Viscoelastic Postseismic Deformation Following the 2012 Mw8.6 East Indian Ocean Earthquake

Abstract:

The 2012 Mw8.6 East Indian Ocean earthquake ruptured multiple planes in an orthogonal conjugate fault system, about 100 km west of the Sumatra subduction zone. It is the largest intra-plate strike-slip event ever recorded. We processed time series of more than twenty continuous GPS stations in the region between the Andaman Islands and southern Sumatra. These GPS stations recorded up to ~30 cm coseismic displacements and up to ~10 cm cumulative postseismic displacements in the first year after the earthquake. The significant postseismic deformation provides a unique opportunity to better constrain the viscosity structure in the Indian Ocean asthenosphere across the subduction slab, to test the contribution of afterslip following the earthquake, and to better understand the interaction between the intra-plate fault system and subduction zone systems. We develop three-dimensional viscoelastic finite element models of the 2012 Sumatra earthquake to study these problems. In our model, we assume that the upper mantle is characterized by a bi-viscous Burgers rheology that is able to describe slow long-term deformation as well as very rapid short-term transient deformation. Our preliminary model indicates that the transient and steady-state viscosities of the oceanic mantle are about one order of magnitude higher than that of the mantle wedge whose long-term viscosity of ~10^{19} Pa s was estimated in a number of studies of deformation following recent subduction zone earthquakes in 2004, 2005 and 2007 along the Sumatra subduction zone.