[S13-5] Earthquake source mechanics V

[S13-5-04] Why did the moderate size 2010 Yushun, China earthquake (Mw=6.8) produce supershear rupture?

^○Shoubiao Zhu, Jie Yuan (Institute of Crustal Dynamics, China Earthquake Administration)

Owing to supershear ruptures, seismic hazards in the 2010 Yushu, China earthquake(Mw=6.8) is particularly severe. So far, the mechanism remains unclear. To this end, we construct finite element model based on the actual geometry of the seismogenic fault in the earthquake. In the model, the fault consists of two segments, and there is an intersection angle of 10º between them, forming fault bent. The simulation result shows that the main rupture for the Yushu event is composed of two sub-events. Once the rupture was nucleated at the hypocenter, the rupture first propagated along the first fault segment at speed of subshear wave velocity. When the rupture goes over the fault bent, the rupture speed turned into supershear wave velocity immediately along the second fault segment. Moreover, the calculation results suggest that dislocations at fault surface, seismic wave speed and strong ground motion acceleration are largely amplified by supershear ruptures, giving rise to terrible damage. This may be an important reason why severe seismic damage was caused in the Yushu earthquake. In particular, we can see from numerical experiment that rupture speed would not change if the strike of the fault in the model does not bend in the case in which all other model parameters keep unchanged. However, if the intersection angle between the orientation of initial stress and the fault varies, rupture will not necessarily propagate at supershear wave speed, even if there is a fault turning in the fault system. Only if the relationship between the orientation of initial stress and the strike of the fault is just perfect, the fault bent possibly promotes supershear ruptures. Thuse, supershear rupture occurred in the Yushu event may be resulted from optimum state in which perfect relationship between orientations of initial stress and the strike of seismogenic fault is formed. It is presumably one of the reasons why natural earthquakes with supershere ruptures are rarely seen.