Earthq Sci (2018)31: 234–241 234 doi: 10.29382/eqs-2018-0234-3

Phase velocity maps of Rayleigh waves in the Ordos block and adjacent regions*

Shaoxing Hui1,* Wenhua Yan1 Yifei Xu1 Liping Fan2 Hongwu Feng1

1 Earthquake Agency, Xi'an 710068, 2 Institute of Geophysics, China Earthquake Administration, 100081, China

Abstract We analyze continuous waveform data 1 Introduction from 257 broadband stations of the portable seismic array deployed under the "China Seismic Array-northern part of NS In recent years, one of the important methods for seismic belt" project as well as data from a permanent seismic network from January 2014 to December 2015. The phase obtaining the S-wave velocity structure of the crust and velocity dispersion curve of 7,185 Rayleigh waves is obtained upper mantle is ambient noise tomography. Its main with a method based on the image analysis of phase velocity advantages are free of uncertainties of earthquake source extraction, and the inversion is obtained. The period of and a better ray coverage, more dispersion curves with Rayleigh wave phase velocity distribution has a range of 5– moderate and short periods are obtained, and finally the 40 s, and minimum resolution close to 20 km. The results distribution of the observation stations is an important show that the phase velocity structure image well reflects the factor in its resolution. With a broad and dense distribution geological structural characteristics of the crust and of observation stations, the deep tectonic environment can uppermost mantle, and that the phase velocity distribution has be detected, and the crustal structure with better resolution obvious lateral heterogeneity. The phase velocity of the 5– can be achieved in the region with weak seismicity. This 15 s period is closely linked to the surface layer and sedim- method has been widely used in many areas (Fang et al., entary layer, the low-velocity anomalies correspond to loose 2013; Fan et al., 2015; Yan et al., 2016; Guo et al., 2017). sedimentary cover, and the high-velocity anomalies corres- The Ordos block and adjacent regions are located in pond to orogenic belts and uplifts and the boundary between the transitional area between the northeastern margin of high and low velocity anomalies is consistent with the block boundary. The phase velocity of the 5–15 s period is strongly the Tibetan Plateau and the North China block. The main affected by the crust layer thickness, the northeastern Tibetan tectonic units in the area include the Alxa block, the has low-velocity anomalies in the middle to lower block, the Fenwei graben, and the Qinling mountain, and crust, the west side of the Ordos block is consistent with the the area has been affected by the collision of Indian and northeastern Tibetan plateau, which may imply the material Eurasian plates. However, the relatively stable Alxa and exchange and fusion in this area. The velocity variation is Ordos blocks have considerably blocked the push of the inversely related to the Moho depth in the 40 s period of S- Qinghai block (Chao, 2002; Chang et al., 2011). The wave, and the lateral velocity heterogeneity represents the seismogenic structure in this area is intricate and complex, lateral variation of the Moho depth. The Ordos block and the and there is a strong neotectonic movement. Historically, northern margin of Sichuan basin are located in the uppermost strong earthquakes have been very active, and there have mantle at this depth, and the depth in the transition zone is been many devastating medium and strong magnitude still located in the lower crust. earthquakes (Xu et al., 2000). In recent years, different researchers have carried out a lot of study in the region and Keywords: Ordos block; ambient noise tomography; Rayleigh obtained some important results about the deep structure of wave; phase velocity the region (Pan et al., 2017; Ding et al., 2017; Zheng et al., 2018). As a result, the lower crust flow, crustal growth,

and low-speed anomaly in the crust were discussed in * Received 28 August 2018; accepted in revised form 22 November detail. However, resolution has been limited because there 2018; published 11 April 2019. is no combination of permanent and portable stations data * Corresponding author. e-mail: [email protected] © The Seismological Society of China and Institute of Geophysics, in previous study, the fineness of existing research results China Earthquake Administration 2018 needs to be further improved.

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Since September 2013, the Shaanxi Earthquake Admini- each processing step. stration and other departments have deployed 160 broad- 2.1 Data preprocessing and cross-correlation band portable seismographs in central and western Shaa- function calculations nxi, and we collected a total of 257 seismic stations data, including 160 portable broadband stations of the Himal- We first converted the continuous waveform data from ayan array (Phase II) (ChinArray-Himalaya, 2011) and 97 Seed and Pascal files to SAC data files, merged the seismic permanent stations of China Seismic Networks, with an data for each station, split it into hourly segments, and average spacing of about 30 km (see Figure 1). We obta- resampled from 100 Hz to 1 Hz. Next, moved the instru- ined 5–40 s phase velocity maps of Rayleigh wave based mental response, removed the mean and trend of the on ambient noise tomography in Ordos and its adjacent seismic data, then band-pass filtered the seismic data in the regions, our research utilized the observation data of the 5–50 s period. To eliminate the interference from the highest density and the largest number of seismic stations single-frequency signal and make the ambient noise of the in this area. The new research results provide an important signal band more continuous (Bensen et al., 2007), we reference for understanding important issues such as the performed temporal normalization and spectral whitening crust and upper mantle structure, tectonic plate boundaries, for the data to remove the unsteady noise signal around and crust-mantle coupling and its deformation mechanism. each station and signal distortions caused by instrument malfunction. Finally, we calculated cross-correlations and

106°E 108°110° 112° 114° stacked hourly cross-correlation functions to obtain the 38°N time domain cross-correlation function for each station pair (Figure 2). It can be seen from the figure that the symmetrical component has a higher signal-to-noise ratio than the asymmetric component at each period. The more 36° Ordos block cross-correlation functions are stacked, the stronger the surface wave signal, and the lag time of the surface wave North China block signal is positively correlated with the interstation Fenwei graben 34° distance. When the interstation distance is relatively small, Qinling fold belt the body wave precursor signal in the surface wave signal

is relatively obvious (Wang et al., 2011). Northeast margin of Tibetan plateau Tibetan of margin Northeast

700 32° South China block 600

Portable station Paleoearthquake (M>7.5) Permanent station Plate boundary 500

Figure 1 A sketch map of the boundaries and seismic 400 stations in the Ordos block and its adjacent regions 300 2 Seismic data and methods Distance (km) 200 We collected continuous broadband seismic data of 100 the vertical components from 257 seismic stations from January 2014 to December 2015. The continuous seis- 0 mic data was recorded in the portable broadband stations −500 0 500 using Güralp CMG-3T seismometers (0.02–120 s) and Time (s) CMG-3ESP seismometers (0.02–60 s) equipped with Figure 2 Cross-correlation waveforms between station 61012 Reftek-130 seismic recorders. The average spacing bet- and others ween stations was 30 km. All the stations used GPS timing 2.2 Phase velocity measurement to ensure consistency. The station distribution is shown in Figure 1. The data processing flow consists primarily of The phase velocity dispersion calculation formula data preprocessing, calculations of the cross-correlation (Dahlen and Tromp, 1998) is functions, Rayleigh wave phase velocity measurements, ∆ 2D phase velocity tomography, and analysis of the CAB (T) = (1) t − T inversion results. The following is a brief description of 8

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8 CAB(T) is the average phase velocity, Δ is the inter- station distance, T is the period, and t is the travel time of 7 ) 3 the peak after filtering the empirical Green’s function near 6 period T. Interstation phase velocity dispersion are measured using rapid analysis software based on an image 5 transformation technique proposed by Yao et al. (2004). 4 The efficiency of this software is far superior to that of 3 manual extraction. Moreover, the software quickly impro- 2 ves the measurement accuracy of the phase velocity Number of measurements (×10 dispersion while quickly tracking the entire phase velocity 1 dispersion curve and is reliable and fast for batch proce- 0 ssing ambient noise. In order to obtain a reliable and 510 15 20 25 30 35 40 Period (s) accurate extraction phase velocity dispersion curve, the following three conditions must be satisfied simultaneou- Figure 3 Number of rays for inversion at different periods sly: (1) the ambient noise must be continuous waveforms 2.3 Inversion method of more than 1 year for the cross-correlation calculation, (2) cross-correlation functions with low signal-to-noise In this study, 2D Rayleigh wave phase velocity maps ratios (SNR) must be removed, so we only use the cross- for different periods are inverted using the method correlation functions with signal-to-noise ratios greater proposed by Ditmar and Yanovskaya (1987), Yanovskaya than 10 (Bensen et al., 2007; Fang et al., 2010) for the and Ditmar (1990)). This is the traditional Backus-Gilbert extraction of the phase velocity dispersion, and (3) far- one-dimensional geophysical tomography method and field conditions must be satisfied as theory applied in two-dimensional tomography. This ∆ inversion method is a mathematical solution of the phase C × T = λ ≤ (2) AB 3 velocity distribution that satisfies multiple constraints under Tikhonov’s regular law, which is given by CAB is the average phase velocity, λ is the wavelength, w Δ is the interstation distance, and T is the period. For all δ − T −1 δ − + α |∇ |2 = ( t Gm) Ri ( t Gm) m dr min (3) station pairs, equation (2) determines the largest period to r ( ) r S δ = −1 − −1 , = , , = be considered. Assuming an average phase velocity of ( t L )V V0 ds Gm S G (r)m(r)dr m(x y) 3 km/s, the maximum period can be calculated approxi- −1 − −1 V V0 V0, Ri is a covariance matrix and α is a regul- mately as Δ/9 s. In this study, the maximum station pair arization parameter. In this study, we tested different grid distance is approximately 850 km, and therefore we scales for inversion (1°×1°, 0.5°×0.5°, 0.25°×0.25°) and obtained a phase velocity dispersion curve with the period found that the resolution for 0.25°×0.25° was clearly range of 5–40 s. better. After many tests of model smoothness and data In this study, we used common methods to calculate errors, we found that the model error was small, and the the signal-to-noise ratio (SNR). The SNR is defined as the model was smoother when the regularization parameter ratio of the peak within the signal window and the root- was 0.2. During the calculation, the next round of inver- mean-square (RMS) noise in the noise window (the sion selected the dispersion curve of the residual less than waveform data in the range of 150 s after the signal three times the RMS of travel time residual. window). To avoid automatic extraction of the phase velocity dispersion curve influencing the image result, all 3 Tomography results the dispersion curves were manually extracted, and a total of 13,482 dispersion curves were obtained. To further improve the resolution of the inversion results, we The phase velocity distribution with a period from 5 to controlled all the selected dispersion curves based on the 40 s are obtained. Figures 4a–4f respectively shows maps principle that the station pair distance is greater than three of the phase velocity distribution for 5 s, 15 s, 20 s, 25 s, times the wavelength (Bensen et al., 2007) and not less 30 s, and 40 s periods, Figures 5a–4f shows maps of the than 180 km. The cluster analysis method was used to resolution corresponding to these periods. The figures ensure the ray path was evenly distributed on the azimuth show that the horizontal resolution in most areas is better (Fang et al., 2010; Wang et al., 2014). Finally, 7,185 than 60 km. In the central and western part of the study dispersions were obtained for the final 2D dispersion area, the horizontal resolution is even better than 20 km maps. Figure 3 shows the number of rays used for due to the dense distribution of seismic stations. inversion at different periods. The phase velocity of the Rayleigh wave is quite sens-

Earthq Sci (2018)31: 234–241 237

106°E 108° 110° 112° 106°E 108° 110° 112°

T = 5 s (a) T = 15 s (b)

Yan′an Yan′an 36°N Guyuan Linfen 36°N Guyuan Linfen Qingyang Qingyang Pingliang Hancheng Pingliang Hancheng

Yima Yima Tianshui Tianshui Baoji Weinan Xi’an Xi’an 34° 34° Shangluo

Hanzhong ShiyanDengzhou Ankang ShiyanDengzhou Guangyuan Guangyuan 32° 32°

c (km/s) c (km/s) 2.7 2.8 2.9 3.0 3.1 3.2 3.2 3.3 3.4

106°E 108° 110° 112° 106°E 108° 110° 112°

T = 20 s (c) T = 25 s (d)

Yan′an Yan′an 36°N 36°N Guyuan Linfen Guyuan Linfen Qingyang Qingyang Pingliang Hancheng Pingliang Hancheng

Yima Yima Tianshui Tianshui Baoji Weinan Baoji Weinan Xi’an 34° Xi’an 34° Shangluo Shangluo

Hanzhong Hanzhong Ankang ShiyanDengzhou Ankang ShiyanDengzhou Guangyuan Guangyuan 32° 32°

c (km/s) c (km/s) 3.3 3.4 3.5 3.6 3.5 3.6 3.7

106°E 108° 110° 112° 106°E 108° 110° 112°

T = 30 s (e) T = 40 s (f)

Yan′an 36°N 36°N Guyuan Linfen Guyuan Qingyang Qingyang Pingliang Hancheng Pingliang Hancheng

Yima Yima Tianshui Tianshui Baoji Weinan Baoji Weinan Xi’an 34° Xi’an 34° Shangluo Shangluo

Hanzhong Hanzhong Ankang ShiyanDengzhou Ankang ShiyanDengzhou Guangyuan Guangyuan 32° 32°

c (km/s) c (km/s) 3.6 3.7 3.8 3.8 3.9 4.0 Figure 4 Rayleigh wave phase velocity maps for different periods in the Ordos block and its adjacent regions itive to the velocity structure of S-waves, and different peri- waves at different depths: the greater the penetration depth, ods imply the velocity distribution characteristics of S- the longer the period required. The sensitivity kernel of the

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106°E 108°110° 112° 106°E 108°110° 112°

T = 5 s (a) T = 15 s (b)

80 80 60 60 40 40 40

60 36°N 80 36°N 80 60

34° 34°

32° 32° 80 40 80 40 60 60

Resolution/(km) Resolution/(km) 30 60 90 120 06 09 0 120 150 03

106°E 108°110° 112° 106°E 108°110° 112°

T = 20 s (c) T = 25 s (d)

80 60 80 60

60 60 40 80 36°N 40 80 36°N

40

34° 34°

40 32° 32° 40 80 60 60 80

Resolution/(km) Resolution/(km) 03 06 09 0 120 150 03 06 09 0 120 150

106°E 108°110° 112° 106°E 108°110° 112°

T = 30 s (e) T = 40 s (f)

80 60 80 80 60 60 36°N 40 36°N

40 40

40 34° 34°

60

60 32° 80 32° 80

Resolution/(km) Resolution/(km) 30 60 90 120 30 60 90 120 150 Figure 5 Resolution maps of Rayleigh wave phase velocity tomography fundamental Rayleigh wave phase velocity to the S-wave model follows the research results of Xin et al. (2017) and velocity was calculated as shown in Figure 6. The velocity Hui et al. (2018). In this study, we set the crust thickness

Earthq Sci (2018)31: 234–241 239 as 39 km, the crust would be divided into two layers, and correspond to orogenic belts and uplifts, the boundary the upper mantle model used the AK135 continental between high-velocity and low-velocity anomalies is model. consistent with the boundary of the block. These features are consistent with those published surface wave imaging

0 results (Li et al., 2013; Lü et al., 2016). The phase velocity of a Rayleigh wave with a period of 10 15 s (Figure 4b) is sensitive to the structure at a depth of

20 12 to 22 km. The image of the phase velocity distribution of a Rayleigh wave with a period of 15 s is roughly similar 30 to that for 5 s, but the value of the velocity is increased as a whole. Most of the fault basins (the Weihe basin, Tianshui 40 basin, etc.) still exhibit low-velocity anomalies, it indicates that there maybe thicker sedimentary layers in the Weihe 50 basin. In the 15 s phase velocity distribution map, the south Depth (km) 60 of Ankang area also show low-velocity anomalies, which maybe related to the crust uplift that raising the original 70 lower crust to the current upper crust (He et al., 2015). 5 s The phase velocity of the Rayleigh wave with a period 80 15 s 20 s of 20 s (Figure 4c) is sensitive to the structure whose depth 25 s range from 18 to 30 km. The phase velocity mainly reflects 90 30 s 40 s the lateral variation of the mid-lower crust. Most of the 100 study area show low-velocity anomalies, There are differ- −0.05 0.00 0.050.10 0.15 0.20 ent high-velocity and low-velocity anomalies on both sides dc/dv S of the basin near Weinan area, which is consistent with the Figure 6 Depth sensitivity kernels of fundamental Rayleigh block boundary. The Tianshui-Pingliang-Qingyang-Guyuan wave phase velocities with respect to the shear wave velocity area and the south of Ankang area still show obvious low- structure at different periods. The abscissa represents the velocity anomalies. derivative of the phase velocity with respect to the S-wave The phase velocity of Rayleigh waves with periods of velocity 25 and 30 s (Figures 4d and 4e) are sensitive to the structure at a depth of 30 to 40 km. The change of phase The phase velocity of a Rayleigh wave with a period of velocity at this depth is negatively correlated with crust 5 s (Figure 4a) is sensitive to the velocity of S-wave at a depth, that is, the deeper the crust is, the lower the velocity depth of 6 to 14 km, mainly reflecting the shallow crust is. It mainly reflected the lateral variation of crustal structure. In Figure 4a, the Weihe basin and the northeas- thickness. The west side of the study area (Tianshui- tern Sichuan basin show low-velocity anomalies due to the Pingliang-Qingyang-Guyuan) is located in the northeastern distribution of thick sedimentary layers. There are also margin of the Tibetan plateau and belongs to the transition low-velocity anomalies along the Qingyang-Pingliang zone between the Tibetan plateau and the Ordos blocks. area. The northwestern Sichuan basin has the largest low- The crustal thickness of this region is greater than that of velocity anomalies zone and the lowest velocity. Since the Ordos block and the northern margin of Sichuan basin. western margin of the Ordos block has a Cenozoic and The Yan'an area on the east side of the Ordos block is some Mesozoic low-velocity sedimentary caprocks (Liu high-velocity anomalies relative to the west side, which is et al., 2003), it also exhibits obvious low-velocity anom- similar to the results obtained by the former using the alies. In the north of Guangyuan, along the Qinling Moun- receiver function (Chen et al., 2005), and also consistent tains between Baoji and Ankang, and from the Shangluo to with the results obtained using the two-plane-wave method Dengzhou area, the phase velocity is relatively high, and (Li et al., 2012). the southeast side of the study area also exhibits high- The phase velocity of a Rayleigh wave with a period of velocity anomalies. In general, the sedimentary layers in 40 s (Figure 4f) is sensitive to the structure at a depth of 37 the fault basins and graben areas are thicker, and the to 47 km, and its distribution characteristics are mainly sedimentary layers in the mountainous areas are thinner. related to the Moho depth. Similar to 30 s, the low-velocity Therefore, the fault basins and grabens tend to show low- anomalies are mainly distributed in the transition zones at velocity anomalies, while the high-velocity anomalies south of Tianshui and west of Guyuan and from Pingliang

240 Earthq Sci (2018)31: 234–241 to Qingyang. The Qinling Mountains near Xi′an and south nshan fault zone and its two sides, the northeastern Tibetan of Xi′an are relatively low-velocity anomalies. The most plateau and the Ordos block, is obvious lateral heterogen- areas of the Ordos block and the northern margin of the eous. However, the phase velocity of the west side of the Sichuan basin show high-velocity anomalies, which refle- Ordos block is consistent with the northeastern Tibetan cts the lateral heterogeneity of the Moho depth in the study plateau, which may imply the material exchange and area, the Ordos block and the northern margin of the fusion of the northeastern Tibetan plateau and the Ordos Sichuan basin are located at the uppermost mantle at this block through the Liupanshan thrust fold belt. depth and the transition zones of this depth is in the lower (3) The Yan'an area located in the eastern Ordos block crust. The Moho depth of the Qinling orogenic belt is is high-velocity anomalies relative to the western Ordos gradually decreasing from 52–54 km in the western block in the period of 25–30 s, which reflects the lateral Qinling orogenic belt to 42 km in the eastern Qinling heterogeneous of the Ordos block. The crust thickness of orogenic belt. The overall shape of the Moho depth is the transition zone between the Tibetan plateau and the undulating and inclined to the west (Li et al., 2015), which Ordos block is greater than those of the Ordos block and led to the low-velocity anomalies of the West Qinling the northern margin of Sichuan basin, which related to the orogenic belt in the northwestern Hanzhong area relative fact that the block on the Tibetan plateau extruded toward to the East Qinling orogenic belt near the Shiyan area. The the northeast and blocked by the rigid Ordos block, and the different phase velocity of the eastern and western Qinling crust became thick in the transition zone. orogenic belts may be caused by the block uplift of the (4) The 40 s period of S-wave mainly reflects the stru- Tibetan plateau and the expansion of the Tibetan plateau cture of the lower crust and uppermost mantle (30–60 km). block to the northeastern Tibetan plateau. The velocity variation is inversely related to the Moho depth, and the lateral velocity heterogeneity represents the 4 Discussion and conclusions lateral variation of the Moho depth. Most area of the Ordos block and the north of the Sichuan basin are high-velocity The continuous background noise recordings in this anomalies and the transition zones are low-velocity paper are derived from the Himalayan Phase II mobile anomalies, which indicates that the Ordos block and the array and the permanent seismic network. The two-year northern margin of Sichuan basin are located at the continuous observation data was first analyzed, and the uppermost mantle at this depth, and the depth in the Rayleigh wave cross-correlation functions and phase transition zone is still located in the lower crust. The velocity dispersion curves for all stations were then change of phase velocity at this depth reflects that the obtained. The background noise tomography technique lateral heterogeneity of the Moho depth. was then used to calculate the Rayleigh wave phase velocity distribution for periods of 5 to 40 s in Shaanxi and Acknowledgments neighboring regions. The distribution characteristics are summarized and interpreted below. This study was supported by the Science for Earth- (1) There is an obvious lateral heterogeneity in the quake Resilience (Nos. XH17035YSX and XH19041Y) velocity structure of the crust and uppermost mantle in the and Navigation and Innovation Fund of Shaanxi Earth- study area. This heterogeneity exists not only in the quake Agency of 2018 (No. QC201805). The waveform intersection of different blocks but also in the interior of data was provided by China Seismic Array Data Manag- the block. The phase velocity of the 5 s period is charac- ement Center at Institute of Geophysics, China Earthquake terized by obvious low-velocity anomalies and lateral Administration. Parts of the figures were plotted with heterogeneity. The low-speed anomalies correspond to GMT software (Wessel and Smith, 1998). At the same loose sedimentary cover, and the high-speed anomalies time, The authors express our sincere respect and heartfelt correspond to orogenic belts and uplifts and the boundary thanks to the reviewers for the valuable suggestion between high and low velocity anomalies is consistent proposed for this article. with the block boundary. 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