Rheological structure beneath the southern Sumatra constrained from postseismic deformation of the 2007 Mw8.4 Sumatra earthquake

The 2007 Mw8.4 southern Sumatra earthquake provides an opportunity to understand the rheological properties in the southern Sumatra, particularly in the Mentawai gap. In this study, we have derived the first 3-year GPS postseismic observations to study deformation processes based on a three-dimensional viscoelastic finite element model. In the model, a 2-km-thick shear zone attached to the fault is used to simulate the time-dependent and stress-driven afterslip. Model results indicate that a model with a heterogenous shear zone better fits the horizontal GPS observations than a model with a uniform shear zone. This heterogenous shear zone is divided into the southern shear zone and northern shear zone (Mentawai gap), which is separated by the southern edge of the Mentawai gap. The southern shear zone is further divided into an upper (depths of ≤ 20 km) and lower shear zone (depths of > 20 km). The viscosities in these three shear zones are determined to be 5 x 1017 Pa s, 1016 Pa s and 1018 Pa s, respectively. Model results indicate that a weakened mantle wedge is required to better explain the observed uplift in vicinities of the rupture area.