Crustal Structures In The Northeastern Margin Of Tibet Plateau Revealed From Joint Inversion Of Body-wave Travel Times And Gravity Data

The northeastern margin of Tibetan Plateau is the leading edge of the plateau extending to inland,which has strong tectonic deformation and frequent seismic activities since the late Cenozoic.It is a hot-spot area to study the uplift and geodynamic evolution of the Tibetan Plateau,and an ideal place to study the process of strong earthquakes in the mainland.The crustal low-velocity layer and the seismogenic structure are two hot issues in the tectonic and dynamic studies of the northeastern margin of the Tibetan Plateau,and they remain dispute up to now.In this thesis,the seismic and gravity joint inversion of the crustal structure in the northeastern margin of the Tibetan Plateau is perform in order to provide deep constraints for the study of crustal deformation and seismogenic mechanism in the northeastern margin of the Tibetan Plateau.The main research work and achievements of this thesis are as follows:(1)The near-seismic event data recorded by 71 permanent stations and 418 portable stations of the “China Seismic Scientific Exploration Array – North Section of North-South Seismic Belt”,as well as the Bouguer gravity anomaly data in the study area have been collected.Firstly,the 3D crustal P-and S-wave velocity structure and hypocentral relocations in the study area are obtained by using the Double-Difference tomography method.Then,joint inversion of body-wave travel times and gravity data is applied to further improve the 3-D crustal P-wave velocity and density structure.The quality evaluation results show that the joint inversion method is effective in this study.(2)The crustal velocity and density structure are characterized by strong lateral heterogeneity of the study area.At the depth of 5 km,the characteristics of velocity and density structure are basically consistent with the surface geology.At the depth of 10-20 km,the Alxa block shows high-density and high-speed anomalies,and the Songpan-Ganzi block,Qilian orogenic belt and Western Qinling orogenic belt show large-scale low-velocity and low-density anomalies,indicating that the Tibetan Plateau block has significantly different middle crustal structures between the surrounding geological units.At the depth of 30 km,both the velocity and density structures present a high and low alternating,which are dominated by low values.The low-velocity and low-density anomalies of the Songpan-Ganzi block and the Western Qinling orogenic belt are obvious and continuous,reflecting that they may belong to the same basement.At the depth of 50 km,the Tibetan Plateau shows low-velocity and low-density anomalies,and the Alxa block shows high-velocity and high-density anomalies,indicating that the cutting depth of the Haiyuan fault zone between these two blocks has reached the top of the upper mantle.(3)Significant low-velocity anomalies of P-and S-waves appear in the depth of 20-40 km beneath the Songpan-Ganzi block and Qilian orogenic belts.The low-velocity layer below the Songpan-Ganzi block may be related to the crustal partial melting,while that of the Qilian orogenic belt possibly indicates the crustal thickening.(4)After relocation,the hypocenter positions are close to the fault zone,and distributed in banding or cluster along the faults,indicating that the active faults are closely related to the earthquake.The hypocenter depths of both Minzhang M6.7 earthquake and Jiuzhaigou M7.0earthquake after relocation lie around the high-and-low velocity transition zone within the brittle upper crust,since both two earthquakes occurred at the junction of adjacent blocks.The low-velocity layer beneath these two earthquake areas may be partially melted or easier to creep,and the brittle upper crust is more likely to accumulate the energy of deformation,resulting in the occurrence of earthquakes.