Numerical Simulation Study on the Influence of Branching Structure of Longmen Shan Thrust Belt on the Nucleation of Mw7.9 Wenchuan Earthquake

Abstract

On the Longmen Shan thrust belt (LMS) on the eastern margin of Tibet Plateau, the Mw7.9 Wenchuan earthquake occurred in 2008. As for the dynamic cause of the Wenchuan earthquake, many scholars have studied the rheological difference and terrain elevation difference on both sides of the fault. However, previous studies have simplified the LMS as a single listric-reverse fault. In fact, the LMS is composed of four faults with different dip angles in the shallow part, and the faults are Wenchuan-Maoxian fault (WMF), Yingxiu-Beichuan fault (YBF), Guanxian-Jiangyou fault (GJF) and Range Front Thrust (RFT) from west to east. However, the control of the branching structure of these faults on the distribution and accumulation of stress and strain during the seismogenic of the Wenchuan earthquake has not been discussed. In this paper, four viscoelastic finite element models with different fault numbers and combination structures are built to analyze the effect of fault branching structures on the stress distribution and accumulation during the seismogenic of Wenchuan earthquake, and we use geodetic data such as GPS and precise leveling data to constrain our models. At the same time, we also study the influence of the existence of the detachment layer, which is formed by the low-resistivity and low-velocity layer, between the upper and lower crust of the Bayan Har block and the change of its frontal edge position on the stress accumulation and distribution. The results show that the combinations of YBF and GJF is most conducive to the concentration of equivalent stress below the intersection of the two faults, and the accumulated stress on GJF is shallower than that on YBF, which means that more stress is transferred to the surface along GJF; and the existence of a detachment layer can effectively promote the accumulation of stress at the bottom of YBF and GJF, and the closer the frontal edge position of the detachment layer is to the LMS fault, the more favorable the stress accumulation is. Based on the magnitude of stress accumulation at the bottom of the intersection of YBF and GJF, we speculate that the frontal edge position of the detachment layer may cross YBF and expand eastward.