Characteristics of postseismic deformation following the 2003 Tokachi-oki earthquake and estimation of the viscoelastic structure in Hokkaido, northern Japan

Abstract

Postseismic deformation of the 2003 Tokachi-oki earthquake (M w 8.0) has been observed by GNSS. We analyzed the deformation observed in Hokkaido in the 2nd to the 7th year following the 2003 Tokachi-oki earthquake and examined the effect of two major mechanisms (i.e., afterslip and viscoelastic relaxation) for the observed postseismic deformation by fitting it with a model consisting of afterslip and viscoelastic relaxation. The thickness of the lithosphere, the viscosity of the asthenosphere, and the time decaying constant of afterslip were estimated to be 50 km, 2.0 × 1019 Pa s, and 0.110 year, respectively, which are concordant with those in the Tohoku region estimated in previous studies. The revealed characteristics of postseismic deformation are as follows. At most of the used stations, afterslip played the dominant role in the 2nd year and was still sustained near the coseismic area even in the 7th year. However, the calculated velocity due to viscoelastic relaxation was comparable to that due to afterslip at the stations in northern Hokkaido after the 5th year. Because the calculated velocity due to viscoelastic relaxation was landward near the coseismic slip area, afterslip near the coseismic slip area will be biased to be smaller if viscoelastic relaxation is ignored. A systematic spatial pattern of the residuals considering afterslip only highlights an importance for explaining the observation data. We also examined the effect of viscoelastic relaxation due to afterslip for the parameter estimation and found that it was too small to affect the estimated structure parameters.