Interplate coupling in and around the rupture area of the 2011 Tohoku earthquake (M9.0) before its occurrence based on terrestrial and seafloor geodetic observations

In this study, we investigated the state of interplate coupling between the subducting Pacific plate and continental plate around the rupture area of the 2011 Tohoku Earthquake (M9.0) based on seafloor geodetic data as well as terrestrial GPS data obtained before the M9 earthquake in order to evaluate the potential for further seismic activity from the viewpoint of the slip deficit budget balance. Due to the difference between the site spatial densities on land and the seafloor observation networks, it was necessary to utilize a covariance matrix with non-zero off-diagonal components in the inversion to estimate the interplate coupling distribution on the plate interface and to account for the correlations among the observation errors for components and stations. We validated the implementation of the assumed covariance matrix by performing a computational test and we then applied the method to actual geodetic data obtained before the 2011 Tohoku Earthquake. The results demonstrate the persistence of the interplate coupling in the area of a very large coseismic slip near the Japan Trench.