Physical mechanism for extremely serious seismic damage in the Beichuan area caused by the great 2008 Wenchuan earthquake

The Beichuan area suffered the most serious seismic damage during the 2008 Wenchuan earthquake although over 100 km away from the instrumental epicenter of the mainshock. The mechanism for this peculiar phenomenon remains unclear even though nearly 10 years has passed since the event. To address this issue, we construct a spontaneous rupture model in which the Gaochuan right bend (GRB) in the middle of the Yingxiu-Beichuan fault, a major seismogenic structure for the Wenchuan event, is included. The simulation results show that the complex geometry of the GRB plays a first-order role in controlling the rupture propagation. While rupture is initiated at the epicenter of the Wenchuan mainshock, it propagates spontaneously northeastward at a speed slower than the shear wave velocity of local media. When the rupture front spreads near the end of the Yingxiu-Gaochuan fault, a new rupture is re-nucleated at the curve section of the Gaochuan bend, and propagates in the NE direction with the speed greater than the S wave velocity. In particular, this rupture speed transition from subshear to supershear does not need time delay, much different from the case of a fault step over which was studied well by previous work. Due to the curved geometry of GRB, the stress regime on the fault section favors the occurrence of super shear rupture. Once the super-shear rupture occurs along the Beichuan fault, seismic waves are focused and strongly amplified with the form of Mach waves. The numerical results also illustrate that high values of spatial distribution of the strong ground motion acceleration are mainly located in the Beichuan area, directly leading to grave seismic catastrophe. Therefore, this work may give some insight into why the most serious seismic damage occurred in the Beichuan area, and may have important implications for understanding earthquake dynamics and assessing seismic hazards.