Study On The 3D Velocity Structure And Relocation Of Earthquakes In The Menyuan Area

Seismic location and tomography are important methods to study seismogenic structure and earthquake mechanism of preparation and occurrence.On the basis of seismic data recorded by regional digital seismic network and temporary seismic array,earthquake location and 3D velocity structure in the Menyuan area were inverted by using Hypo2000 earthquake localization method and double-difference tomography method.Firstly,we used the Hypo2000 earthquake localization method to relocate the collected earthquakes to obtain high-precision source location parameters in the Menyuan area.Then the relocated seismic data is applied to the double-difference tomography method for the joint inversion of three-dimensional velocity structures and seismic location parameters.Combined with geological tectonic setting,we have studied the deep medium environment of the Menyuan M_S6.4 earthquake and the relationship between velocity structure and seismic activity in this area.Finally,the collected seismic data were divided according to the time period to study the spatial and temporal evolution characteristics of the velocity structure in the Menyuan area before and after the occurrence of the Menyuan M_S6.4 earthquake.We further analyzed the relationship between the change of velocity structure and the process of earthquake preparation and occurrence,try to seek earthquake precursor information and provide the basis for the prediction of future seismic trend in this area.The results indicate that after the relocation of the Hypo2000 earthquake localization method,the location accuracy of the earthquake in the Menyuan area has been significantly improved,eliminating the phenomenon of stratified distribution of the earthquake that not relocation and the source distribution in the vertical direction is clearly a strip distribution.It can more truly reflect the distribution of underground faults.The high-precision seismic source position provides a powerful guarantee for the reliability of the next joint inversion results.After joint inversion of the double-difference tomography,the root-mean-square residuals of travel time have been greatly reduced.The epicenter of the M_S6.4 is37.66°N,101.61°E,and the focal depth is 9.65 km.The aftershock sequence is distributed in the NW-SE direction in the horizontal direction,nearly vertical and slightly inclined in the southwest direction in the vertical direction.There is significant inhomogeneity of the crustal velocity structure in the study area,and the shallow velocity structure is closely related to the surface geological structure and topographic features.The seismic activity in the study area has a strong correspondence with the crustal velocity structure,and earthquakes are mainly distributed in high-velocity anomaly areas and their edges.The P-wave and S-wave velocity structures near the epicenter of the M_S6.4 earthquake show obvious high-speed anomalies,and there is a low-velocity layer of P-waves below the source region.This special tectonic condition may be an important reason for the occurrence of the earthquake.The temporal and spatial variation characteristics of velocity structure in the Menyuan area show that the crustal velocity structure is generally consistent in different time periods,but the velocity of P-wave and S-wave changed obviously in the vicinity of the source area.The results show that the velocity of the source area increased gradually from 2013 to before the Menyuan M_S6.4 earthquake occurred.Then the velocity of the source area decreased significantly after the M_S6.4earthquake occurred on January 21,2016,but it increased again by 2017.It reflects the process of the stress in the source area from accumulation to sudden release before and after the Menyuan earthquake.