are the same as in Fig. 1 delay of the slow shear wave at each station. Sitedependent polarizations of fast shear wave are clearly observed. North of the study area, the average polarizations at stations such as BAD, XAZ, CAD, FTZ, and DOH are nearly in E W direction (Fig. 6). Station YUF presents only two records (Table 1) which provide different polarizations (Fig. 5) and despite of it the average polarization direction is too E W (Fig. 6). However, some stations, EWZ and LUT, in the central part of the explored area show polarization W NW, although other stations (ZTZ, CHT, HAG) show clearly polarizations in E W direction in spite of the observed scatter, for example, at station CHT (Fig. 5). In the northern half of the study area, the average polarizations differ 20 from the horizontal principal compressive stress in North China (N71.6 E), thus being roughly consistent with the horizontal principal compressive stress in the region (Fig. 6). In contrast, a variety of polarization patterns can be observed in the southern half of the test area (Fig. 6). With the exception of station CHH which also shows clear fast-shear-wave polarization in E W direction, other stations such as JIH, T23, and BET exhibit average polarization nearly in N S direction, and another stations such as ANK, NHZ, and WAK in N E direction. Lastly, station WEA exhibits fast-shear-wave polarization in nearly N W direction. Station QIG shows very scattered polarizations (Fig. 5) and the reliability of the polarization data is not good enough. Most of the 20 control stations used in this study provide consistent fast-shear-wave polarizations and predominant directions of polarization (Fig. 5). However, some stations present scattered polarizations, large standard errors, and low reliability, such as YUF, CHT, QIG, NHZ, and T23 (Figs. 5 and 6). There are many possible reasons by which scattered polarization may happen independently from the data quality. At stations such as BAD, CAD, FTZ, DOH, and HAG, some analyzed waveforms taken in the limit of the shearwave window can be the cause of biased results

8 284 J Seismol (2009) 13: and therefore of less accuracy (Fig. 5). Anyhow, because of the large number of useful records, the final results are not influenced by this reason. Other scattering factor may be the complex stress field owing to crossing faults, which can result in an unclear pattern with more than one predominant polarization. For instance, the admissible reason for such a situation at stations NHZ and T23 (Fig. 5) is the existence of nearby crossing faults in N S and E W directions, respectively (Fig. 6). Moreover, unknown deep faults may be the other possible reason too. Station QIG shows, for example, various polarizations (Fig. 5) which are not easy to understand. Complex scattered polarizations patterns at stations such as QIG and NHZ (Fig. 5) need to be studied in detail in the future and likely with more data. 6 Preliminary observations on the splitting results The results support that the fast-shear-wave polarizations at stations in the northern half of the Capital area are rather consistent and exhibit nearly E W direction (Fig. 6). In particular, stations BAD, XAZ, CAD, FTZ, and DOH are all very close to the Baodi fault (F2 in Fig. 1) and their respective polarizations are very similar and parallel to the strike of this fault. Even a station with so low reliability as YUF, which is far from these five stations, presents a similar polarization in E W direction. But in all these cases the polarizations are different from the maximum horizontal principal compressive stress in the zone. In consequence of the splitting results, the wave polarizations at stations on or nearly-around active faults are usually parallel to the strike of the faults (Crampin et al. 2002; Peng and Ben-Zion 2004; Gao et al. 2008; Shi et al. 2006;Wuetal.2007). In change, the predominant fast-shear-wave polarizations at stations in the southern half of the Capital area are quite different ones from others (Figs. 5 and6), likely in accordance with local variations regarding the tectonic of this area. Stations WAK, JIH, NHZ, ANK, CHH, and QIG show certainly different polarizations, albeit the first four of them depict a predominant polarization in N NE or nearly N S direction, following more or less the strike of a nearby long fault which crosses the region from south to north. As before, the polarizations are all different from the maximum horizontal principal compressive stress in the zone but nearly parallel to the strike of the long fault, so that the influence of this fault on the shear-wave splitting results seem to be a real possibility. With respect to station WEA, the average fastwave polarization is N123.3 E ± 6.2 (Fig. 5), obviously very far from the dominant direction of the regional stress field. It is interesting to observe that the predominant polarization at WEA (Fig. 6) is also different from the wave polarizations at other nearby stations (WAK, JIH), which possibly is related with recent seismic activity: a M L 5.1 earthquake occurred in Wen-an in At present, it is not possible to say whether the splitting result at WEA is due to a nearby active fault in W NW direction or not. Furthermore, there is a clear wave polarization in E W direction at station CHH which is the nearest one to the city. But it is indeed premature to attribute this result to a hidden active fault with strike in the same direction. A conclusion of this nature needs more research. 7 Conclusions By applying systematically a method based on time-delay correction, computation of the particle motion, and polarization analysis of split shear waves to seismograms recorded during the period , we have determined the splitting parameters at 20 stations belonging to the CASN array installed in the southeast Capital area of North China. The average polarization of fast shear wave is N88.2 W ± 40.7 and the normalized average time delay of slow shear wave is 3.55 ± 2.93 ms/km. Since the horizontal principal compressive stress in North China is N71.6 E, it is evident that the observed predominant polarization in the northern half of the test area is not quite different from the direction of the regional stress field and is roughly consistent with the regional

9 J Seismol (2009) 13: maximum compressive strain, which indicates that shear-wave splitting depicts reasonably well the principal compressive stress and strain affecting the region. An overall view of the fast-shear-wave polarizations allows us to appreciate that they are very distinct depending on the location of the CASN stations either in the northern half or in the southern half of the study area. Unlike this last area where the polarizations exhibit a variety of possible azimuths, in the north, the polarizations are rather consistent and show E W direction, thus suggesting dissimilar stress field and tectonic frame in both areas. As the polarization of fast shear wave is consequence of the principal compressive stress in situ and the time delay of slow shear wave express the degree of seismic anisotropy in situ, the faults near the measurement sites can influence the shearwave splitting results; thus, the uneven distribution of faults in the explored region may be the cause of the scatter affecting some estimates of polarization, as expected. Furthermore, different tectonic structures can result in different shear-wave splitting measurements. So, since the average fast-wave polarization in the northwest Capital area is N69.9 W ± 44.5 (Wu et al. 2007), there is a difference in polarization of about 20 between the southeast and northwest parts of the Capital area. Furthermore, the average time delay of slow shear wave in the southeast Capital area, that is 3.55 ms/km, is obviously less than the normalized time delay of 4.44 ms/km in the northwest Capital area (Wu et al. 2007), which implies that the magnitude of crustal seismic anisotropy is not the same in the two zones. In addition, the standard polarization errors in the southeast part are smaller than the standard errors in the northwest part. As the southeast Capital area lies on the basin and the northwest Capital area lies where uplifts and basin converge, the shear-wave splitting results permit to do a distinction regarding the tectonics and stress field affecting the two regions. Acknowledgements We thank Dr. Zhan-wu Gao, Prof. Jin-li Huang and Prof. Rui-feng Liu for their help in geological setting and data acquisition. Helpful comments and suggestions from two anonymous referees that led to significant improvement of the early manuscript are gratefully acknowledged. The IES-CEA Project , the Seismic Professional Science Foundation (Grant ), and the NSFC Project supported this research. References Crampin S (1978) Seismic wave propagation through a cracked solid: polarization as a possible dilatancy diagnostic. Geophys J R Astron Soc 53: Crampin S, Peacock S (2005) A review of shear-wave splitting in the compliant crack-critical anisotropic Earth. Wave Motion 41:59 77 Crampin S, Gao Y (2006) A review of techniques for measuring seismic shear-wave splitting above small earthquakes. Phys Earth Planet Inter 159:1 14 Crampin S, Volti T, Chastin S, Gudmundsson A, Stefánsson R (2002) Indication of high pore-fluid pressures in a seismically-active fault zone. Geophys J Int 151:F1 F2 Gao Y, Crampin S (2006) A further stress-forecast earthquake (with hindsight), where migration of source of earthquakes causes anomalies in shear-wave polarizations. Tectonophysics 426: Gao Y, Hao P, Crampin S (2006) SWAS: a shear-wave analysis system for semi-automatic measurement of seismic shear-wave splitting above small earthquakes. Phys Earth Planet Inter 159:71 89 Gao Y, Zheng S, Sun Y (1995) Crack-induced anisotropy in the crust from shear wave splitting observed in Tangshan region, North China. Acta Seismol Sin 8(3): Gao Y, Zheng S, Zhou H (1999) Polarization patterns of fast shear wave in Tangshan region and their variations. Chinese J Geophys 42(2): (in Chinese) Gao Y, Liu X, Liang W, Hao P (2004) Systematic analysis method of shear-wave splitting: SAM software system. Earthq Res China 18(4): Gao Y, Wu J, Cai J, Shi Y, Lin S, Bao T, Li Z (2008) Shearwave splitting in the Southeast of Cathaysia Block, North China. J Seismol (this issue) Huang J, Zhao D (2005) Three dimensional P wave velocity tomography and deep structure related to strong earthquake in Capital area. Chin Sci Bull 50(4): (in Chinese) Jia S, Qi C, Wang F, Chen Q, Zhang X, Chen Y (2005) Three-dimensional crustal gridded structure of the Capital area. Chin J Geophys 48(6): (in Chinese) Jiang Z, Zhang X, Chen B, Xue F (2000) Characteristics of recent horizontal movement and strain stress field in

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