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Determination of Three-Dimensional in Situ Stresses from Anelastic Strain Recovery (ASR) of Wenchuan Earthquake Fault Scientific Drilling Project Hole-1 (WFSD-1) and Formation Mechanism of The “5.12” Wenchuan Earthquake
详细信息   
摘要
The stress existing in the crustal rock mass is called in-situ stress. The ASR ( short for anelastic strain recovery) technique is widely used indeep3-D stress measurements of rocks in crustaldrilling,especially in seismic fracture zones with complexgeologic conditions and broken formations. The 3-D ASR measurement results of the well WFSD-1 show that there are major differences between the Longmenshan foreland obduction zone and its underlying Longmenshan foreland basin and overlying Songpan-Garzê block in terms of structure and stress state.0n the whole,during the Wenchuan Earthquake,the Longmenshan foreland obduction zone displayed SW to NE "laminar"flow ofdeep materials along the inter-crustal detached layer,and then turned into rapid vertical extrusion along the Yingxiu-Beichuan fault in the upper crust;while the Songpan-Garzê block on itswest side showed SE to NW gravitational slipping,and the Longmenshan foreland basin on its east side showed NE to SW strike-slip ordextral rotation. There has notoccurred apparent or large-scale intracontinental subduction of the Yangtze block toward the Longmenshan orogenic zone on the eastern margin of the Qinghai-Tibet Plateau since the Late Cenozoic. The rapid and vertical up-going flow and extrusion ofdeep high-temperature low-viscosity materials in the Longmenshan foreland obduction zone is thedirect cause for the occurrence of the May-12 Wenchuan Earthquake,while the intensive compression and crustal thickening of the Longmenshan foreland area as well as accumulation ofdeep stress and seismic energy as a result of E-trending expansion of the Songpan-Garzê block are the main cause inducing abrupt change of thedeep-seated shift field and rapid vertical extrusion of materials. The latter is the necessary condition for the former and not thedirect result. The orientation of the principal compressive stress obtained by the ASR stress measurement isexactly parallel to the shiftdirection of the GPS coseismal velocity field,which indicates that the in-situ stress measured with the ASR technique may actually reflect or most approach the tectonic stress stateduring the earthquake.

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