Abstract
Determinations of afterslip distribution from postseismic deformation observed with GPS are important to infer frictional characteristics of faults on plate boundary zones . To obtain a more reliable afterslip, however, we should also model the contribution of viscoelastic deformation and subtract it from the observed displacement before the inversion. This indicates that it is also important to increase the accuracy of estimation of viscoelastic relaxation. However, in theories of viscoelastic postseismic deformation based on spherical Earth models, which should enable us to rigorously evaluate far-field deformations, compressibility has been neglected so far. In this paper, we use a method which was recently developed to account for compressibility in a spherically symmetric Earth model and show that its effect on post seismic deformation caused by a typical large earthquake in a subduction zone is detectable with GPS. Next, we apply this method to the 2003 Tokachi-Oki Earthquake as an example. A preliminary result of a forward calculation shows that the viscoelastic deformation can explain the spatial variation of the postseismic trend in the GPS data, except for the first few months after the event. The good agreement between the theory and the model suggests a possibility that viscoelasticity has begun to take effect at a detectable signal level in the observed postseismic deformation.
