The shear-wave splitting in the crust and the upper mantle around the Bohai Sea, North China

Journal of Asian Earth Sciences xxx (2015) xxx–xxx

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The shear-wave splitting in the crust and the upper mantle around the Bohai Sea, North China Shi Yutao ⇑, Gao Yuan, Tai Lingxue, Fu Yuanyuan Institute of Earthquake Science, China Earthquake Administration, Beijing 100036, PR China

a r t i c l e

i n f o

Article history: Received 7 January 2015 Received in revised form 8 May 2015 Accepted 9 June 2015 Available online xxxx Keywords: Bohai Sea area Seismic anisotropy Crust Upper mantle Tan-Lu fault belt

a b s t r a c t In order to infer the distribution of local stress and the deep geodynamic process in North China, this study detects seismic anisotropy in the crust and upper mantle beneath the Bohai Sea area. A total of 535 local shear-wave and 721 XKS (including SKS, PKS and SKKS phases) splitting measurements were obtained from stations in permanent regional seismograph networks and a temporary seismic network called ZBnet-E. The dominant fast polarization orientation of local shear-waves in the crust is nearly East-West, suggesting an East-West direction of local maximum compressive stress in the area. Nearly North-South fast orientation was obtained at some stations in the Tan-Lu fault belt and the Zhang-Bo seismic belt. The average fast orientation from XKS splitting analysis is 87.4° measured clockwise from the North. The average time-delays of XKS splitting are range from 0.54 s to 1.92 s, corresponding to a 60–210 km thick layer of anisotropy. The measured results indicate that upper mantle anisotropy beneath Bohai Sea area, even the eastern part of North China, is mainly from asthenospheric mantle flow from the subduction of the Pacific plate. From the complicated anisotropic characteristics in this study, we infer that there might be multiple mechanisms in the crust and upper mantle around the Bohai Sea area that led to the observed anisotropy. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Seismic anisotropy describes the directional dependence of seismic velocity as an important characteristic feature in the Earth’s interior structure. It widely exists at different depth ranges in the crust, mantle and inner core. Crustal anisotropy is most likely the result of aligned micro-cracks referred to Extensive-dilatancy Anisotropy (EDA) or layer fabric (Crampin et al., 1980; Crampin, 1994; Crampin and Peacock, 2005; Crampin and Peacock, 2008). The dominant polarization of the fast shear-wave splitting equates with the orientation of aligned crack by in situ tectonic stress, and the time-delay equate with the degree of crack density for each source-receiver path (e.g., Crampin and Zatsepin, 1997; Boness and Zoback, 2004; Gao et al., 1998, 1999; Cochran et al., 2003; Shi et al., 2009). Alternately, if anisotropy is due to layered fabric, the anisotropic parameters relate to direction of fabric and degree of layering (e.g., Cochran et al., 2006; Margheriti et al., 2006; Peng and Ben-Zion, 2004; Shi et al., 2009, 2013). Upper mantle anisotropy is a consequence of the strain-induced lattice preferred orientation (LPO) of intrinsically anisotropic ⇑ Corresponding author. E-mail address: [email protected] (S. Yutao).

mantle minerals (principally olivine). In principle, the observations of shear-wave splitting in the upper mantle can be used to constrain the lithospheric and sublithospheric mantle deformation (Silver and Chan, 1991; Vinnik et al., 1992; Savage, 1999). The polarization orientation of the fast shear-wave can characterize the orientation of asthenospheric flow, and the time-delay can characterize depth extent of mantle strain fields (e.g., Silver, 1996; Savage, 1999; Conrad et al., 2007). The Bohai Sea area is situated in the eastern North China Craton, which experienced thermotectonic reactivation and destruction during the Late Mesozoic–Cenozoic. Since then, it has been tectonically active with high heat flow values (e.g., Ai and Zheng, 2003; Kusky and Li, 2003), thinned lithospheric mantle (e.g., Griffin et al., 1998; Chen et al., 2008; Chen et al., 2010), and frequent strong earthquakes such as the Ms7.3 Haicheng earthquake and the Ms7.8 Tangshan earthquake (Fig. 1). The Tan-Lu fault belt, which is a giant rift-slip fault belt across the Bohai Sea area, was formed by the continent–continent collision between the North China block and the Yangtze block probably began in the Middle Triassic (e.g., Xu et al., 1987; Li, 1994; Menzies et al., 2007). As an important asthenospheric upwelling channel (Chen, 2010; Zhu and Zheng, 2009), the Tan-Lu fault belt separates the Bohai Sea area into two lithospheric blocks with different velocity structure in the crust and mantle (Wang et al., 2000; Ai and Zheng, 2003;

http://dx.doi.org/10.1016/j.jseaes.2015.06.015 1367-9120/Ó 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Yutao, S., et al. The shear-wave splitting in the crust and the upper mantle around the Bohai Sea, North China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.015

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S. Yutao et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

Fig. 1. Distribution of seismic stations and active faults around the Bohai Sea area and its vicinity. The black and blue triangles are the seismic stations from regional seismograph networks (RSN) and ZBnet-E operated by Institute of Earthquake Science (IES) of China Earthquake Administration. The red dots indicate earthquakes with Ms = 5.0 in this region, and the two yellow dots indicate the Ms7.8 Tangshan Earthquake on July 28, 1976 and Ms7.3 Haicheng earthquake on February 04, 1975, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Station: BH01 2 Event: 2010/09/22-08:01:08

1 0 -1 -2 1 0.5

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Fig. 2. Shear-wave splitting analysis of seismic event (20100922080108) recorded by station BH01. The depth of this event is 5.0 km with ML3.6. The upper panel shows the original vertical, north-south and east-west waveforms. The lower-left is particle movement of shear-wave of original record and shear waveforms at north-south (NS) and east-west (EW) direction. S1 and S2 indicate the start position of fast shear-wave and slow shear-waves respectively. The lower-left is fast shear-wave (F) and slow shearwave (S) and the trail of particle movement of fast shear-wave and slow shear-wave, which have eliminated the effect of time delay. The ordinate is the count value of amplitude. The abscissa is the number of sampling points. The two vertical lines mark the segment of shear waveforms showed in the polarization diagrams.

Please cite this article in press as: Yutao, S., et al. The shear-wave splitting in the crust and the upper mantle around the Bohai Sea, North China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.015

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S. Yutao et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

Chen et al., 2006; Li et al., 2010). Azimuth variations of mantle anisotropy were accompanied from east to west across the Tan-Lu fault belt (Zheng et al., 2008). Moreover, the Tan-Lu fault belt is cut by the NW trending Zhangjiakou-Bohai seismic zone (Zhang-Bo Seismic Belt) in the Bohai Sea area, which is a seismically active belt in North China. The Zhang-Bo seismic belt is not a continuous fault zone, but a complex tectonic belt composed of about 20 consecutive secondary faults (Gao et al., 2001). There are numerous studies about the regional tectonic conditions, stress field, dynamic mechanism and the geological interactivities in eastern North China (e.g., Iidaka and Niu, 2002; Gao et al., 2011; Chen et al., 2010; Li and Niu, 2010; Zheng et al., 2008; Jiang et al., 2000; Zhao et al., 2007; Zhao, 2015). Due to the limited coverage of seismic stations in this area, especially the lack of the seismic stations in Bohai Sea, high resolution images of crustal structure beneath the area is absent (Xu and Zheng, 2005). In order to understand anisotropic and heterogeneous area and associated tectonic processes beneath Bohai Sea area, we analyzed shear-wave splitting from local and teleseismic events to estimate the first-order anisotropic patterns in the crust and upper mantle anisotropy around the Bohai Sea area based on seismic data recorded by regional seismograph networks and a temporary

seismic network. Based on the resulting shear-wave splitting parameters, we investigated the distribution of local stress around the Bohai Sea area, as well as the mantle flow pattern and geodynamics. These results could provide geophysical basis for future studies on the evolution of the Tan-Lu fault belt area. 2. Data and methods To study features of shear-wave splitting beneath Bohai Sea area, we collected and examined seismic data recorded by permanent regional seismograph networks including Hebei, Shandong, Liaoning, and Beijing (Zheng et al., 2010), and data from 14 broadband temporary seismic stations, called ZBnet-E (Fig. 1), which was deployed by the Institute of Earthquake Science, China Earthquake Administration, between December 2010 and April 2012. The seismometer type of ZBnet-E is CMG-3ESP with the sampling rate of 100 Hz. In this study, we collected local events smaller than ML4.0 between August 2007 and July 2012 around the Bohai Sea area. Shear-waves need to be recorded in the shear-wave window, which is defined by incidence angle is 45° in this study (Crampin and Peacock, 2005). The seismic-waves with high signal to noise

Station: BH02 2 Event: 2010/10/21-00:23:25

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Fig. 3. Same as Fig. 2 but for event 201001021002325 recorded by BH02. The depth of this ML2.2 event is 8.0 km.

Please cite this article in press as: Yutao, S., et al. The shear-wave splitting in the crust and the upper mantle around the Bohai Sea, North China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.015

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S. Yutao et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

ratio (SNR) were filtered in the frequency range of 0.5–10 Hz to calculate the anisotropic parameters of the upper crust using the shear-wave splitting system analysis method-SAM (Gao et al., 2008), which is based on the correlation function, concluding three contents of the calculation of cross-correlation function, elimination of time-delay and analysis of polarization and has function of self-examination. Examples of analysis using the SAM method are showing in Figs. 2 and 3. We use high-quality XKS (including SKS, PKS and SKKS) data from teleseismic events with magnitude greater than Ms5.5 from

85° to 140° to measure the anisotropic parameter of the upper mantle. The broadband seismic data used for the mantle anisotropy were recorded from August 2007 to January 2012 by the regional seismograph networks and the ZBnet-E array. The seismic data were band-pass filtered in the frequency range of 0.04–0.5 Hz to improve quality of the signal. The optimal pair of XKS splitting parameters is measured using the procedure of the Liu and Gao (2013), which is based on the minimization tangent energy approach (Silver and Chan, 1991). Fig. 4 is a schematic diagram of analysis of XKS splitting.

Station: BH04 Event: 2011/01/04 Lat: −52.08; Lon: 139.51; Dep: 16km

Station: DLI Event: 2007/09/15 Lat= −44.80; Lon= 167.55; Dep= 18km

SKS

Original R Original T

Original T

Corrected R

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1420

1430

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1440

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Lag (s) φ = -86.0 +/- 7.5deg. δ t= 1.55 +/−0.27s

Fig. 4. Shear-wave splitting analysis using an SKS phase recorded by stations BH04 and DLI. The top panel shows the original and corrected radial and transverse components, the middle panel shows the resulting fast and slow components and the corresponding particle motion patterns, and the bottom panel shows the contour diagram of the energy on the corrected transverse component.

Please cite this article in press as: Yutao, S., et al. The shear-wave splitting in the crust and the upper mantle around the Bohai Sea, North China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.015

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S. Yutao et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

3. The distribution of crustal anisotropic characteristics To ensure the reliability of the results of statistics, we only consider stations with the three or more reliable measurements. A total of 535 shear-wave splitting parameters results were obtained beneath 35 stations in this research area. The mean polarization orientations and the time-delays beneath each of the stations are given in Table 1. For each of the stations, a rose diagram was produced using the resulting fast orientations (Fig. 5). The consistent results of fast orientations beneath most stations indicate that our quality control process was effective. Station GAX shows the inconsistence polarizations from 3 events, more data will be helpful to identify detailed anisotropic characteristic beneath this station. In general, the fast polarization orientation is nearly East-West direction (Fig. 6), and is consistent with the local maximum horizontal principal compressive stress, which dominate the crustal anisotropy (e.g., Lai et al., 2006; Gao et al., 2010, 2011). Due to the oblique subduction of the Pacific plate and the northeast collision of Indian plate, the local stress field is nearly East-West in the eastern North China (e.g., Xu, 2001; Zhang et al., 2004; Gao et al., 2011). In this study, most of the rose diagrams of mean fast polarization orientation are consistent with the direction of the principle compressive stress in North China. Moreover, complex geologic structure can result in azimuthal and spatial variations of splitting parameters. Numerous studies of shear-wave splitting indicate that anisotropy varies with fault location and activity (e.g., Cochran et al., 2003, 2006; Peng and Ben-Zion, 2004; Shi et al., 2009, 2013; Liu et al., 2008b). In this study, the predominant fast polarization orientations beneath some stations are not

consistence with the orientations of local principle compressive stress, even not uniform. Some stations (CHD, DOH, JIH, GAX, SJT, B05, B08, and B09) located near the Tan-Lu faults show the N-S fast orientations, which are consistent with the strike of the fault belt (Fig. 5). Structure anisotropy, as another mechanism of crust anisotropy, could dominate the anisotropic parameters in the fault zone or sedimentary layers. Previous studies of shear-wave splitting in the crust suggested that the fast orientation is influenced by the trend of the boundary between the Uplift and the Basin, and the strike of faults and mountain topography (Gao et al., 2011; Tai et al., 2009; Sun et al., 2013). In this study, the dominant fast orientations beneath stations SHC, MDY, SSL, SZJ, BAD, CAD, FTZ, CHT, HAG and CHH are within 20° from the orientation of the local compressive stress. However, some stations (LQS, DSQ, LUT, YTA and RCH) exhibit mean a fast orientation that is consistent with the trend of the Zhang-Bo seismic belt, which is composed of numerous North-West and North-East trend faults produced by the near East-West directed of local stress field (Fig. 7). The polarizations with North-East orientations beneath two stations (MAF, DAX) are parallel to the surrounding faults, and with high angle to the Zhang-Bo seismic belt. Stations CHD, BH08, and BH09 located in the junction of the Tan-Lu fault belt and the Zhang-Bo seismic belt exhibit nearly the North-South fast orientation. The time-delay of shear waves splitting is a cumulative result along the ray paths through the upper crust. Thus the complex geological structure would seriously impact the magnitude of time-delay. In this study, the mean time-delay is 3.80 ± 1.85 ms/km, which is consistent with the measurements in North China (Wu et al., 2007, 2009; Gao et al., 2011).

Table 1 Crustal anisotropic parameters from local S splitting measurements beneath the stations around the Bohai Sea area. Station

Longitude (°)

Latitude (°)

Number of effective data

Polarization (°) Average direction ± standard error

Time-delay (ms/km) Average values ± standard error

CHD CHY GAX

120.7 120.5 122.4

37.9 41.6 40.4

JIX JUX KUC RCH SJT SNY XMN XYN YKO YTA CXT DAX DSQ LQS MDY SHC MAF SS L SZJ BAD CAD CHH CHT FTZ HAG JIH LUT BH01 BH02 BH05 BH08 BH09

117.5 118.9 118.5 122.4 123.4 123.6 122.9 123.3 122.6 121.4 114.0 116.3 116.4 116.1 116.5 115.5 117.0 116.2 116.3 117.3 117.5 117.2 117.4 117.8 117.8 116.9 117.7 123.4 123.1 122.0 121.2 120.8

40.1 35.5 40.6 37.2 41.6 41.8 42.1 40.2 40.8 37.5 36.4 39.8 40.1 40.1 40.4 40.4 40.0 40.3 40.0 39.7 39.6 39.1 39.2 39.6 39.2 38.9 39.4 40.6 39.9 39.4 38.8 38.3

7 3 2 1 4 5 3 7 4 23 3 11 220 4 4 4 16 16 5 15 28 5 14 8 7 5 3 8 10 9 19 6 3 8 14 31

17.9 ± 19.5 45.0 ± 5.0 20.0 ± 5.0 130.0 80.0 ± 9.4 58.0 ± 19.5 113.3 ± 5.8 142.1 ± 14.5 23.8 ± 19.3 70.7 ± 11.5 100.7 ± 1.1 63.2 ± 18.7 76.3 ± 27.0 126.3 ± 17.2 65.0 ± 17.3 35.0 ± 28.8 138.0 ± 16.8 114.6 ± 30.1 82.0 ± 22.8 88.0 ± 41.6 38.2 ± 19.8 68.0 ± 27.9 90.3 ± 12.0 81.2 ± 9.1 72.8 ± 12.8 95.0 ± 32.6 78.3 ± 2.8 74.3 ± 21.9 100.0 ± 17.1 14.2 ± 26.3 137.1 ± 24.6 88.3 ± 27.1 80.0 ± 5.0 20.0 ± 17.6 25.9 ± 19.0 170.6 ± 14.5

3.70 ± 1.10 3.13 ± 0.62 1.90 ± 0.50 4.10 2.08 ± 1.51 3.89 ± 2.21 3.84 ± 1.23 2.89 ± 0.91 3.08 ± 1.26 5.17 ± 3.64 2.03 ± 1.04 3.42 ± 1.69 3.23 ± 1.95 2.18 ± 0.85 3.36 ± 0.90 4.65 ± 3.28 2.95 ± 2.11 2.95 ± 2.11 1.18 ± 0.19 4.67 ± 2.78 5.22 ± 2.49 2.50 ± 1.90 7.16 ± 1.91 5.21 ± 1.64 4.22 ± 1.48 4.33 ± 2.90 3.08 ± 0.88 4.37 ± 3.36 4.86 ± 1.59 3.14 ± 2.17 4.15 ± 2.11 2.94 ± 1.18 4.14 ± 1.14 2.78 ± 0.64 5.27 ± 2.70 4.90 ± 3.13

Please cite this article in press as: Yutao, S., et al. The shear-wave splitting in the crust and the upper mantle around the Bohai Sea, North China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.015

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S. Yutao et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

Fig. 5. Map of local s-wave splitting results in the area around the Bohai Sea area. For each station, individual splitting parameters are shown by blue rose diagrams. Orientation and length of red bars indicate the mean polarization direction and time-delay of shear-wave splitting, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Recommended the depths of small earthquakes are distributed between 10 km and 30 km in North China (Wu et al., 2009; Gao et al., 2011), the estimated time-delay is between 0.04 s and 0.11 s. In this study, larger time-delays are mainly distributed near the Tan-Lu fault zone and Zhang-Bo seismic belt, such as those observed at stations SNY, DSQ, SSL, CAD, CHH. Previous studies suggested that the distorted geological structures, such as large faults and irregular topography, could seriously disturb the measurement of crustal anisotropy (Gao et al., 2011). Beneath the irregular topography of the Zhang-Bo seismic belt and the Tan-Lu fault zone, the spatial distribution of mean time-delays as well as exposited a complicated relationship between time-delay of each station and fault where it located (Figs. 5 and 7).

4. Spatial distribution of mantle anisotropic characteristics In this study, the P-to-S converted phases XKS, including PKS, SKKS, and SKS, from the core-mantle boundary and collected for obtaining the mantle anisotropy around the Bohai Sea area. A total of 721 measurements of XKS splitting beneath 84 stations were obtained from 2524 seismograms. The mean polarization orientations and time-delays beneath each of the stations are given in Table 2. The mean results of shear-wave splitting for the entire data were measured by the unbiased estimator of mean and sample standard deviation. The mean polarization of XKS splitting is 87.4° ± 9.5°, which is subparallel to the local stress field. The

Please cite this article in press as: Yutao, S., et al. The shear-wave splitting in the crust and the upper mantle around the Bohai Sea, North China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.015

S. Yutao et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

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Fig. 6. Map of mean polarization of local s-wave splitting beneath each station in the Bohai Sea area. Orientation and length of red bars indicate the mean orientation of polarization and time-delay of shear-wave splitting. The two blue bars indicate the two dominate orientations beneath station GAX. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

N

E

Fig. 7. Superimposed equal-area rose diagrams of fast polarization orientation in study area for 535 records from all 35 stations for data from between August 2007 and July 2012.

time-delays range from 0.54 s to 1.92 s beneath each station with a mean value of 1.16 ± 0.3 s, which is proportional to 60–210 km thickness of the anisotropic layer assumed average of 4% anisotropy (Silver, 1996). The mean fast orientations and the time-delays from this study are mostly consistent with those from previous studies using the seismic events recorded by the NSNC (National Seismograph Network of China), IRIS (Incorporated Research Institutions for Seismology) and temporary seismic array (Chang et al., 2008, 2009; Iidaka and Niu, 2002; Liu et al., 2008a; Luo et al., 2004; Zhao and Zheng, 2005; Zhao et al., 2011). The superior azimuthal coverage of seismic events ensures detailed and reliable measurements of mantle anisotropy in this study (Fig. 8). The distribution of the teleseismic events used for mantle anisotropy study is

shown in Fig. 9. Moreover, according the depth-dependent P-wave azimuthal anisotropy, the dominant fast velocity directions in the range of 200 km depth is also consistent with our SKS splitting results (Tian and Zhao, 2013). The polarizations of XKS beneath most stations are mostly in the East-West, which is consistent with the orientation of the absolute plate motion (APM) direction. Therefore, this result reveals that upper mantle anisotropy in the Bohai Sea area, even in the eastern North China, is mainly driven by asthenospheric mantle flow from the subduction of the Pacific plate beneath the Eurasian plate. The fast orientations XKS splitting in the orogenic belts are parallel to the tectonic features (Nicolas, 1993). As the Central Orogenic belt of the North China Craton, the Shanxi Graben display complicated lithosphere (Tian et al., 2011; Chen et al., 2009) and strong anisotropy in the upper mantle (Zhao et al., 2011). The fast orientations at four stations (KAB, ZHB, ZJK, and YAY) located at the junction of the Shanxi Graben and Zhang-Bo seismic belt present the complex orientation of polarization with North-East and North-West directions (Figs. 9 and 10). In contrast, two stations (XFN and TIL) located in the Songliao Basin show North-South oriented polarization of XKS splitting (Figs. 9 and 10). The measured polarizations at stations XFN and TIL are close to those observed by a previous study with 137° and 177° (Li and Niu, 2010), suggesting an asymmetric pattern across the Songliao basin based on results observed at stations in the Northeast China. In this study, we are able to infer complex mantle anisotropy in the Songliao Basin. However, more data are necessary for detailed analysis of anisotropy there. The NNE trending Tan-Lu fault belt is the largest continental fault belt in North China (Xu et al., 1987). According to the SKS

Please cite this article in press as: Yutao, S., et al. The shear-wave splitting in the crust and the upper mantle around the Bohai Sea, North China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.015

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Table 2 Upper mantle anisotropic parameters from XKS splitting measurements beneath the stations around the Bohai Sea area. Station

Longitude (°)

Latitude (°)

Number

Polarization (°) Direction ± error

Time-delay (s) Time ± error

ANQ BHC CSH DSD HAY JIN JIX JUN JUX LAS LAY LIS LOK LQU LSH LZH NLA RCH RSH RZH TCH TIA WED WUL XIT YSH ZCH BBS DHC JIX LBP LLM MIY NKY SSL XBZ ZHT BZH CHY DLI FXI HUR HXQ JCA JZH KDN LYA NAP SHS TIL XFN CLI CXT FEN FUC GAN HNS HST JNX KAB KUC LIC LUQ QIL SXT WAT WEC XIL XLE XTT YAY YUS ZAH

119.2 120.9 118.1 116.8 121.3 117.0 116.4 118.9 118.9 119.3 120.7 118.7 120.5 118.5 116.1 121.0 118.0 122.4 121.6 119.5 118.5 117.1 121.9 119.2 117.8 118.6 117.0 116.2 116.1 117.5 116.2 116.6 116.8 116.0 116.2 116.0 115.7 121.8 120.5 121.6 121.6 125.4 122.8 119.9 121.1 124.8 119.4 120.8 121.0 123.9 124.7 119.1 114.1 116.6 116.2 114.0 114.7 114.0 114.2 114.6 118.5 114.4 114.3 118.9 113.6 114.05 117.75 117.47 114.66 114.3 114.17 114.4 114.4

36.4 38.4 34.9 35.1 36.8 36.6 35.4 35.2 35.5 35.1 37.0 35.0 37.6 36.4 35.8 37.2 35.5 37.2 36.9 35.4 34.7 36.2 37.2 35.8 36.0 35.8 35.4 40.0 39.9 40.1 40.6 40.6 40.5 39.7 40.3 40.3 40.0 41.6 41.6 38.9 42.1 41.3 42.6 40.8 41.1 40.7 41.2 41.1 40.7 42.3 42.7 39.8 36.4 41.2 37.9 38.4 37.4 36.2 38.0 41.9 40.6 37.5 38.0 40.4 36.1 36.8 41.9 40.4 38.4 37.1 40.2 37.8 37.7

12 5 15 9 4 7 10 14 6 5 6 8 12 9 10 7 8 5 3 6 7 10 5 6 7 4 14 10 8 12 20 19 12 3 23 8 16 4 3 8 3 3 3 11 6 3 16 5 7 3 3 3 22 17 5 11 12 32 10 11 9 14 9 6 15 11 5 14 3 8 6 13 13

96.4 ± 9.37 90.6 ± 6.3 82.1 ± 9.6 97. 7 ± 10.1 86.8 ± 5.9 105.7 ± 10.9 121.6 ± 6.4 83. 8 ± 7.7 80.2 ± 5.3 92.2 ± 7.7 96.1 ± 8.9 78.4 ± 3.9 104. 0 ± 6.3 94.7 ± 16.2 112.1 ± 7.1 119.3 ± 8.4 85.3 ± 8.1 97.2 ± 8.1 84.3 ± 4.0 100.5 ± 10.2 74.3 ± 9.9 122.9 ± 11.5 88.8 ± 5.1 85.8 ± 9.3 86.7 ± 8.5 82.8 ± 11.9 79.4 ± 8.1 77.1 ± 9.5 68.5 ± 7.4 61.0 ± 9.7 92.4 ± 8.9 63.3 ± 7.4 77.8 ± 6.6 67.0 ± 15.2 88.8 ± 10.0 66.0 ± 12.6 84.4 ± 11.9 72.8 ± 9.6 94.0 ± 12.2 103.0 ± 19.5 114.0 ± 12.7 114.7 ± 12.7 124.0 ± 9.8 72.2 ± 12.7 80.8 ± 13.0 72.8 ± 9.6 114.7 ± 12.7 99.0 ± 9.1 63.1 ± 6.5 21.0 ± 11.5 36.3 ± 7.8 71.0 ± 11.3 73. 6 ± 8.9 93.5 ± 10.4 86.3 ± 10.7 105.0 ± 7.9 98.9 ± 10.0 87.1 ± 6.7 95.5 ± 9.5 109.7 ± 5.9 108.2 ± 5.2 88.6 ± 11.0 97.6 ± 11.4 100.2 ± 9.5 70.4 ± 7.9 67.2 ± 11.9 141.2 ± 9.5 65.8 ± 8.2 89.7 ± 3.3 99.8 ± 8.9 40.5 ± 10.0 90.8 ± 9.2 75.6 ± 8.3

1.10 ± 0.27 1.92 ± 0.37 1.26 ± 0.32 1.33 ± 0.29 1.35 ± 0.28 1.09 ± 0.33 1.35 ± 0.41 1.36 ± 0.32 1.58 ± 0.26 1.18 ± 0.23 1.10 ± 0.24 1.69 ± 0.24 1.35 ± 0.26 0.91 ± 0.41 1.50 ± 0.35 1.71 ± 0.25 1.13 ± 0.23 1.07 ± 0.25 1.37 ± 0.28 0.97 ± 0.20 1.38 ± 0.27 1.05 ± 0.32 1.70 ± 0.27 1.35 ± 0.29 1.34 ± 0.30 1.41 ± 0.46 1.36 ± 0.28 0.94 ± 0.22 1.20 ± 0.30 1.16 ± 0.42 1.19 ± 0.29 1.27 ± 0.30 1.09 ± 0.20 1.08 ± 0.56 0.99 ± 0.28 0.94 ± 0.58 0.54 ± 0.18 0.59 ± 0.16 1.03 ± 0.30 1.05 ± 0.57 0.90 ± 0.32 0.90 ± 0.47 1.40 ± 0.43 0.84 ± 0.26 1.19 ± 0.44 0.59 ± 0.16 0.90 ± 0.47 1.06 ± 0.25 0.97 ± 0.19 1.15 ± 0.58 1.07 ± 0.42 0.88 ± 0.23 1.34 ± 0.32 0.98 ± 0.27 0.75 ± 0.18 1.42 ± 0.42 1.26 ± 0.37 1.48 ± 0.23 0.93 ± 0.25 1.23 ± 0.28 1.27 ± 0.19 1.06 ± 0.30 1.23 ± 0.36 1.07 ± 0.28 1.49 ± 0.39 1.25 ± 0.45 1.22 ± 0.41 1.04 ± 0.46 1.61 ± 0.16 1.19 ± 0.37 1.10 ± 0.62 1.06 ± 0.24 1.16 ± 0.27

Please cite this article in press as: Yutao, S., et al. The shear-wave splitting in the crust and the upper mantle around the Bohai Sea, North China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.015

9

S. Yutao et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx Table 2 (continued) Station

Longitude (°)

Latitude (°)

Number

ZAQ ZDN ZHB ZJK BH02 BH03 BH04 BH06 BH07 BH09 BH13

115.9 114.6 114.5 114.9 123.1 120.4 120.0 123.1 121.2 120.7 119.7

37.4 38.2 41.4 40.8 39.9 40.9 40.5 39.1 38.8 38.2 37.6

3 3 7 14 3 4 4 2 3 3 3

Polarization (°) Direction ± error 79.3 ± 6.5 99.7 ± 9.7 60.7 ± 13.1 62.3 ± 11.8 98.6 ± 15.3 64.5 ± 6.9 81.4 ± 11.9 91.8 ± 12.5 88.3 ± 14.1 69.8 ± 7.8 88.0 ± 9.6

Time-delay (s) Time ± error 1.43 ± 0.36 1.40 ± 0.37 0.71 ± 0.35 0.83 ± 0.27 1.07 ± 0.71 0.95 ± 0.28 1.02 ± 0.21 0.82 ± 0.24 1.25 ± 0.47 1.14 ± 0.51 0.76 ± 0.37

⁄ BH01, BH02, BH04, BH05, BH06, BH07, BH08, BH09, BH13 as the ZBnet-E.

lithosphere around the Bohai Sea area, perhaps in the eastern North China.

5. Discussions and conclusions

135

90

45

Fig. 8. An azimuthal equidistant projection map of the distribution of earthquakes used for XKS splitting analysis (red dots). Blue dashed circles and corresponding labels show the distance (in degree) to the center of the study area, which is the green square. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

splitting measurements beneath 30 temporary seismic stations, there is small difference of anisotropic parameters on east and west sides of the Tan-Lu fault (Zheng et al., 2008). With more teleseismic events and good coverage of stations in this study, excellent consistent splitting parameters were obtained on the east and west sides of the Tan-Lu fault belt. The Tan-Lu fault belt, as an upwelling channel due to the Archean lithosphere delamination (Zhu and Zheng, 2009; Xu and Zhao, 2009; Wang et al., 2014), separates North China into two lithospheric blocks. In the west of Tan-Lu fault belt, the orientations of polarization beneath four stations (JIN, TIA, LSH and JIX) at the Luxi Uplift are North-West, within 20° difference with other results (Figs. 9 and 10). Moreover, the subduction of the Pacific slab, which is the causes of anisotropy observed in North China, has reached to the Shanxi graben of the North China Craton (Huang and Zhao, 2006). Therefore, according to the results of our study, the consistent East-West polarizations of XKS splitting on both sides of the Tan-Lu fault indicate the coherent deformation of asthenosphere corresponding to the corner flow caused by the subduction of the Pacific plate, which is the dominant factor of anisotropy in the

Shear wave splitting parameters were observed using shear-wave splitting observations from local seismic events and teleseismic events recorded by regional seismograph networks and a temporary seismic network called ZBnet-E. A total of 535 measurements of crustal anisotropy recorded at 35 stations and 721 measurements of upper mantle anisotropy recorded at 84 stations were obtained in this study. For the crustal anisotropy, the dominant orientations of fast shear-wave beneath most stations are within 20° from the East-West. It suggests that the direction of local maximum compressive stress is about East-West in the Bohai Sea area. Moreover, several stations located on the active faults or seismic belt have nearly North-South orientations, which are probably caused by structural anisotropy. For the mantle anisotropy, we obtained 721 measurements of XKS splitting beneath 84 stations. The mean fast orientation of XKS splitting is 87.4° ± 9.5°, indicating that the upper mantle anisotropy is mainly driven by asthenospheric flow from the subduction of the Pacific plate. The mean time-delay is 1.16 ± 0.3 s for all records, and range from 0.54 s to 1.92 s, implying about 60–210 km thickness of the anisotropic layer. Within Bohai Sea, there is a small island chain from Dalian of Liaodong peninsula to Yantai of Shandong peninsula. It is clear that the orientations of the polarizations of local shear-wave in the crust are quite different to fast polarizations of XKS splitting in the upper mantle in this area (Figs. 5 and 10). This small island chain is the junction of the Tan-Lu fault belt and Zhang-Bo seismic belt. The orientations of polarization in the crust reveal the intense crustal deformation by the collision between the North China block and the Yangtze block, which generated structural anisotropy in this area. The near East-West fast orientations of polarizations of XKS in the upper mantle indicate coherent deformation of asthenosphere in the eastern and western sides of the Tan-Lu fault belt by the subduction of the Pacific plate. Because of the steep incidence of the ray paths for shear-wave splitting, including the local S and XKS phases, the measurements of shear-wave splitting, have a good lateral resolution but a low vertical resolution. However, the upwelling cell would generate anisotropy with a vertical symmetry from the asthenospheric flow. It is difficult to observe the detailed anisotropy along the entire path from a single measurement. Generally, the time-delay of shear-wave splitting caused by the crustal anisotropy is about 0.l–0.3 s, and the measured time-delay in the upper mantle anisotropy is 1.0–2.0 s by ignoring influence of crustal anisotropy

Please cite this article in press as: Yutao, S., et al. The shear-wave splitting in the crust and the upper mantle around the Bohai Sea, North China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.015

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S. Yutao et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

44˚

XFN TIL

42˚

FXI KAB BZH

CHY ZHB FEN

LYA

LBP

ZJK

KUC

XIL

MIY

40˚

BH02

JIX

BBS DHC NKY

ZHT

KDN

SHS

BH04

QIL

SSL

HUR

JZH

JCA BH03

LLM

XBZ YAY

NAP

CLI

BH06 DLI

BH07

BHC

XLE

GAN

38˚

BH09

ZDN

JNX LUQ

FUC

YUS ZAH LIC HNS XTT WAT

LOK LZH

ZAQ

WED RCH

LAY

JIN

CXT

LQU

ANQ

TIA

HST

XIT

LSH

JUX

NLA

ZCH

36˚

RZH

JUN LAS LIS

CSH

DSD

BH13

WUL

YSH

JIX

RSH

HAY

SXT

TCH

Station 1.0 s

PFS(XKS)

34˚

Faults

112˚

114˚

116˚

118˚

122˚

120˚

124˚

126˚

Fig. 9. Individual (blue bars) and mean (red bars) XKS splitting measurements in the study area. The orientation of the bars represents the fast polarization orientation, and the length is proportional to the splitting time. Black triangles are stations used in the study. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

44˚ So

ng

-L

iao

Ba TIL

XFN

sin

42˚

FXI KAB BZH

CHY ZHB

lt t ul

CLI

Bohai Sea

ZAH LIC HNS XTT WAT

BH06

BHC BH09

ZDN YUS

40˚

DLI

BH07

XLE

JNX LUQ

KDN BH02

Fa

JIX

BBS DHC NKY

Be

SHS

BH04

QIL

North China Basin GAN

HUR

JZH

BH03

n-

xi an

ZHT

Sh

KUC

XIL

MIY SSL

YAY

NAP JCA

Lu

ra G

XBZ

LYA LLM

Ta

be

n

FEN LBP

ZJK

Zh

FUC LOK LZH

ZAQ

an

g-

WED

38˚ Bo

RCH LAY

JIN

SXT

HAY

Luxi Uplift LQU

CXT HST

RSH

Se

ism

ic

ANQ BH13

Be

lt

TIA XIT

LSH

YSH

JIX

ZCH

NLA

DSD CSH

36˚

WUL

JUX RZH JUN LAS LIS TCH

Station 1.0 s

112˚

114˚

116˚

118˚

120˚

122˚

34˚

PFS(XKS)

124˚

126˚

Fig. 10. Mean XKS splitting measurements in the study area. The orientation of the red bars represents the fast polarization orientation, and the length is proportional to the splitting time. The green, light blue and dark blue bars indicate the polarization of shear-wave splitting beneath stations at Shanxi Garben, Luxi Uplift, and Song-Liao Basin respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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S. Yutao et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

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