The 2013 MS7.0 Lushan earthquake, Sichuan, China, occurs on a blind thrust fault in the southern Longmenshan fault belt. The terrestrial hybrid gravity observations enable us to investigate how the deep mass changes before and after the earthquake. Based on the gravity measurement data, we find a transient increase in the gravity field about 2 years before the earthquake and a drop after the mainshock. To find out the cause, a Bayesian inversion method with spatiotemporal smoothness prior is employed to extract apparent density changes. We hypothesize that the significant increase of apparent density over the region on the south of the focal zone might be related to mass transfer in the deep crust. Therefore, we introduce a disc-shaped equivalent source model with a homogeneous density to address this hypothesis. Based on Markov Chain Monte Carlo simulations, we also estimate that the disc-shaped model has a radius of about 96 km, with a thickness of about 1.2 km and a depth of about 14 km. As a fluid diffusion footprint is indicated by seismicity migration in this region, with a fitted diffusion rate of 10 m2/s, we conclude that such deep crustal mass transfer may be caused by fluid diffusion.

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