CRUSTAL ANISOTROPY AND ITS TECTONIC IMPLICATIONS IN THE CHONGQING REGION

doi:10.3969/j.issn.0253-4967.2020.01.010

Abstract

Abstract:

The receiver function which carries the information of crustal materials is often used to study the shear-wave velocity of the crust as well as the crustal anisotropy. However, because of the low signal-to-noise ratio in Pms(P-to-S converted phase from the Moho), the crustal anisotropy obtained by shear-wave splitting technique for a single receiver function usually has large errors in general. Recent advance in the analysis method based on Pms arrival time varying with the back-azimuth change can effectively overcome the above defects. Thus in this paper, we utilize the azimuth variations of the Pms to study the crustal anisotropy in Chongqing region for the first time. According to the earthquake catalogue provided by USGS, seismic waveform of earthquakes with magnitude larger than 5.5 and epicenter distance range of 30°~90° between January 2015 and December 2016 are collected from 14 broadband seismic stations of Chongqing seismic network. We carry out the bootstrap resampling to test the reliability of the radial maximum energy method for the observation data. In addition, we also applied the receiver function H-Kappa analysis in this paper to study the crustal thickness and Poisson's ratio.
Our results show the crustal thickness ranges from 40~50km, and there is a thin and thick crust in the southern and northern Chongqing, respectively. The crustal average Poisson's ratio ranges from 0.23~0.31, the Poisson's ratio reaches the maximum value in the central part of Chongqing, while the Poisson's ratio in the northern and southern parts of Chongqing is obviously low. We obtain the crustal anisotropy from 9 stations in total. The delay time of crustal anisotropy distributes between 0.08s and 0.48s, with the average value of 0.22s. Among them, the CHS, QIJ and WAZ stations in central Chongqing have relatively large crustal delay time(>0.3s), followed by ROC station in the western Chongqing(0.25s), while the delay time in CHK station in northern Chongqing and WAS station in southern Chongqing are 0.08s, showing relatively weak crustal anisotropy. The fast polarization directions(FPDs)also change obviously from south to north. In southern Chongqing, FPDs are dominant in NNE-SSW and NEE-SWW, while the FPDs in WAZ station change to NWW-SEE, and the FPDs appear to be NW-SE in CHK in the northern Chongqing. In general, the FPDs are sub-parallel to the strikes of faults in most areas of Chongqing areas.
Combined with other results from GPS observations, tectonic stress field and XKS splitting measurements, the main conclusions can be suggested as following: The cracks preferred orientation in the upper crust is not the main source of crustal anisotropy in Chongqing area. The crust and lithospheric upper mantle in the eastern Sichuan fold belt(ESFB)and Sichuan-Guizhou fault fold belt(SGFFB)are decoupled, and the deformation characteristics in the north and south parts of ESFB and SGFFB is different. The complex tectonic deformation may exist beneath the mountain-basin boundary, causing the fast directions of crustal anisotropy different from that in other areas of ESFB and SGFFB. The faults with different strikes may weaken the strength of average crustal anisotropy in some areas. The crustal deformation in southern Dabashan nappe belt(DNB)may be mainly controlled by the fault structure.

Key words: Chongqing region, receiver function, seismic anisotropy, crustal structure

摘要：

文中搜集了重庆地震台网14个宽频带地震台站记录的远震波形资料, 利用接收函数Pms到时的方位变化首次获取了重庆地区的地壳各向异性结果, 同时采用接收函数H-Kappa方法获得了研究区的地壳厚度及泊松比。结果显示: 重庆地区的地壳厚40~50km, 在大巴山推覆构造带附近存在明显的梯度变化; 地壳泊松比平均为0.23~0.31, 在重庆中部泊松比值达到最大。重庆地区的地壳快慢波时间延迟为0.08~0.48s, 平均为0.22s; 绝大部分地区地壳的快波方向与断裂走向比较一致。结合GPS观测、构造应力场和XKS分裂测量等研究资料, 分析认为: 川东褶皱带和川黔断褶带的地壳和岩石圈上地幔为解耦变形, 其南、北段具有不同的变形特征; 盆山边界可能存在复杂的深部构造变形, 影响了局部地区平均方位各向异性的快波方向, 不同走向的断裂活动还弱化了部分区域的平均各向异性强度; 大巴山推覆构造带南部的地壳变形可能主要受到断裂带构造的影响。

关键词: 重庆地区, 接收函数, 各向异性, 地壳结构

CLC Number:

P315.2

GAO Jian, YANG Yi-hai, HUANG Shi-yuan, YANG Cong, ZHANG Yuan-sheng, LIU Cun-xi, LI Shao-rui, HUA Qian. CRUSTAL ANISOTROPY AND ITS TECTONIC IMPLICATIONS IN THE CHONGQING REGION[J]. SEISMOLOGY AND GEOLOGY, 2020, 42(1): 147-162.

高见, 杨宜海, 黄世源, 杨聪, 张元生, 柳存喜, 李少睿, 花茜. 重庆地区地壳各向异性及其构造启示[J]. 地震地质, 2020, 42(1): 147-162.

Figures/Tables 10

Fig. 1 Tectonic setting and location of seismic stations in the Chongqing region.

Fig. 2 Distribution of tele-seismic events used in this study.

Fig. 3 H-Kappa analysis result of receiver functions at station CHS.

Fig. 4 Receiver functions recorded at station CHS by binned in 10° azimuthal caps(a)and Pms phases after the crustal anisotropy correction(b).

Fig. 5 Ratio diagram of stacked Pms energy before and after the anisotropy correction from the grid search(station CHS).

Table1 Single layered horizontal crust model used for the synthetic receiver function data

地壳厚度/km	V_P/km·s^-1	V_S/km·s^-1	快波方向/(°)	各向异性/km·s^-1
50	5.95	3.5	30	0.2

Fig. 6 Distributions of fast-wave direction(a)and delay time(b) from bootstrap resampling method.

Table2 Crustal thickness, average VP/VS ratio, fast wave polarization direction and delay time beneath each station determined by this study

序号	台站名	地壳厚度/km	地壳平均V_P/V_S	地壳泊松比	快波方向/(°)	时间延迟/s
1	CHK	50.0±2.3	1.749±0.081	0.257±0.012	130	0.08
2	CHS	41.2±1.6	1.829±0.072	0.287±0.011	26	0.33
3	CQT	41.9±1.5	1.721±0.060	0.245±0.009	69	0.18
4	FUL	42.9±2.1	1.791±0.088	0.274±0.013
5	HCB	48.4±1.8	1.800±0.068	0.277±0.010
6	QIJ	41.8±2.4	1.860±0.106	0.297±0.017	39	0.48
7	ROC	39.9±1.3	1.731±0.057	0.250±0.008	60	0.25
8	SHZ	40.8±1.5	1.920±0.070	0.314±0.011
9	WAS	44.0±2.2	1.680±0.084	0.226±0.011	85	0.08
10	WAZ	45.4±1.8	1.812±0.070	0.281±0.011	106	0.34
11	WUL	43.6±1.9	1.689±0.072	0.230±0.010	180	0.13
12	WUX	50.0±1.9	1.738±0.065	0.253±0.009
13	XIS	41.6±1.8	1.702±0.074	0.236±0.010
14	YUB	40.7±1.7	1.771±0.073	0.266±0.011	174	0.10

Fig. 7 Thickness(a) and average Poisson's ratio(b) of the crust beneath Chongqing region.

Fig. 8 Comparison of crustal anisotropy determined in this study with GPS observation(ITRF 2008, International Terrestrial Reference Frame), P-axis and seismic anisotropy derived from XKS splitting measurements.

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