Estimation of the long-term effect of viscoelastic relaxation induced by the 2011 Tohoku earthquake and other interplate earthquakes around northeastern Asia

After the large earthquakes, postseismic deformation is induced by their stress disturbance. The large coseismic and postseismic deformation caused by the 2011 Tohoku-oki earthquake observed by GNSS not only in Japan, but also in the northeast (NE) Asia. Even though six years passed, several IGS sites in NE Asia still show clear postseismic signals. The postseismic deformation in far-field is mainly caused by the viscoelastic relaxation. And, the effect lasts for several decades or longer period. Therefore, stable tectonic movement, such as plate motion, must be contaminated by this phenomenon. Quantitative estimation of this effect with preferable rheological structure is important to understand the tectonic behavior with the earthquake cycle around the focal area. In this study, we assume the long-term widespread postseismic deformation is mainly caused by the viscoelastic relaxation. Then, we predict the spatio-temporal evolution of the viscoelastic relaxation by the 2011 Tohoku-oki earthquake using VISCO-1d. The signal of surface displacement depends on rheological structure (thickness of elastic layer and viscosity of the asthenosphere). We search appropriate sets of those parameters. As a result, when we set 65 km of elastic thickness and 1018 Pa s of viscosity of the viscoelastic layer, observations are reproduced. The estimated elastic thickness corresponds to a boundary of the high/low seismic velocity distribution beneath the Sea of Japan. Japan is a typical plate subduction zone, therefore many large interplate earthquakes have already occurred here. There are 15 earthquakes over magnitude 8.0 have occurred since 869 in and around Japan. In order to check the effect of their possible postseismic deformation, we also estimate the surface displacement induced by the viscoelastic relaxation for those earthquakes with estimated rheological structure. As a result, five earthquakes may contaminate the displacement field around Vladivostok, Russia. The displacement signal from each postseismic deformation depends on the magnitude of earthquake, focal distance, and so on. This estimation implies that we cannot ignore the effect of these postseismic deformation in extensive region of NE Asia. (This study is supported by RFBR grant No. 17-55-53110, KAKENHI No. 26109007, and KAKENHI No. 25257204.)