Application Of Gravity-magnetic Joint Inversion In Deep Structure Research Of Longmenshan Fault Zone

The Longmenshan fault zone is located on the eastern edge of the Qinghai-Tibet plateau and the western side of the Sichuan basin.The geological structure of the region is complex and prone to earthquakes.Since the Wenchuan earthquake in 2008 and Lushan earthquake in 2013,the Longmenshan fault zone has become a hot research spot for scholars.There is a seismic void of about 40 km length between the two seismic sources.In this paper,I study the deep structure of the seismic void and the two source regions based on joint inversion of the gravity and magnetic data.Which help understand the seismicity of the seismic void and the uplift mechanism of Longmen mountain.In this paper,the gravity and magnetic data are jointly processed to obtain the density and magnetic profiles of this region.Based on these profiles and some geological information,the deep structure is understood and inferred.I collect and process the gravity and magnetic data from the research zone.Joint inversions are conducted to provide three gravity and magnetic profiles to obtain deep structure models,which respectively pass through the Wenchuan earthquake source,Lushan earthquake source and earthquake void region.Based on the existing geophysical data in the research area and previous research results,the following conclusions can be drawn:(1)For the two earthquakes,the common point is that the source is located at a place with high magnetic density and high magnetic properties,and the difference is that their stress and power sources are different.(2)The comparison between the source region and the seismic void shows that,compared with the source region and the source region,there are no high-density and high-magnetic blocks at the same depth as the two sources.The change of lateral density around the source area of the two earthquakes is more intense than that of the seismic void,but the change of lateral density in the seismic region is relatively less,which may indicate that the surface of seismic void is relatively smooth,or the surface uplift of seismic void is mainly plastic rheology,while the surface uplift of the two earthquake sources is mainly elastic fracture.Partial melting exists in the upper mantle of the seismic void.The above results indicate that the likelihood of a major earthquake is low.(3)I try to discuss Longmenshan uplift mechanism and a possible model is supposed.The upper and lower crust decoupling of Songpan-Ganzi block and the lower crust of the Yangtze block are passively wedge into the weaker Songpan-Ganzi block with the middle crustal slip surface,which further leads to the shortening of the upper crust and the uplift of Longmenshan.