Earthquake Relocation And Tomography Of Weihe Basin And Its Adjacent Areas

Weihe basin is located at an important tectonic boundary in China,connected with the Ordos plate in the north,Qinling fold belt in the south,Shanxi uplift belt in the east,and arcuate fracture bundles in the southwest boundary of Ordos in the west.The seismic relocation and tomography of Weihe basin and its adjacent areas are helpful to study the basin structure and explain the distribution and development of earthquake and geological disasters.By introducing the residual error of seismic travel time,the source location parameters with high accuracy can be obtained and the errors caused by the velocity model can be reduced without station correction.Double-difference tomography(DD)is an extension of Doubledifference mapping.It is used to solve three-dimensional velocity structures and seismic event positions.In this paper,the initial velocity model is selected by the checkerboard test and the mesh partitioning accuracy is determined.The smooth weight and damping factor are determined by the L-curve method.In addition,1236 seismic events and 32,472 seismic phase data from 97 stations in Weihe basin and its surrounding network from the national seismological data sharing center and Shaanxi seismological bureau were used to carry out double-differential repositioning and tomographic imaging of Weihe basin and its adjacent areas.The results of seismic relocation show that the seismic distribution is more concentrated after the relocation,which better shows the close relationship between seismic activity and geological structure.The results of double difference imaging in Weihe basin and its adjacent areas show that,on the whole,there is a big difference between S wave velocity structure and P wave velocity structure in the same section.In the northeastern margin of the Qinghai-Tibet block,there is a high speed anomaly in the northwest near Qingchuan,and a low speed anomaly near Longnan,which may be related to partial material melting.There is a relatively high speed anomaly in the Ordos block,and the Weihe basin and the Winling orogenic belt show a relatively high speed characteristic.From the perspective of depth,the velocity structure of each structural unit in the study area has a certain distribution law.The results at 0km and 10 km in the study area mainly reflect the velocity structure of P-wave and S-wave in the shallow part of the crust and the upper crust,and show the characteristics of high-speed and weakly high-speed interphase on the whole.The imaging results from the range of 15 km to 65 km show that the high speed and low speed regions alternate with each other,which generally shows the lateral nonuniformity of the velocity structure.The imaging results at 0km showed that the distribution of velocity anomalies was correlated with the surface geological tectonic units to a certain extent,and the distribution of high and low velocity anomalies basically reflected the regional geological structural characteristics of the thickness of the sedimentary layer and the burial depth of the crystalline basement.The slice at 15 km shows that the average velocity of the Swave increases overall in the imaging region.The slice at 39 km shows that the P-wave velocity on both sides of Weihe basin is high,indicating that there is obvious transverse asymmetry at the bottom of Weihe basin.The section at 33.5 °N indicates that the Qinling orogenic belt has a high-speed upwelling of material from the upper mantle,which is presented as a high-speed uplift,and a low-speed zone on the northeast edge of the Qinghai-Tibet plateau,which is cut down to the middle crust.The high-speed material upwelling in the upper mantle exists in Weihe basin near Tianshui-Baoji and Weinan-Hua,and the high-speed material upwelling in the upper crust and the relatively low speed of the middle and lower crust appear near Xi'an between these two regions.The section of east longitude 108.5° shows that in Weihe basin,the velocity of Pwave in the middle and upper crust and surface layer is low on the whole,and the low-speed zone extends to about 25 km,while there is a relatively high-speed uplift area of S-wave from the upper mantle to the middle crust,which may be related to the upwelling of deep material.