Deep mass redistribution prior to the Mw 8.8 Maule earthquake (Chile, 2010) revealed by GRACE satellite gravity

Subduction zones are places of intense seismic activity where the largest ruptures occur. Studies on the causal mechanisms of subduction earthquakes generally focus on the accumulation of tectonic stress and strain at the shallow plates interface, which can be documented from surface displacements and seismic activity. The control exerted by deeper sudbuction processes is however not well understood. It can be addressed from time-varying satellite gravity data, that provide a new and unique means of studying mass redistributions at intermediate spatial and temporal scales throughout the volume around plate boundaries, and in particular at depht.

Here we use gravity gradients from GRACE geoid to probe slow deep mass variations and their possible interactions with intraplate seismicity along the Chilean margin. We work with three different GRACE geoid models (GRGS, CSR,ITSG) from 2003 to 2014, over a large region surrounding the rupture zone of the Mw 8.8 2010 Maule earthquake. From these data we reconstruct the Earth’s gravity gradients at different spatial scales in order to better separate signals associated with mass sources of different sizes, shapes or orientations in the GRACE geoids. Our analysis reveals an anomalous gravity gradient signal north-east of the epicentral zone, which amplitude progressively increases during the months preceding the earthquake. This signal is consistently detected in all 3 GRACE solutions and we show that it cannot be explained by a water mass redistribution nor artefacts. Instead, it could be explained by an extension of the plunging Nazca plate near 150 km depth along the subduction direction. The migration of the gravity signal laterally and from the depths to the surface from a weakly coupled zone in the North to a strongly coupled zone in the South suggests that the Mw 8.8 earthquake may have originated the propagation of this deep slab deformation towards the surface. Our results highlight the importance of time series of satellite observations of the Earth’s gravity field, to detect and characterize mass redistributions at depth of major plate boundaries at timescales of month to years.