Evidence for power-law rheology of the Wharton Basin asthenosphere

Abstract:

Laboratory experiments indicate that the viscous deformation of olivine-rich mantle material is thermally activated and controlled by a power law, whereby the effective viscosity depends on stress. The rheology of olivine controls the depth to the asthenosphere where ductile flow becomes weaker than the frictional strength of rocks. Evidence for nonlinear viscoelastic deformation has been found in the continental mantle (Freed & Burgmann, 2004), but never before in the oceanic context. Here, we analyze the transient deformation following the 2012 Mw 8.6 Wharton Basin earthquake that was recorded by continuous geodetic stations along Sumatra to show strong evidence for power-law flow in the Wharton Basin asthenosphere. A combination of afterslip on the reactivated fracture zones and viscoelastic relaxation in the asthenosphere can fully explain the geodetic time series. But whereas Newtonian viscoelastic relaxation and afterslip around the main shock imply widespread subsidence around Banda Aceh, the models that include power-law flow successfully reproduce the regional postseismic uplift. These observations provide us with unprecedented insight about the mechanism of deep, far-reaching, and time-dependent stress transfer between seismic events along the Sumatra subduction zone.