Joint Inversion Of Ambient Noise Surface Wave And Gravity Data And Its Application In Northeastern Margin Of Tibet Plateau

The surface wave dispersion data is mainly sensitive to the shear wave velocity of underground media,while the gravity data reflects the density distribution of underground media.Single geophysical inversion has non-uniqueness,and joint inversion using different geophysical data and methods effectively reduces the nonuniqueness of inversion.According to the empirical relationship between seismic wave velocity and rock density,this paper unifies the surface wave dispersion data and gravity data into one inversion framework for inversion.The inversion model can fit the two data simultaneously.Integrating various geophysical data for inversion can fully play the advantages of different geophysical data,help to improve the inversion resolution and obtain more accurate and reliable physical parameter structure imaging.The northeast margin of the Tibet Plateau is the latest uplift of the plateau and the frontier area extending to the interior of the continent,which is one of the most seismically active areas in China.Its lithosphere has undergone substantial deformation.Therefore,this region is not only a hot spot for the study of uplift mechanism,tectonic deformation and block interaction of the Tibet Plateau but also an excellent place to study the strong earthquake preparation environment.Although many studies have been carried out on the tectonic and dynamic processes in the northeast margin of the Tibet Plateau,there are still significant disputes on the distribution and genesis of the crustal low-velocity layer and the inducing mechanism seismicity.This paper develops and realizes the joint inversion method based on the relationship between rock physical properties using background noise surface wave and gravity data.We applied to the study of the correlation between geological structure characteristics,depth velocity structure and seismicity in the northeast margin of the Tibet Plateau.Combined with the previous research results in this area,the distribution characteristics and causes of regional velocity structure,material migration channels and the relationship between seismic distribution and velocity structure are analyzed and speculated.The results show that:(1)the results of the surface wave dispersion inversion,surface wave dispersion,and gravity joint inversion are generally similar.Still,the joint inversion shows more details in the study area's shallow structure and sparse ray coverage area.It can better fit the surface wave dispersion and gravity data simultaneously,indicating that the inversion resolution is effectively improved after the introduction of gravity data.(2)The crustal velocity structure in the study area is complex,showing strong heterogeneity related to structural units.(3)Obvious lowvelocity anomaly areas are widely distributed in the middle and lower crust of Qiangtang Orogen and Songpan-Ganzi Orogen,which may be related to the partial melting of deep crustal materials.There is a low-velocity anomaly area in the northwest of Qilian Orogen,which may be related to crustal thickening.(4)It is considered that there may be no northeast crustal material channel flow in the middle and lower crust of the northeast margin of the Tibet Plateau.(5)Most of the strong earthquakes in the northeastern margin of the Tibet Plateau occurred in the transition zone of high and low-velocity anomalies.The Maduo Ms7.4 earthquake and the Menyuan Ms6.9earthquake sources are located in the brittle high-speed layer of the upper crust,and there is a low-speed layer in the crust below.Due to the compression of the Indian plate on the Eurasian plate,the stress accumulates for a long time,and the relatively weak low-speed materials in the middle and lower crust make it easier for the brittle upper crust to store release energy,which eventually leads to earthquakes.The research results of this paper provide a new basis for the study of crustal structure,material migration,and seismogenic structure in the northeast margin of the Tibet Plateau and provide a reference model for further research in the later stage.