Evidence of sudden rupture of a large asperity during the 2008 Mw7.9 Wenchuan earthquake based on strong motion analysis

We investigate the rupture process of the 12 May 2008 Wenchuan earthquake using the records of 26 strong‐motion stations, located 20–120 km from the seismic fault and with a good azimuthal coverage. The wave velocity model required to conduct this analysis has been validated against aftershocks, for which the point source hypothesis is a very good approximation. The inversion of the main shock rupture process confirms the slip distribution and the average rupture velocity (∼3 km/s) previously determined. However, a very peculiar behavior is clearly resolved by the extensive data set used in this study: the major slip area of the Wenchuan earthquake, located at 20–50 km North‐East of the epicenter, is shown to break almost simultaneously, 25 s after earthquake initiation. This implies that slip in this part of the fault cannot be understood by simple stress release at the rupture front. A more likely interpretation is the presence of a strong asperity, which could break only when it was completely surrounded by stress increase, resulting in a delayed but brutal rupture.