Physical modeling of postseismic deformation following the 2008 Iwate-Miyagi Nairiku Earthquake

Abstract:

The Iwate-Miyagi Nairiku Earthquake (IMNE) occurred on June 14, 2008 (Mw 6.8). Large postseismic deformation following the mainshock has been detected by GNSS and InSAR (e.g., Iinuma et al., 2009; Ohzono et al., 2012; Takada et al., 2011). But physical models of the postseismic deformation of IMNE previously proposed only explain GNSS data (Iinuma et al., 2009; Ohzono et al., 2012). In this study, we try to construct a physical model that explains InSAR data as well as GNSS data.

To detect the post-seismic deformation, we used ALOS PALSAR data taken from descending orbit. A series of post-seismic interferograms that we processed display the LOS increase in the hypocentral area indicative of a long-term afterslip, and the LOS shortening to the west of Mt.Kurikoma, an active volcano, which strongly implies the existence of large-scale crustal heterogeneity.

Next, we construct a physical model of the postseismic deformation following IMNE based on a half analytic solution, named Unicycle (Barbot et al., 2017; Moore et al., 2017). With Unicycle, we consider (1) mutual interaction between the afterslip and the viscoelastic relaxation, (2) afterslip following the rate-and-state friction law (e.g., Dieterich, 1979; Ruina, 1983), and (3) horizontal heterogeneity of rock rheology that follows nonlinear power-law creep predicted from room experiments (e.g., Kirby, 1983). We started from a simple model with a stratified rheological structure proposed by Ohzono et al. (2012). The fault model consists of east and west dipping (i.e. conjugate) thrust faults proposed by Abe et al. (2013). The calculated postseismic displacement field well explains the far-field GNSS data as well as the near-field GNSS data in the eastern side of the hypocentral area. However, there are large discrepancies between the calculated and the observed displacements along the western side of the hypocentral area.