Crustal deformation mechanism exploration in the near-source area before Wenchuan M_S8.0 earthquake

Various deformation data showed that the deformation characteristics of near-source area were not obvious and a short-time quiet state existed before the Wenchuan M_S8.0 earthquake. In order to study the deep dynamic factors and deformation mechanism of the phenomenon mentioned above, based on the 13-issue repeated gravity data of Chengdu area from 1996 to 2007, which have been dealt with by adjustment processing, we obtained the spatio-temporal evolution characteristics of crustal density variation in near-source area along horizontal and vertical directions ten years before the Wenchuan earthquake by 3-D inversion of crustal density. The density variation distribution is ordered in space and mainly concentrates on the Longmenshan fault zone and its vicinity. Additionally, the range of density variation in deep is more obvious than sha-llow, which may mean the fault activity in deep is more significant. From the view of temporal distribution, the density variation rate is uneven, that is, it is severe 3--8 years before Wenchuan earthquake and unobvious in short term before the event. Based on the deformation characteristics and the dynamic evolution process of density variation in different depth before the Wenchuan earthquake, we deduced that the layered crust movement of Longmenshan fault zone results in small deformation in shallow and obvious density variation in deep from the dynamics perspective. In addition, we selected and improved the combination-hardening model as the appropriate seismogenic model for the Wenchuan earthquake according to the density variation characteristics of Longmenshan fault zone and its adjacent areas. Finally, we combined the dynamic process with generating mechanism of earthquakes so as to reveal the deformation mechanism in near-source area before Wenchuan earthquake.