Broad-scale gravity changes following the 2011 Tohoku-Oki thrust and 2012 Indian Ocean strike-slip earthquakes and implications for coseismic dilatation and viscoelastic relaxation

Friday, 19 December 2014: 8:45 AM
Shin-Chan Han, NASA Goddard Space Flight Center, Greenbelt, MD, United States, Jeanne M Sauber, NASA Goddard SFC, Greenbelt, MD, United States, Frederick F Pollitz, USGS, Menlo Park, CA, United States, Riccardo Riva, Delft University of Technology, Delft, Netherlands and Emile A Okal, Northwestern University, Evanston, IL, United States
We analyzed spatially- and temporally-continuous GRACE gravity observations by decomposing the gravity field changes into five independent moment tensor elements to understand the regional surface and interior deformation in response to post-earthquake stress/strain redistribution. For the 2011 Tohoku-Oki thrust earthquake, the GRACE data revealed postseismic gravity increase by 6 microGal over a 500-km scale within a couple of years, which is nearly 40–50 % of the coseismic gravity change. It originates mostly from changes in the isotropic component corresponding to the Mrr moment tensor element. For the 2012 Indian Ocean strike-slip earthquakes, the postseismic gravity change was similar to the coseismic change (including the Mw 8.6 and Mw 8.2 ruptures) with the magnitude smaller by ~80 % in two years. The gravity change corresponding to the Mtp moment tensor element is dominant (and also Mtt–Mpp to a lesser extent). In both earthquakes, the exponential decay with rapid change within a year and gradual change afterwards is a characteristic temporal pattern. We also compared the two earthquakes in terms of their respective seafloor vertical and interior deformation (Bouguer gravity). The processes responsible for the coseismic and postseismic gravity changes at a such spatial scale are, respectively, the density change (dilatation) and the viscoelastic deformation without much perturbation in density. The postseismic gravity variation is best modeled by bi-viscous relaxation with a transient and steady-state viscosity of 10^18 and 10^19 Pa s, respectively, for the asthenosphere. Furthermore, we found viscoelastic relaxation triggered by the partially-ruptured elastic lithosphere is a main driver of the local subsidence above the rupture region reported from the GPS-acoustic seafloor surveying after the 2011 Tohoku-Oki earthquake.