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Deep processes and dynamic responses associated with the Wenchuan M_s8.0 earthquake of 2008

TENG Ji-Wen,BAI Deng-Hai,YANG Hui,YAN Ya-Feng,ZHANG Hong-Shuang,ZHANG Yong-Qian,Ruan Xiao-MinInstitute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China  
The Wenchuan Ms8.0 earthquake of 2008 occurred on the Longmenshan tectonic zone which seems not very active from available data.And it is reported that no noticeable precursors have been observed before the event.It might imply that the crustal deformation at depth associated with this gigantic shock had no expression on the surface or shallow subsurface which was detectable.In this article,we attempt to analyze the processes of brewing and generation of this event at depth that can account for the reason of this devastating earthquake.Tectonically,it is obvious that the India-Eurasia plate collision serves as the most important dynamic setting of major earthquakes in southwestern China.The eastern end of the Himalayan chain is moving rapidly in NNE direction into the Tibetan plateau,forcing deep material to flow toward east.Due to the obstruction of the NE trending Longmenshan tectonic zone in the northwestern margin of the Sichuan basin,the partial flow turns to southeast.Longmenshan is a conspicuous boundary of topography and crust structure,across which the elevation difference is as large as 3500±500 m,with crustal thickness 60±5km in its west and 40±2 km in its east,respectively.Thus it must experiences intense deformation and stress accumulation,though very slow without appreciable manifestation.Based on available data,it is inferred that material of lower crust and covering strata of upper mantle is flowing eastward along two deep slide surfaces,of which one is the layer of low-velocity and low-resistivity(20~25 km depth),and the other is the top of mantle asthenosphere.Hindered by rigid mattter beneath the Sichuan basin,the flow of lower crust and covering strata of upper mantle material turns overthrusting at high angles at the Longmenshan.On the other hand,three thrust faults visible west of the Longmenshan extend downward to depth 20 km,where they converge into one active zone.Thus the Longmenshan tectonic zone is a place where occurs the coupling between slippage on faults of shallow subsurface and material flow of lower crust and covering strata of upper mantle.Under the intense exchange of matter and energy,the source medium ruptures suddenly at a large scale to release a huge amount of elastic strain,resulting in the Wenchuan Ms8.0 earthquake.
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