B.1

Retrieving short-term preseismic signals before the occurrence of great subduction earthquakes is a major goal for seismic hazard mitigation. It requires a continuous monitoring of seismic and aseimic deformations within the entire subduction system, which cannot be obtained from space geodetic measurements of ground deformations and seismology only. With its unique sensitivity to mass redistributions at all depths along and around the descending slab, satellite gravity can bring key information to overcome this observational gap.

Taking the example of the 2011 Tohoku earthquake, we develop here a global and systematic analysis of time series of GRACE-reconstructed gravity gradients, to test whether the deep gravity signals preceeding the earthquake (Panet et al., 2018 ; Bouih et al. 2022) can be detected before the occurrence of the event, as a specific feature originating from solid Earth. Our approach is based on a refined detection of short-term gravity variations aligned with the subduction direction, and consistency tests between signals shared by different GRACE geoid models, allowing us to evaluate the signals robustness with respect to errors and oceanic corrections in the GRACE gravity solutions. Considering the GRGS and the CSR solutions, the results of these tests underline the unique character of the deep pre-Tohoku gravity variations compared to the usual variability of gravity, confirming its interpretation as pre-seismic signal.

We compare the preseismic gravity signals of the 2011 Tohoku and 2010 Maule earthquakes, and conclude that our approach can be applied to the systematic monitoring of the Pacific subduction belt.

How to cite: Panet, I., Narteau, C., Lemoine, J.-M., Bouih, M., Bonvalot, S., and Remy, D.: Detecting Preseismic Signals in GRACE Gravity Solutions: application to the 2011 Tohoku earthquake, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-76, https://doi.org/10.5194/gstm2022-76, 2022.

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.

How to cite: Bouih, M., Panet, I., Remy, D., Longuevergne, L., and Bonvalot, S.: Deep mass redistribution prior to the Mw 8.8 Maule earthquake (Chile, 2010) revealed by GRACE satellite gravity, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-47, https://doi.org/10.5194/gstm2022-47, 2022.