Detection of Plastic Strain Using GNSS Data of Pre- and Post-Seismic Deformation of the 2011 Tohoku-oki Earthquake

In general, there are three mechanisms causing crustal deformation: elastic, viscous, and plastic deformation. The separation of observed crustal deformation to each component has been a challenging problem. Meneses-Gutierrez and Sagiya (2016, EPSL) have successfully separated inelastic deformation from observed geodetic data from the comparison of GNSS data before and after the 2011 Tohoku-oki earthquake in the northern Niigata-Kobe tectonic zone (NKTZ), central Japan. In this study, we further succeed in separating plastic deformation as well as viscous deformation in the northern NKTZ using GNSS data before and after the 2011 Tohoku-oki earthquake, under the assumptions that elastic deformation is principally caused by the plate coupling along the Japan trench and that plastic deformation ceased after the Tohoku-oki earthquake due to the stress drop caused by the earthquake. The cease of plastic deformation can be understood with the concept of stress shadow used in the field of seismic activity. The separated strain rates are about 30 nanostrain/yr both for the plastic deformation in the preseismic period and for the viscous deformation in both the pre- and post-seismic periods, which means that the inelastic strain rate in the northern NKTZ is about 60 and 30 nanostrain/yr in the pre- and post-seismic periods, respectively. This result requires the revision of the strain rate paradox in Japan. The strain rate was exceptionally faster before the Tohoku-oki earthquake due to the effect of plastic strain, and the discrepancy between the geodetic and geologic strain rates is much smaller in usual time, when the plastic strain is off. In oder to understand the onset timing of plastic deformation, the information on stress history is essentially important.