Abstract

On 12 May 2008, the Mw 7.9 Wenchuan earthquake ruptured two northeast-striking imbricated reverse faults and one northwest-striking reverse fault along the middle Longmen Shan thrust belt, at the eastern margin of the Tibetan plateau. The morphology of the coseismic scarp varies drastically along strike. We distinguish eight different categories: (1) simple thrust scarp, (2) hanging-wall collapse scarp, (3) simple pressure ridge, (4) dextral pressure ridge, (5) fault-related fold scarp, (6) backthrust pressure ridge, (7) local normal fault scarp, and (8) pavement suprathrust scarp. The coseismic surface ruptures confirm that the Wenchuan earthquake is dominated by reverse faulting with some right-lateral component that varies from site to site. The surface rupture can be divided into two parts, the Yingxiu segment and the Beichuan segment. When the earthquake is split into two subevents accordingly, they correspond to an Mw 7.8 event and an Mw 7.6 event, respectively. These two segments can in turn be divided into four second-order subsegments, which are equivalent to four subevents of Mw 7.5, Mw 7.7, Mw 7.0, and Mw 7.5, respectively. The segmentation of the rupture is consistent with a cascading-rupture pattern, responsible for the total 110 s of the earthquake rupture. In addition to surface ruptures, the focal mechanisms determined for the aftershocks recorded by the local seismic network are used to constrain the fault geometry of the subsegments. They show that the dip of the fault responsible for the earthquake varies along strike, and the fault tends to flatten at depth. In addition, the fault plane gets steeper northward, enabling the fault to accommodate a larger strike-slip component along a lateral ramp. This major earthquake confirms that crustal shortening could be the main driving force for the growth of high topography along the eastern margin of the Tibetan Plateau and that lower crustal flow is not required.

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