Study On 3D Crustal Velocity Structure Of Crust In Qilian Mountains

Seismic positioning is a basic problem of seismology and the basis of many studies.Seismic tomography(Seismic Tomography)can obtain the spatial distribution of the earth's internal velocity structure.Seismic precise positioning method and seismic tomography method can study issues such as seismicity,seismogenic structure and occurrence mechanism of seismogenesis.This paper integrates the original waveform data recorded from 40 short-period stations of the active source observation network and 14 broadband fixed stations of the digital seismic network of Gansu Province from July 2014 to July 2018 Processed to form an earthquake event file in EVT format,combined with the seismic phase data reports from July 2014 to July 2019 at fixed stations,using MSDP seismic analysis software,using simplex location method to initially locate and identify natural seismic events,identification of seismic phase with Pg,Sg,Pn,Sn,PmP,SmS,etc.The number of existing earthquakes in the official earthquake catalog of Gansu Province is 6077,and the number of seismic phases is 91223.After manually identifying and sorting the data,the final number of earthquake observation reports is 6224 and the number of seismic phases is 158589.Relocated using the double-difference positioning method,and collated to form an earthquake observation report,obtained about 87% of the seismic relocation.The relocation results show that the root-mean-square residual error is reduced from the original 0.25 s to 0.07 s,and the positioning accuracy is improved,which shows more accurate source parameter information in the Qilian Mountains area and obtains a more refined epicenter distribution map.The relocated earthquake epicenter distributions are more clearly clustered,showing the close relationship between seismic activity and active structure in the study area.The focal depth is concentrated between 0 and 25 km,indicating that the seismogenic layer in the Qilian Mountains area is shallow.According to the relocation results,part of the original data that has not been relocated is merged to generate a data format file that satisfies seismic tomography.Using GABWIT seismic tomography software and genetic algorithm for inversion calculation,the three-dimensional crustal velocity structure of Qilian Mountains was obtained.The resolution of this paper reaches 30Km?30Km?5Km,which is higher than the minimum resolution(39Km?39Km?5Km)studied by the predecessors in this area,which improves the accuracy.Combined with the test board test results,below 25 Km depth,due to the small number of Pn,Sn and reflected wave phases,the test board has lower resolution and lower reliability.Therefore,this paper mainly discusses the situation of 0-25 Km.The tomography results show that the earthquakes in the study area are spatially distributed along the NW-SE direction,which is basically consistent with the trend of large faults in the area.The depth of the crystalline basement is relatively uniform,about 7Km.The basement depression of the Qilian Mountain fold belt,the Alxa block is relatively stable and has little change.Shallow velocity has a corresponding relationship with the distribution of the main surface fault structure and geological structure characteristics.Shallow velocity is related to the stratum exposed on the surface,and has a corresponding relationship with the distribution of the main fault tectonic zones and geological structure features.Most of the low-speed areas are located in areas where the terrain is basins or valley depressions,and the high-speed areas are corresponding to areas where the terrain is the edge of the basin or the intersection with mountains.The collision of the Indian plate and the Eurasian plate in the early period and the northward pushing effect have continued to this day,and have been affecting the Qilian Mountain area,resulting in complex intra-plate structures in the Qilian Mountain area,severe deformation and active seismic activity.Seismic activity is relatively active in the high-speed anomaly area and the high-low speed transition area,with more distribution,indicating that there is a certain correlation between the distribution of crustal velocity structure and seismic activity,which can provide a certain amount of future seismic risk judgment and seismic trend research reference.