Deep mass redistribution prior to the Maule earthquake revealed by GRACE satellite gravity

The control on megathrust earthquake generation exerted by deeper subduction processes remains poorly understood and still insufficiently documented. Here, we use the 2003-2014 space-time variations of the Earth’s gravity gradients derived from the GRACE geoids in order to probe aseismic mass variations at depth and their possible interactions with intraplate seismicity along the Chilean margin. We work with three different datasets of GRACE geoid models over a large region surrounding the rupture zone of the Mw 8.8 2010 Maule earthquake. In order to separate signals associated with mass sources of differents sizes, shapes or orientations, we reconstruct each month the Earth’s gravity gradients at different spatial scales from these geoid models. Our analysis emphasizes a highly anomalous, large-amplitude gravity gradients signal that appears three months prior to the earthquake North of the epicentral zone, and progressively increases until the megathrustal rupture, in all three datasets. We show that this large signal cannot be caused by a shallow hydrological source nor by GRACE striping artefacts and dealiasing models. Instead, we conclude that its most likely origin is in mass redistributions within the solid Earth on the continental side of the subduction zone. These anomalous gravity gradient variations could be explained by a deep extensional deformation of the slab around 150-km depth along the Nazca Plate subduction direction, driving large-scale fluid motion in the subduction zone and into the overriding lithosphere. Our results highlight the importance of observations of the Earth’s time-varying gravity field from satellites to probe aseismic mass redistributions in-depth major plate boundaries . The detection of such mass redistributions at depth by GRACE and their interactions with interplate seismicity opens a new field of research to better characterize and understand the dynamics of the seismic cycle at megathrusts.