Absolute and relative gravity data during 1998 to 2008 were used to study gravity field and temporal variation in the North-South seismic-belt region, and their correlation with seismic activities before and after Wenchuan Ms8.0 earthquake. The temporal variation of gravity field shows that the portentous information of the gravity field reflects the development and occurrence of earthquake more clearly. The variations of gravity field are inhomogeneous in the space-time distribution, and are associated with the development and occurrence of the Wenchuan Ms8.0 earthquake, also closely connected with active fault tectonics.
Based on leveling data in 1972 -2011 and relative-gravity data in 1993 -2011, we obtained a longterm vertical crustal-deformation rate of 1.62mm/a and a relative-gravity variation rate of 0.62 × 10^-8 ms^-2a^-1 for the northeastern margin area of Qinghai-Tibet plateau. After removing the contributions from the observed vertical movement and inferred surface denudation, we obtain a gravity-variation rate of 0.73 × 10^-8 ms^-2a^-1 attributable to the mass changes beneath the crust. This positive change suggests that the total mass under the observation stations was gradually increasing. We consider this result to be the gravitational evidence of underplating beneath the study area, and propose that the underplating was caused by collision betwen the Indian plate and Tibetan plateau and by gravitation-potential induced deviatoric stress.
