DISCUSSION ON EARLY FEATURES OF WENCHUAN M8.0 EARTHQUAKE SEQUENCE AND ITS CAUSES ON MAY 12,2008

The features of Wenchuan M8.0 earthquake and judgment of the sequences are useful to develop the work of earthquake relief.The research of the causes of great earthquakes are useful to realize the earthquake prediction and forecast.The study results show that: ① The aftershocks of the earthquake distributed along the Longmenshan mountain fault zone,and mainly in the north side of the mainshock,the earthquake zone is 100km width,about 330km length.②There were two rapid energy attenuation in early period of the sequence.③ The type of Wenchuan M8.0 earthquake sequences was judged to be main-aftershock pattern.The max aftershock is Qingchuan M6.4 earthquake on May 25,2008.④ Time-space evolution,the focal mechanism of bigger aftershock and the results of aftershock precisely location in the sequence proved that the sequence shows an obvious segmented feature.⑤ The Longmenshan mountain fault zone is major triggering seismic fault with spade fault profile.A step of moho depth form Tibet to Sichuan basin,crossed through Longmenshan mountain,was revealed by the result of geophysical exploration and inversion of Crust-Mantle velocity structure.The step is a barrier of the crust of Tibet eastward movement and makes the movement rate slowed down and great energy of M8.0 Wenchuan accumulated.The broken east-westward stress balance generates the earthquake.The aftershocks of the sequences show zonal distribution along to Longmenshan mountain fault zone,and it's the result of north-eastward movement under the broaching of India plate.The south section and north end of the Longmenshan mountain fault zone moved simultaneously and the all fault zone was actived.