Study On The Three-Dimensional Velocity Structure Of The Crust In Southeast Gansu

Seismic tomography based on model-based data fitting,is a geophysical tool that uses seismic wave data to invert deep Earth information to determine subsurface fine structure and local nonuniformity.With the continuous efforts of seismologists,the method has been gradually developed and perfected,and applied to the deep crustal studies in many regions of the world with fruitful results.Southeast of Gansu Province is an important part of the northeast margin of the Tibetan Plateau.The area is in the northern part of the South-North Seismic Belt with strong tectonic activity,and is the intersection area of the Tibetan Plateau block,Ordos block,Alxa block and South China block.The region has complex topography and geomorphology,large faults are developed,and there are steep deformation zone of crustal thickness and gradient zone of gravity change at depth.It has the deep tectonic background and historical background for strong earthquakes,which is a key area of interest for geological researchers.In this paper,we briefly describe the basic tectonic situation in the southeast of Gansu area and the basic principle of seismic tomography method,and use the regional seismic data recorded by fixed and mobile stations in the study area for 52 years since the observation records.This paper jointly use P_g?S_g?P_m?S_m?P_n?S_n seismic phases,and perform joint inversion of velocity structure and earthquake location parameters based on the P-wave initial velocity model.A total of24544 seismic events and 219559 effective S-P records are screened out,and the minimum standard deviation of the total travel time is 0.790691.The velocity structure model in the southeastern Gansu region is obtained,and the model is used as the input model.The earthquakes recorded by more than three stations in the study area are relocated by genetic algorithm,35880 seismic events are selected,and the seismic location results in the region are obtained.Focal depth concentration advantage distribution range is 5-20km.Combining the information on the distribution relationship of fault and earthquakes and tectonic activities in the region,it is concluded that there are three obvious depth gradient zones in the Moho surface in the study area,and the Ruoergai area may be relatively intact,blocking the eastward movement of the Tibetan Plateau,forming a gradient zone at the eastern edge of the region,with increased stress accumulation by pushing,which is an area of concern for future strong earthquake prediction;the deep and shallow tectonic relationships of several large active fractures in the western part of the study area,including the West Qinling fault in the north,the Lintan-Tanchang fault,the East Kunlun fault in the east,the Mado-Gande fault,and the Diebu-Bailongjiang fault.Combining the velocity structure,earthquake distribution and surface fault distribution,we have analyzed the fault of the two historical M8 earthquakes in southeast Gansu.After analysis,it is speculated that the"Minxian-Lixian-Liangdang Fault"may exist,which is the seismic fault of the 1654Lixian M8 earthquake;It is speculated that the main seismogenic structure of the 1879South of Wudu M8 earthquake may be affected by the Hanan-Qingshanwan-Daoqizi fault and the Wenxian-Kangxian-Lueyang fault,and the focal depth is about 15-25 km.After comparing and analyzing the results,it is concluded that the results of this paper are basically consistent with the regional geological structure and previous related studies,and have high credibility.This paper differs from previous studies in that the seismic data used in this paper are comprehensive,and the seismic phase data in the study area have been recorded since the observation.In addition,this paper contributes positively to the study of active ruptures and seismogenic environment in the southeastern part of Gansu and the determination of seismic hazard zones,and provides an important reference for the formation and extension mechanism of the northeastern margin of the Tibetan Plateau.