Construction Of Crustal Velocity Structure Model In Southern California Using Seismic Body Wave Travel-time Tomography

Seismic tomography is one of the important tools to study the heterogeneous structure and geodynamics inside the earth.Among them,seismic body wave travel-time tomography technology is the most widely used,the travel-time information of the seismic wave can reflect the internal structure of the earth,because the travel-time of seismic wave propagating in the underground non-uniform medium is different with the travel-time obtained in the ideal uniform medium,By analyzing and comparing this change,we can extract information about the internal structure of the earth from it.The Southern California area of the United States is located at the intersection of the Pacific Plate and the North American Plate.It has severe seismic activities,complex geological structures,and dense seismic stations.It is a typical area for natural seismic wave velocity imaging to explore the earth's internal velocity structure.The purpose of this study is to obtain the crustal velocity structure of the study area using seismic body wave travel-time tomography technology through the travel-time information of seismic wave data,and to study the crustal P wave velocity structure of the area is of great significance for understanding the geological structure of the area.In this paper,in view of the problem that the acquired seismic waveform data has poor quality and it is difficult to obtain the observed travel-time,a method to improve the quality of the waveform and automatically pick up the first arrival wave of the near earthquake is explored.Finally,this paper collected the seismic waveform events that occurred in the Southern California area of the United States from 2015 to 2018.The AIC method was used to pick the first arrival travel-time.Through quality screening and error analysis,35600 first arrival P and Pn wave travel-time were finally picked.Then,using seismic body wave travel-time tomography to study the crustal at a depth of 0-40 km in Southern California,a P-wave velocity structure model with a resolution of 0.5 ° × 0.5 ° × 5km in this area was obtained,combine the existing geophysical research results in the area to analyze the chromatographic imaging results.By analyzing the velocity distribution at each depth layer,it is revealed that the P-wave velocity in Southern California not only changes with the depth,but also the crust in this region has obvious lateral heterogeneity.The inversion results show that near the surface,the P-wave velocity distribution in this area is in good agreement with the surface geological structure and topographic fluctuations,that is,the alpine mountains have a higher P-wave velocity in this layer,while the sedimentary basin is low.In the middle crust,the tomographic results show that most areas at this depth show high-speed distribution.The velocity structure of the crust in this area is related to the formation of crustal rock formations and to previous geological tectonic activities.In the lower crust,the inversion results show lateral heterogeneity of the P-wave velocity.In the upper mantle,tomographic results show that low velocity distribution is present in most areas at this depth.