Why did the most severe seismic hazard occur in the Beichuan area in the 2008 Wenchuan earthquake, China? Insight from finite element modelling

The Beichuan area suffered the most serious seismic damage in the 2008 Wenchuan earthquake although the Beichuan is over 100 km away from the instrumental epicenter of the mainshock. The mechanism for this peculiar phenomenon remains unclear even though 10 years has passed since the Wenchuan event. For this purpose, we construct a spontaneous rupture model in which Gaochuan right bend (GRB) is included in the middle of Yingxiu-Beichuan fault, a major seismogenic fault for the Wenchuan event. The simulated results show that the complex geometry of the GRB played a controlling role in the rupture propagation. Once the rupture was initiated at the epicenter of the Wenchuan mainshock, it propagated spontaneously, and the rupture speed on the first segment of the fault is ∼2.79 km/s, slower than the shear wave speed of local medium. When the rupture front spread near the end of the Yingxiu-Gaochuan fault, a new rupture was re-nucleated at the curve section of the Gaochuan bend, and propagated in the northeast direction with the speed of 5.02 km/s, greater than the S wave velocity. In addition, this rupture speed transition from subshear to supershear does not need any time delay, much distinct from the case of fault stepover in which delay is needed in supershear transition. The result also demonstrates that the relation between the high values of peak ground acceleration (PGA) and the supershear rupture is strong. The large area with high values of PGA distributed in the Beichuan directly led to grave seismic catastrophe there. 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.