Abstract

Thrust and reverse faults pose significant earthquake hazards in convergent plate margins around the world, but have proven difficult to study given the complex nature of their ruptures, which often involve multiple along-strike and vertically stacked fault segments. The 2013 Mw 6.6 Lushan earthquake exemplified this complexity, rupturing a blind thrust fault in the southern Longmen Shan, which border the western Sichuan Basin in China. This event occurred 80 km south of the epicenter of the destructive 2008 Mw 7.9 Wenchuan earthquake. The Wenchuan earthquake produced surface ruptures on two parallel fault splays, the Pengguan and Beichuan faults. In contrast, the Lushan earthquake was generated by a ramp in the Range Front blind thrust (RFBT), which is in the footwall of the Wenchuan rupture. We use seismic reflection profiles, petroleum wells, and relocated seismicity to construct a three-dimensional model of this imbricated fault system. Our model illustrates that the 2013 Lushan earthquake ruptured <10% of the RFBT, which extends for 250 km along the Longmen Shan range front and into the western Sichuan Basin. Analysis of growth strata in structures above the RFBT fault along strike shows clear evidence of Quaternary activity and constrains the middle Pleistocene to current slip rate at two locations on the fault. Single segment and multisegment fault rupture scenarios involving the RFBT suggest the potential for large earthquakes (M7.8) that would affect the densely populated western Sichuan Basin. Assessing the hazards posed by such complex thrust systems, which occur in convergent margins worldwide, requires subsurface characterization of fault segments that can be independently associated with geologic and seismologic evidence of fault activity.

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