Deep mass redistribution prior to the Maule earthquake revealed by GRACE satellite gravity
Résumé
The impact of deep subduction processes on the generation of megathrust earthquakes is not well understood. An important limitation comes from the incomplete descriptions of aseismic deformations and mass redistributions at depth in the subduction system. In our study, we analyse the space-time variations of the Earth’s gravity gradient derived from the GRACE geoids from 2003 to 2014, to probe deep aseismic mass variations and their possible interactions with interplate seismicity along the Chilean margin. We use three different series of GRACE geoid models over a large region surrounding the rupture zone of the Mw 8.8 2010 Maule earthquake. From these data we reconstruct each month the Earth’s gravity gradients over a range of spatial scales in order to separate signals associated with mass sources of different sizes and orientations in the GRACE geoids. By analyzing theire short-term variations before and during the rupture we highlight an anomalous signal of large amplitude that appears ~2-3 months before the earthquake north of the epicentral zone and gradually increases until the seismic rupture. We show that this signal cannot be generated by a shallow hydrological source nor artefacts, and more likely originates from a mass redistributions within the solid Earth on the continental side of the subduction. The gravity gradient variations can be explained by extension of the subducted slab at ~150 km depth along the subduction direction of the Nazca plate. The lateral migration of the gravity gradient signal towards the epicenter from January to March 2010 suggests that the Mw 8.8 Maule earthquake could have originated from the propagation up to the surface of this deeper slab deformation. Our results underline the importance of time-varying Earth gravity field satellite observations to probe aseismic mass redistributions in depth major plate boundaries and their interaction with interplate seismicity.