Isostatic Gravity Anomaly Of The Sichuan-Yunnan Region,China

Isostatic gravity anomaly(IGA),caused by anomalous sources within the crust,acts as a sign of equilibrium state of the crust and is closely related to seismic activity.The geological structure of the Sichuan-Yunnan region is complex and the seismic activity there is also frequent.The crust of this region has been active since the Wenchuan earthquake,and earthquakes in Yushu,Lushan,Lutian and Jinggu have occurred subsequently.Research on the isostasy of the Sichuan-Yunnan region can provide references and background information for its seismic tectonic environment.Based on four different models(local,regional,experimental and dynamic models),the IGA of the Sichuan-Yunnan region is calculated and then compared.At last,we analyze the crustal activity(such as earthquakes)in the study region with reference to the IGA.The main work and conclusions are as follows:1.The formulae of the four models are deduced;corresponding program is developed and then availability-verified.2.Based on the four models,gravity-terrain data are processed to obtain the IGA.The result shows that most of the study region is in an isostatic state while the Tibetan Plateau and the Longmenshan region do not reach the equilibrium state.The negative IGA in the Tibetan Plateau are relatively large and should be related to the uplift and crustal thickening of the Tibetan Plateau caused by the Indian-Eurasian continental collision.The anti-isostatic phenomenon in Longmenshan region can be associated with the eastward migration of crustal materials of the Tibetan Plateau.3.The results show that the distributions of IGA in the four models resemble each other in a large scale.Among them,the result of the experimental mechanism is the most isostatic while the regional one is consistent with the crustal structure in the study region.4.IGA is closely related to seismic activity.The distribution of the IGA in the study region is extremely uneven,showing a significant characteristic of block difference.The earthquakes are mainly distributed in areas where the IGA varies rapidly,especially in the Longmenshan,Xianshuihe-Zemuhe,Xiaojiang fault zones.