Dynamic Mechanism Of Crustal Deformation In Sichuan-Yunnan Region And Interaction Among Strong Earthquakes

In order to discuss the dynamic mechanism of crustal deformation in Sichuan-Yunnan region and strong earthquake interaction, we employ the precise three-dimensional viscoelastic model in Sichuan-Yunnan region to explore the effect on crustal deformation that derived from the lower crust flow, different crust rheologies and active faults, through the numerous numerical simulation experiments, and finally obtain a relatively best-fitting dynamic model in Sichuan-Yunnan region. Then, we analyze the seismic interactions among the fifteen strong earthquakes following the Tonghai M7.7 earthquake in 1970, and obtain the stress evolution along the main active faults after the fifteen earthquakes, according to the combined(tectonic loading, coseismic and postseismic viscoelastic) stress change.The preliminary results show that the lower crust flow plays an important part in the crustal deformation in Sichuan-Yunnan region, and the modeled results correspond well with the GPS observation data when the lower crust flows faster about 10~11mm/a than the upper crust, whatever the magnitude, orientation or the fitness. The increasing viscosity in the middle-lower crust of South China block causes the kinematics features change in the adjacent blocks, which results in the modeled results corresponding better with the GPS observation data. Meanwhile, the active faults assimilate the relative differential movement between blocks, and the modeled results indicate that the left-lateral strike-slip rate of Xianshuihe fault zone reaches 9.55mm/a, while the slip rate changes from 3.1mm/a to 9.4 mm/a along the Anninghe fault, Zemuhe fault and Xiaojiang fault. Additionally, the maximum principal compressive stress expresses the significant differences among these sub-blocks. The stress orientation is nearly east-west in the Bayan Har block, which translates into the southeastern in South China block, northeastern in western Yunnan block, and finally the divergent radiation distribution in southern Yunnan block, according to the ideal dynamic model.When the Longling double earthquakes and Lancang-Gengma double earthquakes simplified as a single event, we calculate the Coulomb failure stress change caused by these strong earthquakes, the results indicate that six earthquakes occur in static stress increase region caused by the former earthquakes, two occur in the stress shadow and four uninfluenced, which suggests that one half is advanced for the twelve strong earthquakes following the Tonghai M7.7 earthquake in 1970, whatever the apparent frictional coefficient assuming 0.4 or 0.6. Concerning the effect of coseismic stress change, when the apparent frictional coefficient assumed 0.4, the Coulomb failure stress change on the fault plane and slip direction of the Songpan M7.2 earthquake on August 23 that imparted by the former Songpan M7.2 earthquake on August 16 is 0.217 MPa, which exceeds the threshold of 0.01 MPa, indicating the triggering effect. Results from finite element modeling show significant Coulomb stress growth following these strong earthquakes on northern segment of Longling-Lancang fault, northeastern segment of Menglian-Lancang fault, fault intersections along the northwestern of Xianshuihe fault, middle segment of Huya fault and the seismic gap between Wenchuan and Lushan earthquakes along the Longmenshan fault, and the maximum increase reaches 1.5MPa, which are suspected to be vulnerable to future seismic risks.