Joint Inversion Of Seismic Traveltime And Gravity Data And Its Application In Imaging Crustal Structure Around Southern Section Of North-south Earthquake Belt

The southern section of North-South Earthquake Belt(SNSEB)is located at the intersection of Indosinian Block,Tibetan Plateau and Yangtze Block,where earthquakes mainly occur.Due to complicated geological features and intensive tectonic activity,large differences of velocity structure,gravity anomaly,Moho depth and lithosphere structure are presented among these geological units.The reconstruction of crustal structure in the SNSEB benefits in understanding seismogenic environment,geodynamics and composition of deep crust.In this thesis,we firstly developed a new method for the joint inversion of seismic traveltimes and gravity data,and we also established a framework and a set of code for the joint inversion.Secondly,we assembled 38 DSS profiles and a 3D S-wave model for constructing an initial P-wave model around the region.Thirdly,we assembled the satellite gravity anomaly and picked 169625 P-wave arrivals from 22370 earthquake events recorded by 108 fixed seismic stations and 367 mobile seismic stations.After that,the traveltime inversion,sequential inversion and simultaneous inversion of seismic and gravity data were respectively applied to obtain the 3D P-wave velocity and density crustal structure with mesh of 0.5°×0.5°×5 km around the region.The results showed that it is difficult to converge the misfits for sequential inversion,which failed to improve the inversion result.In contrast,simultaneous inversion can effectively reduce the non-uniqueness to improve the inversion result.The simultaneous result showed that Moho depth in the study area presents a large variation from less than 30 km in the southeast to 65 km in the northwest.This region is dominated by deep faults which control the distribution of velocity anomaly in the crust.In the study area,the low velocity layer is generally widespread,so that the average crustal velocity value is relatively low.Huge differences exist between the southeastern Tibetan Plateau and Sichuan Basin on crustal structure.Nevertheless,the crustal structure of the northwestern Yunnan Province is analogous to the southeastern Tibetan Plateau.The low velocity anomaly in Tengchong volcanic area extends from shallow crust to the mantle,which is related to the composition and the difference of temperature of upper mantle.