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.
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