Study On The Density Distribution Of The Seismic Source Area And Peripheral Medium In Wenchuan Earthquake By Using Gravity Data

On May 12,2008,the Wenchuan Ms8.0 earthquake occurred in the Longmenshan fault zone,Longlingshan fault zone,to its north are the Qinling orogenic belt,to its south are Zemuhe fault zone,the Anninghe fault zone and the Daliangshan fault zone,to its west are Qinghai-Tibetan Plateau,to its east is Sichuan Basin,and also is an important part of China's north-south seismic belt.Based on the pre-earthquake deformation data and gravity data,this paper analyzes its changing characteristics,the ground displacement,vertical displacement and gravity change of surface cose caused by Wenchuan Ms8.0 earthquake were simulated by PSGRN / PSCMP software.Then,based on the variable density rectangular model and the Particle Swarm Optimization Algorithm,the density distribution of the underground medium in the focal area of Wenchuan earthquake is obtained by using the regional residual gravity anomaly.The main research results are as follows:(1)The pre-earthquake gravity observation data better records the gravitational field from stable and then rise and then descend the final gentle change process,gravity point value of the timing changes in the vicinity of Yingxiu the first occurrence of gravity extreme area,and then vicinity of Beichuan also appeared gravity extreme,the relative gravity changes on both sides of the Longmenshan fault zone indicate that the gravity field of the Sichuan Basin is relatively stable before the earthquake,and the gravity change in the western Sichuan Plateau is significant.(2)The simulation results show that the Longmenshan fault zone is characterized of thrust and dextral strike slip,the vertical displacement and the coseismic gravity have good consistency,the uplift area is uplifted and the gravity value is small,and the displacement and gravity of the far-earthquake area are basically unchanged,the change of medium density also causes the co-seismic gravity to play a certain role,the results of the simulation and the measured values of the horizontal displacement and gravity values of the coseismic are relatively accurate,but the vertical displacement is quite different.(3)Free air gravity anomalies are closely related to terrain.Bouguer gravity anomalies are closely related to crustal thickness.Most of the regional performance of the imbalance,especially the Longmenshan fault zone not only did not decline,but in the uplift state,therefore,in the crust of the Longmenshan fault zone or the upper mantle must have a greater force than the equilibrium force to promote the Longmenshan fault zone uplift,which may be caused by the intense underground tectonic movement.The gravity anomaly caused by Moho fluctuation is closely related to the regional crustal thickness.The residual gravity anomaly basically reflects the horizontal distribution of the density of the crust.(4)The density distribution characteristics of each depth are closely related to the regional residual gravity anomalies,the density of different depth layers is characterized by lower northwest region,higher southeast region and increasing gradually from west to east.The inversion results are basically consistent with CRUST1.0 crustal density model,the western Sichuan plateau crust material is weak,prone to "east" and "backlog" in the Longshan fault zone,the Longmenshan fault zone is in the steep zone of regional density change,and the density difference between east and west is large,which is the strong zone of density "exchange",the Sichuan Basin is a craton crust,and the crustal material is stable and effectively blocks the expansion of the western Sichuan plateau.