Persistent Aseismic Deformation in Central Japan Revealed by GPS Observation Before and After the 2011 Tohoku-Oki Earthquake

Abstract:

On March 11th, 2011 the occurrence of the Tohoku-oki Earthquake (M_w 9.0) considerably changed the observed crustal deformation pattern in Central -Eastern Japan, providing the opportunity to study the mechanical properties in the area, since elastic and inelastic mediums would have different response to the coseismic instantaneous displacement as well as transient postseismic displacement. In the near field of the 2011 Tohoku-oki earthquake, it is difficult to extract a pure deformation response since direct effects from the postseismic deformation source are mixed. In order to focus on the response at the far field, we analyzed postseismic crustal deformation in Central Japan. Before the quake, we observe general East-West contraction with localize deformation in the eastern margin of the Japan Sea, known as the Niigata Kobe Tectonic Zone (NKTZ) (Sagiya, 2000), whereas the coseismic response, which corresponds to the instantaneous elastic strain release, do not indicate highly concentrated strain feature, as expected if the deformation could be explain only by an elastic behavior. This implies that the interseismic and coseismic strains are not balanced and there is significant amount of contribution of inelastic processes. In the Earth’s crust, inelastic processes are expected to take a form of aseismic faulting, and should be distinguished from elastic behavior by its spatial wavelength. Also, the inelastic behavior can be persistent before and after the quake since the coseismic stress change in the far field is considered to be too small to change the regional stress that drives the inelastic process. Thus we review temporal variation of crustal deformation pattern in Central Japan based on daily coordinates of 837 GPS sites from GEONET, before and after the Tohoku-oki earthquake in order to distinguish elastic and inelastic behavior within the NKTZ. We found that the deformation in the area can be explained as a combination of two processes: an elastic general pattern, in response to the plate boundary activity, and a source of inelastic deformation such as aseismic faulting driven by the regional stress field.