CRUSTAL ANISOTROPY AND ITS TECTONIC IMPLICATIONS IN THE CHONGQING REGION

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.