Fault-scarp featuresand cascade-rupturing model for the Wenchuan earthquake(M W 7.9),eastern Tibetan Plateau,China

Guihua Yu,Xiwei Xu(徐錫偉),Yann Klinger,Guilin Diao,Guihua Chen,Xiangdong Feng,Chenxia L i,Ailan Zhu,Renmao Yuan,Tingting Guo,Xinze Sun,Xibin Tanand Yanfeng An

1)National Center for Active Fault Studies,Institute of Geology,CEA,

Beijing 100029 2)Institut de Physique du Globe de Paris,CNRS-INSU,Paris 3)Earthquake A dministration of Hebei Province,Shijiazhuang 050021 4)Earthquake A dm inistration of Shanghai Municipality,Shanghai 200062

The post-earthquake field investigations reveal that the MW7.9 Wenchuan earthquake of 12th May 2008 ruptured three NE-striking imbricate reverse faults and another NW-trending reverse fault,along the middle Longmenshan fold-and-thrust belt at the eastern margin of the Tibetan plateau.The fault-scarp features can be categorized into eight group s:simple thrust scarp,hanging-wall collapse scarp,simplepressure ridge,dextral pressure ridge,fault-related fold scarp,back-thrust pressure ridge,local normal fault scarp and crocodile-mouth-like scarp.The local normal scarp is first discovered in the reverse-faulting earthquakes as ever reported in the world.Field observation indicates that the Wenchuan earthquake surface rupture is dominated by reverse faulting with a minus right-lateral component,but the relative ratio varies from site to site.Also,the surface ruptures can be divided,for the first order,into two segments,the Yingxiu and Beichuan segments,corresponding to MW7.8 and MW7.57 events,respectively.The two segments further can be divided,for the second order,into four sub-segments in total,which are equivalent to four sub-events of MW7.46,MW7.69,MW6.99 and MW7.52,respectively.The fault segmentation,for different orders,shows a cascade-rupturing pattern and can exp lain why the quake time of the Wenchuan earthquake was so long as up to 100 second.A ftershock focal mechanisms are also used to constrain the fault geometry for the sub-segments,indicating that the seismogenic faults are listricat depth and in general,the fault plane becomes steeper northward,which enables the fault to accommodate larger strike-slip motion.This earthquake also confirm s that the crustal shortening across the Longmenshan fold-and-thrust belt should be responsible for the grow th of high topographic relief along the eastern margin of the Tibetan Plateau.