Deep ruptures around the hypocenter of the 12 May 2008 Wenchuan earthquake deduced from aftershock observations

Abstract On 12 May 2008, a great earthquake of Mw 7.9 occurred in Wenchuan, China. To record its aftershocks and investigate the distribution of deep ruptures associated with the event, we deployed a temporary short-period seismic network around the epicenter soon after the earthquake had occurred. A total of 6036 aftershocks were located from 108 days of seismic data with a high signal-to-noise ratio, collected from July to October 2008. These aftershocks were further processed using a double-differential location method, and 2878 aftershocks were retained in the relocated clusters. According to the distribution of aftershocks, we refined the location of the main shock hypocenter to 31.001°N, 103.280°E, 17 km deep. The spatial distribution of aftershocks reveals interesting features of the deep ruptures: (1) most aftershocks are located in the upper crust at depths shallower than 20 km, beneath surface outcrop of the Pengguan massif, indicating that the upper crust is brittle and seismogenic and that the brittle–ductile transition is located at the transition between the middle and upper crust; (2) a NW–SE-striking low-seismicity zone crossing through Yingxiu Town divides the study area into two blocks with contrasting distributions of deep seismicity, suggesting different rupture characteristics either side of the line; and (3) upward extrapolation of aftershock clusters intersects with the southern and northern arms of the Zipingpu Reservoir, indicating that aftershocks and future seismicity along the ruptures around the reservoir that were activated by the Wenchuan earthquake will be influenced by reservoir impoundment/drainage for several decades to come.