Abstract
We evaluate the effect of the viscoelasticity in the upper mantle, especially possible viscous wedge mantle, on the postseismic deformation, by comparing the displacements following the 2004 Sumatra-Andaman earthquake obtained by continuous GPS during three and a half years. A three-dimensional finite element model is constructed to incorporate three dimensional heterogeneous structures considering the active back-arc opening in the Andaman Sea.
The results show that large postseismic displacements observed at GPS sites in Thailand, can be explained only by viscoelastic relaxation with a mantle viscosity ranging 5-10×1018 Pa s, except for the first six months. Postseismic displacements during the first six months may be caused by other mechanisms such as afterslip. Postseismic displacements computed for models with a zone of extremely low viscosity, which is expected from the active back-arc spreading in Andaman Sea, are inconsistent with the observed ones. This result implies the viscosity in the mantle wedge is not significantly lower than that in the surrounding mantle.
