Temperature- and stress-dependent rheologic structure beneath Japan inferred from postseismic displacements following the 2011 Tohoku-oki earthquake

Abstract:

A dense GPS network throughout Japan as well as several seafloor stations have recorded significant horizontal and vertical postseismic displacements following the 2011 Tohoku-oki earthquake. We use these observations to constrain finite element models of viscoelastic relaxation and afterslip that take into account the complex geometry of both the Pacific and Philippian Sea Plate subduction zones. Our results suggest that postseismic deformations can be explained primarily by viscoelastic relaxation of coseismic stresses associated with a very heterogeneous rheologic structure influenced by the regional thermal structure and the stress state. Significant afterslip, generally considered to be the primary postseismic mechanisms following subduction zone earthquakes, appears to be limited to two specific zones within the deeper slab interface, where the two most prominent aftershocks occurred. Observed postseismic subsidence in the volcanic arc region of northern Japan is best explain by a shallow low viscosity zone consistent with high heat flow. Significant westerly seafloor displacements can be accounted for by viscoelastic flow either within a narrow asthenospheric channel below the Pacific plate (as previously reported) or within the lower half of the bending portion of the slab, indicative of low-temperature plasticity (Peierls mechanism). Results also indicate significant postseismic flow within the cold nose of the mantle wedge (temperatures less than 700°C), suggesting the low temperature plasticity may be active throughout the highly stresses regions of subduction zones. A muted postseismic response in the southern reaches of the coseismically-stressed region is consistent with the influence of the presence of strong, but thin subducting Philippian Sea Plate.