Rheological Structure Beneath NE Japan Inferred from Coseismic Strain Anomalies Associated with the 2011 Tohoku-oki Earthquake (Mw9.0)

Abstract

Strain concentration zones (SCZs), in which the E–W contraction under a slow tectonic loading is larger than the surrounding area, in NE Japan have been attributed to low viscosity anomalies (LVAs) in their lower crust. The 2011 Tohoku-oki earthquake (Mw9.0) induced a stepwise stress change over NE Japan. The coseismic E–W extension in the SCZ along the Ou backbone Range (OBR) was smaller than theoretical one, whereas it was larger in the forearc SCZ (FSCZ). This suggests variation in rheological structure beneath the SCZs. We numerically evaluated responses of variety of rheological models to the tectonic slow loading and the coseismic instantaneous unloading. A model with a viscoelastic upper crust, which is caused by high temperature relating to magmatic processes along volcanic front, below the OBR can reproduce the observed deformations in the OBR. The surface deformations in the FSCZ were explained as enhanced deformations of thick, compliant sediment. The viscoelasticity in the upper crust was not allowed beneath the FSCZ. LVA in the lower crust was not essential to reproduce the preseismic and the coseismic deformation anomalies in the FSCZ. However, the postseismic deformation was strongly affected by the existence of LVA in the lower crust. Precise observations of the postseismic deformations should provide key clues to elucidate the rheological structure beneath the FSCZ.