Afterslip distribution following the 2003 Tokachi-oki earthquake: An estimation based on the Green's functions for an inhomogeneous elastic space with subsurface structure

摘要

We derive the 1-yr afterslip distribution following the 2003 Tokachi-oki (Hokkaido, northeastern Japan) earthquake (M_w 8.0) by inverting geodetic data, i.e., horizontal and vertical displacements, at 142 land stations of the Global Positioning System (GPS) in Hokkaido and northernmost Tohoku districts, together with vertical displacements at two offshore stations of pressure gauge (PG) off the Pacific coast of Hokkaido. We use the Green's functions (GFs), calculated with a finite element method, for an inhomogeneous elastic (IE) model incorporating subsurface structure. Obtained results show a striking feature of the distribution pattern of significant afterslip, namely, a U-shaped afterslip zone encircling the co-seismic rupture zone of the 2003 event. Amounts of the 1-yr afterslip reach up to 0.9 m, and total seismic moments released from all afterslip zones are of the order of 1021 N m, corresponding to an earthquake of M_w 8.0. For comparison, we also estimate the 1-yr afterslips based on GFs for the homogeneous elastic (HE) model to find that the total seismic moment with GFs for the IE model is larger than that with GFs for the HE model by ～33% (when the most probable values are compared) if we assume a rigidity of 40 GPa. This result implies that inhomogeneities due to subsurface structure have an important role in geodetic inversions.