Postseismic Uplift Along the Pacific Coast of Tohoku and Kanto Districts Associated with the 2011 off the Pacific Coast of Tohoku Earthquake

Abstract

The 2011 off the Pacific coast of Tohoku earthquake (M9.0) produced up to 1.2 m subsidence along the Pacific coast in northeastern Japan. Based on Global Positioning System (GPS) observations, continuing postseismic coastal uplift has been detected in the past six years after the main shock. By applying a 3D spherical Earth viscoelastic finite element model using the postseismic seafloor and terrestrial GPS observations as constraints, I demonstrate that this uplift is mainly caused by the postseismic viscoelastic relaxation of the asthenosphere. Although the model was constrained only based on horizontal crustal deformation, the vertical displacements predicted for six years after the 2011 Tohoku earthquake agree reasonably well with the time series of the observed uplift at sites along the Pacific coast including the southern Sanriku coast and Kanto district. I estimated the time at which the cumulative postseismic uplift will fully compensate the coseismic subsidence. The results show that large coseismic coastal subsidence on the southern Sanriku coast will be fully offset by the postseismic uplift within several decades. To the immediate north, the model underpredicts the postseismic uplift and possibly indicates unaccounted postseismic fault creep. Farther south, the postseismic uplift of the coast of the Kanto district has already exceeded the small amount of coseismic subsidence over the past six years, as predicted by the model. To prepare for future earthquakes, especially with respect to the coastal construction at fishery ports, it is important to construct a comprehensive rheological structure model based on geophysical observations including GNSS positioning.