Gravity field changes reveal deep mass transfer before and after the 2013 Lushan earthquake

Abstract

The 2013 MS7.0 Lushan earthquake, Sichuan, China, occurred on a blind thrust fault in the southern Longmenshan fault belt. The terrestrial hybrid repeated gravity observation enables us to investigate the redistribution of both surface and deep mass. Here, we find a transient increase in the gravity field about 2 years before the earthquake and a drop after the mainshock. A Bayesian inversion method with spatiotemporal smoothness is employed to extract the apparent density changes. The increase of apparent density on the south of the focal zone is assumed to be related to crustal mass transfer. We introduce a disc-shaped equivalent source model with a homogeneous density to address this hypothesis, and estimate the model parameters by Markov Chain Monte Carlo simulations. As a fluid diffusion footprint is indicated by the seismicity migration in this region, with a fitted diffusion rate of 10 m2 s−1, we conclude that such deep crustal mass transfer may be caused by fluid diffusion.