A gravitational image of the 2011 Tohoku-Oki earthquake from GRACE.

Understanding how and when far-field continuous motions lead to giant subduction earthquakes remains a challenge. An important limitation comes from an incomplete description of aseismic mass fluxes at depth along plate boundaries. Here we analyse Earth’s gravity field variations derived from GRACE satellite data in a wide space-time domain surrounding the M 9.0 2011 Tohoku-Oki earthquake. We show that this earthquake is the extreme expression of initially silent deformation migrating from depth to the surface across the entire subduction system. Our analysis indeed reveals large scale gravity and mass changes throughout three tectonic plates and connected slabs, starting a few months before March 2011. Before the Tohoku-Oki earth- quake rupture, the gravity variations can be explained by aseismic extension of the Pacific plate slab at mid-upper mantle depth, concomitant with increasing seismicity in the shallower slab. For more than two years after the rupture, the deformation propagated far into the Pacific and Philippine Sea plate interiors, suggesting that subduction accelerated along 2,000 km of the plate boundaries in March 2011. This gravitational image of the earthquake’s long-term dynamics provides unique information on deep and crustal processes over intermediate timescales, which could be used in seismic hazard assessment. This presentation will describe the science objectives and related simulations, the EntrySat system and mission, the current status of the CubeSat platform and ground segment, and provide a return of experiment at this stage of the project.