Study On Current Crustal Deformation And Stress Charateristics In Southeastern Gansu

This paper selects the eastward extension of the northeast margin of the Qinghai-Tibet Plateau-southeast Gansu as the research area.The geological structure in this area is extremely complex,geological disasters occur frequently since ancient times.This article focuses on the 2013 Minxian-Zhangxian Ms6.6 earthquake in southeastern Gansu as the main study object.The study of the current crustal deformation and stress characteristics of this area can play a positive role in understanding seismic activity in this area.It also provides a certain reference for the subsequent earthquake research in this area.The main content of the paper is as follows:(1)First,use GAMIT/GLOBK software to process the data of 42 GPS monitoring stations in the study area(2009~2017),and then use Euler vector method to convert to GPS horizontal motion velocity fields which deduct the rotation speed of the Eurasian plate under the framework of ITRF2008,including 2009~2013(before-earthquake),2009-2015(long-term average),and 2015-2017(post-earthquake);The maximum shear strain rate,surface expansion rate,main strain rate are calculated by using the GPS data in the preceding three periods combing the least squares configuration method respectively.Through analysis,it can be seen that the horizontal movement of the regional crust as a whole exhibits the characteristics of clockwise arc deflection.In addition,the GPS horizontal motion velocity field and strain rate field in the epicentral areas of Minxian County and Zhangxian County during the pre-earthquake period exhibited anomalies.This kind of anomaly is very likely to cause the Ms6.6 earthquake in 2013.From the perspective of the long-term average GPS horizontal motion velocity field and strain rate field,the epicentral area still maintains a certain anomaly;in the post-earthquake period,the horizontal movement of the epicentral area seems to return to normal,but the crustal movement in Minxian County,Dangchang and other regions is extremely active,which may be caused by the adjustment and recovery of local crustal movement in the post-earthquake region.(2)The GPS horizontal motion velocity field of the above-mentioned regional crust in three periods is processed in cross-fault zone sections,and the movement rate of the fault strike in two directions of vertical and parallel are calculated by using the difference between the weighted average and the arc tangent dislocation model and the least square method of damping.Through specific analysis,it can be seen that the fault zone in the northern margin of the West Qinling Mountains and the Lintan-Dangchang fault zone have different degrees of left-lateral strike-slip and squeezing movement characteristics in different periods;In the average period,the left-handed slip rate increases,which is different from the Guomatan section and the Yuanfeng-Tianshui-Baoji section compared to the pre-earthquake period.This may be related to the Ms6.6 earthquake in this area.The activity of the Lintan-Dangchang fault zone in the post-earthquake period showed abnormal activity compared to the pre-earthquake and long-term average periods,indicating that the degree of blocking of the fault is reduced,thereby increasing the activity.(3)Based on the fault static Coulomb stress rupture criterion,the Coulomb3.3 software was used to calculate the static Coulomb stress generated by the Ms5.2 earthquake in Minxian,Ms5.0 in Minxian-Zhuoni and Ms6.6 in Minxian-Zhang County,respectively,which impacts on the subsequent seismic events,aftershock sequences and surrounding faults;the results show that the static Coulomb stress change caused by the superposition of the Minxian Ms5.2 earthquake and the Minxian-Zhuoni Ms5.0 earthquake plays a more crucial role in the earthquake of Minxian-Zhangxian Ms6.6;and the aftershock sequence of Ms6.6 earthquake in Minxian-Zhang County is most likely the result of the joint action of three moderate and strong earthquakes in this area;The resulting static Coulomb stress changes have a certain stress loading effect on the western part of the Zhangxian section of the northern Qinling fault zone and the eastern part of the Lintan-Dangchang fault zone,which should be noted.