Geodetic evidence of viscoelastic relaxation after the 2008 Iwate-Miyagi Nairiku earthquake

Continuous GPS observations, for over two years, detected long-term postseismic deformation after the 2008 Iwate-Miyagi Nairiku earthquake (M _j 7.2). The displacement field exhibits ESE-WNW shortening and subsidence near the focal area. These features are attributed to a viscoelastic relaxation caused by the mainshock. A simple two-layered structural model, which consists of an elastic layer having a thickness of 19.0-23.5 km and an underlying Maxwell viscoelastic layer having a viscosity of 2.4-4.8 × 10¹⁸ Pa s, explains the far-field deformation pattern, which probably reflects the viscoelastic response exclusively. These estimated parameters are consistent with the deeper limit of the seismogenic layer in the upper crust and the previous rheological model in northeastern Japan. However, near-field deformation requires additional sources in order to reproduce the observed postseismic deformation, such as long-term afterslip and/or a complicated response due to the highly heterogeneous structure suggested by seismic tomography studies.