Numerical simulation study on the impact of the Wenchuan earthquake to the seismic environment of the Lushan earthquake

Abstract

The finite element numerical simulation method was used to investigate the impact of the Wenchuan earthquake on the seismic environment of the southern Longmenshan Mountain. The three-dimensional viscoelastic geological model, including the Longmenshan front-range fault, central fault, back-range fault, the Bayan Kala block, and the South China block, was constrained by the results of the geological survey, coseismic dislocation, and the GPS observation of Wenchuan and Lushan earthquakes. We studied how the calculation results were influenced by gravity potential energy caused by high altitude topography, the low velocity layer in the middle crust and the rupture of the Beichuan-Yingxiu and Jiangyou-Guanxian faults, by modifying the parameters of a model. The Wenchuan earthquake was simulated by reducing the Young’s modulus of the possible generating fault. We calculated the Coulomb stress of the Dayi blind fault, Dachuang-Shuangshi fault, and the “Y”-shaped blind thrust fault between the two faults above. The simulation results show that the Coulomb stress has a similar distribution on the fault planes of the southern Longmenshan faults zone caused by the Wenchuan earthquake, considering the low-velocity layer or the double-rupture surface. The Coulomb stress on the top of the Dachuan-Shuangshi fault surface was about 0 MPa, and it gradually reduced towards the southwest direction. At the source of the Lushan earthquake, the Coulomb stress was about −0.02 MPa. For the west branch of “Y”-shaped fault, the Coulomb stress on the fault surface was significantly reduced and was about −0.02–−0.03 MPa at the source of the Lushan earthquake; after the Wenchuan earthquake, the Coulomb stress on the east branch of the “Y”-shaped fault plane was about −0.01–0.03 MPa; for the Dayi fault, the Coulomb stress was increasingly obvious on the fault surface from the southwest to the northwest after the Wenchuan earthquake. Therefore, the Wenchuan earthquake could reduce the risk of an earthquake on the Dachuan-Shuangshi fault and the west branch of “Y”-shaped fault earthquake, but it increases the risk on the east branch of “Y”-shaped fault and the Dayi fault. The maximum Coulomb stress near the source was located on the Dayi fault. Considering the terrain and the simultaneous failure of the Beichuan-Yingxiu and Jiangyou-Guanxian, the presence of the low velocity layer increases the Coulomb stress on the four faults, and obviously increases towards the east branch of the “Y”-shaped fault and the Dayi fault. If the terrain and the low velocity layer are considered, compared with only the ruptured Beichuan-Yingxiu fault, the Beichuan-Yingxiu and Jiangyou-Guanxian faults’ simultaneous rupture increased the risk of earthquakes on the Dachuan-Shuangshi fault, the east branch of “Y”-shaped fault, and the Dayi hidden fault, but reduced the risk of the west branch of “Y” type fault. In any of the above models, the Coulomb stress changes on the four fault planes of southern Longmenshan Mountains were influenced by terrain. In the case of certain crust, mantle medium properties, and spatial structure, the existence of the low velocity layer on the west of the Longmenshan fault zone, the topographic features of the Qinghai-Tibet Plateau to the Sichuan Basin, and the conditions for the simultaneous rupture of the central fault and the front-range fault during the Wenchuan earthquake, are the important factors that affect the Wenchuan earthquake to the Lushan earthquake. Among them, the terrain and low velocity layer have the most significant influence on the Coulomb failure stress for the area around the Lushan earthquake area caused by the Wenchuan earthquake. The effect of terrain features is more obvious in the north section of the selected section, yet the effect of the Wenchuan earthquake simultaneous rupture planes is smaller.