Abstract:

The 2008 Mw 79 Wenchuan earthquake had caused the rupture of two NEstriking imbricated reverse faults and one NWstriking reverse fault along the Longmenshan range, which forms the eastern boundary of the Tibetan Plateau. On the basis of the morphology of the coseismic fault scarp, we distinguish eight different categories of fault scarps, namely: the simple thrust scarp, the hangingwall collapse scarp, the simple pressure ridge, the dextral pressure ridge, the faultrelated fold scarp, the backthrust pressure ridge, the local normal fault scarp and the pavement suprathrust scarp. The surface rupture zone can be divided into two parts, the Yingxiu segment and the Beichuan segment. The Yingxiu segment is dominated by reverse deformation with a smaller component of rightlateral motion, and the Beichuan segment is dominated by right obliqueslip faulting with comparable horizontal and vertical motion. These two segments correspond to a Mw 78 event and a Mw 76 event, respectively. They can in turn be divided into four secondorder subsegments, which are equivalent to four subevents of Mw 75, Mw 77, Mw 70 and Mw 75, respectively. The cascadingrupture can be used to explain why the earthquake lasted for 110 seconds. The analytical results of the aftershock focal mechanism show that the seismogenic fault dip angle is slowing down with depth, and is gradually steepening from southwest to northeast, which could be used to explain the increases of strikeslip component. This major earthquake confirms that upper crustal shortening can produce the high topography of the Longmenshan without calling on other uplift mechanisms.

Key words: Wenchuan earthquake, earthquake surface rupture zone, fault segmentation, cascadingrupture pattern, Longmenshan thrust belt, Tibetan Plateau