Asymmetry Between Intersesimic Strain Accumulation and Coseismic Strain Release in Northeastern Japan Associated with the 2011 Tohoku Earthquake

During the 2011 Tohoku Earthquake, the 200km-wide northeast Japan island arc was extended by about 4 m in the east-west direction, resulting average extensional strain of about 20 ppm. Coseismic strain release shows a smooth spatial decay from the source region, implying that the elastic property is rather homogeneous over the whole island arc crust. We have also been monitoring interseismic strain accumulation in the same area for last 15 years. Interseismic strain accumulation was more uniformly distributed over the whole island arc with an average strain rate of 0.1-0.2ppm/yr. In addition, we have noticed concentrated strain zones along the Ou Backbone range and the Japan Sea coast. In these strain zones, interseismic strain rate is larger than the surrounding area by a factor of 2-10, but the elastic response to the coseismic deformation is not very prominent, implying that inelastic deformation process is prevailing in interseismic period. We analyze GPS data from the nationwide GPS network and dense GPS networks in Niigata prefecture to extract contribution of inelastic deformation. The result is crucially important in estimating long-term seismic hazard in the inland fault zones. Correct evaluation of inelastic component is important for estimation of elastic strain accumulation in the plate boundary. The comparison between interseismic and coseismic deformation pattern clearly indicates that many of previous analyses overestimated the interplate slip deficit by attributing inland inelastic contribution to the plate interface.