Studying The Deep Structure Of The Central Shanxi Rift And The Western Sichuan Region

Shanxi rift zone is located at the intersection of Ordos and North China block.It was formed by the compression of Indian Ocean plate and the westward subduction of Pacific plate.A series of faults developed in the Shanxi rift zone,resulting in its complex structure,is a strong earthquake active zone in the west of North China.Western Sichuan is located in the eastern margin of the Tibet Plateau.Due to the strong collision between the Indian plate and the Eurasian plate,the crust of Sichuan Yunnan region has undergone strong deformation and frequent seismicity,making it the most significant area of strong earthquake activity in China.Both regions are strong earthquake areas,but they have their own characteristics.Therefore,it is of great significance to study the high-precision deep tectonic environment for further understanding the seismogenic environment and tectonic evolution of the region.In this paper,we collected a large number of high-density temporary array and routine seismic station data in the two study areas,and used the double difference tomography method.The main results and understanding are as follows:(1)Based seismic events of routine stations and the data of high-density temporary array,A total of 7455 seismic events from January 2008 to June 2015 were used to the tomographic inversion,The resolution is up to 0.2 °×0.2 °.The results of the velocity structure in the middle and upper crust reveal the deep structure features of the Xin Ding and Taiyuan basin,which are isolated basins and separated by obviously high velocity anomalies,which is Shi Ling Guan uplift.Although Taiyuan Basin and Linfen basin are separated by Ling Shi uplift,there is little difference in velocity between Ling Shi uplift and the two basins.The reason may be that in a long period of history,Ling Shi uplift is a geological activity dominated by sedimentation,and there are a lot of sediments in this area.(2)The profile of the middle of Tai Yuan basin shows the deposit characteristics of Taiyuan basin,the deposition layer was gradually thinned from west to east.The result of profile of Xin Ding basin shows that there are obvious velocity anomalies in the middle crust and the earthquake occurred at the top of the low velocity body;The section through Taiyuan Basin reflects the relative sedimentary depth and sedimentary environment of Taiyuan Basin.The dip angle of sedimentary layer in the west side is larger,and that in the east side is smaller,showing that the sedimentary layer gradually becomes thinner from west to East.(3)The focal mechanism shows that there are nearly pure fault earthquakes in the lower part of the central Shanxi rift system,and most of the focal mechanism solutions are distributed in NEE direction.The focal mechanism characteristics also verify that the central Shanxi rift basin is caused by dextral shear extension,which further indicates that the Shanxi rift basin is still affected by dextral shear extension;SKS anisotropy results show that the fast and slow wave delay time in Xin Ding basin is fast.It show that the lithosphere has no obvious thinning and strong deformation.The delay time is slower in Tai Yuan and Lin Fen basin than the Xin Ding basin.The lithosphere has obvious thinning and strong deformation.(4)The results show that there are earthquakes in the middle and lower crust of Taiyuan Basin;The comprehensive analysis of focal mechanism,anisotropy and the occurrence of earthquakes in the middle and lower crust shows that the central Shanxi rift basin dominated by Taiyuan basin is not only affected by dextral shear tensile stress,but also by the thermal material activity of lithosphere.(5)Based seismic events of routine stations and the data of high-density temporary array,A total of 20542 seismic events from January 2008 to June 2015,Pwave velocity structure with the resolution of 0.25° is obtained by double difference tomography.Based on 5290 earthquakes from January 2010 to June 2020,P-wave velocity structure with the resolution of 5km is obtained by using local earthquake double difference tomography.(6)The shallow velocity structure of the Western Sichuan region shows that the Emeishan show that the high velocity anomaly,and the low velocity anomaly occurs in the Sichuan Basin.In the north of the study area,The low velocity anomaly is obvious in the West of 102°E;the high velocity anomaly is obvious in the East of 102°E;The high velocity anomaly of Panxi rift in the south of the study area continued to the depth of 30 km;Through The profile of Pan Xi,presents a clear shape with a wide bottom and a narrow top.The wave velocity ratio result is high anomaly,which is supposed to be the Seismological Evidence retained during the Permian magmatic intrusion.(7)The results of velocity structure in the area of moderate and strong earthquakes in Western Sichuan show that there is a strong heterogeneity in this area,which show that the underground medium structure in this area is extremely complex,and the velocity structure controls the distribution of active faults;The obvious low velocity anomaly exists in the north of Qiaojia,which is mainly due to the fact that the area is located at the intersection of Daliangshan fault,Lianfeng fault,Zemuhe fault and Xiaojiang Faul,which the rocks were damaged in the upper crust,resulting in the low wave velocity in the area;Near the source area of Ludian earthquake,there are a high velocity anomaly with a maximum velocity of 6.4km/s in the shallow part,and a low velocity anomaly with a minimum velocity of 5.8km/s below 10 km.It is speculated that the high velocity anomaly in the shallow part is related to the basic rocks with high mechanical strength,and the low velocity anomaly may be related to the normal middle upper crust rocks rich in fluid.