Abstract: The time-varying gravity signal obtained by land gravity observation is an important basis for studying the medium change in the seismogenic region of the crust. This paper employs Bayesian adjustment method to process the mobile gravity observation data of West Sichuan from 2010 to 2012. It is found that the scale factor of the relative gravimeter can change up to 2×10^-4 in two years, and the instrument has a small nonlinear drift rate. The cumulative dynamic change image of gravity field shows that the 2013 Lushan M_S7.0 earthquake occurred near the intersection of zero value line of gravity gradient zone and Longmenshan fault zone. Furthermore, the equivalent source method is used to delineate the areas with high field source resolution centered on Ya'an, Kangding and Shimian, and to inverse the crustal apparent density changes before the earthquake. The results of dynamic variation of field source apparent density show that: during the two years before the 2013 Lushan M_S7.0 earthquake, a large area of increasing equivalent apparent density was formed in the south side of Longmenshan fault zone and the east of the southern section of Xianshuihe fault zone, and small-scale of increasing apparent density was also found in the area from Xiaojin to Kangding and around Dujiangyan. In this paper, the equivalent apparent density change in the crust before the 2013 Lushan M_S7.0 earthquake is obtained by inversion based on the land time-varying gravity measurements, which can provide a reference for the study of the characteristics of the seismogenic environment in the deep crust.

Key words: Terrestrial gravimetry, Bayesian adjustment, Gravity variation, Equivalent source, Field source apparent density inversion