Stable forearc stressed by a weak megathrust: Geodynamic implications of a stress reversal caused by the M=9 Tohoku-oki earthquake.

The rupture-zone averaged static stress drop in the 2011 M=9 Tohoku-oki earthquake wasless than 5 MPa according to all published slip models, but it caused a reversal of the state ofstress in most of the offshore forearc, from predominantly compressive to predominantlyextensional. In this work, we demonstrate that this stress reversal has the followingimportant geodynamic implications. (1) The reversal unequivocally indicates avery weak subduction megathrust. In order to reproduce this reversal in a finiteelement model of force balance that quantifies the effects of gravity and megathrustfriction, the effective coefficient of friction of the megathrust has to be about 0.032. Alower value would make the forearc too extensional before the earthquake, and ahigher value would make it too compressive after the earthquake. (2) The very weakmegathrust results in very low differential stresses in the upper plate, consistent withpreviously reported sensitivity of the state of stress to small perturbations. Applying thedynamic Coulomb wedge model, we demonstrate that the inner wedge, and byinference the nearshore-onshore forearc, is in a stable state throughout subductionearthquake cycles, far from failure. (3) The outer wedge is normally in a stable state butmay reach an extensionally or compressively critical state during an earthquake,depending on the behavior of the shallow megathrust. Gravitational collapse of theouter wedge is prevented by a finite strength of the underlying shallow megathrust.Therefore, complete stress drop (i.e., effective coefficient of friction decreases tozero) is unlikely to have happened over the main part of the shallow megathrustexcept for limited local areas. (4) The presence of permanent deformation such asearthquakes and active faulting in the overall stable forearc can be explained bystructural and stress heterogeneities. We propose that for the most part, the upper platenear a subduction zone is much below its yield stress and is an elastic body, butpermanent deformation can locally occur in areas of low strength, high stress, orhigh fluid pressure. (5) The concept of dynamic Coulomb wedge (Wang and Hu,2006, JGR) needs to be expanded to address timescales much beyond subductionearthquake cycles. The strength of the megathrust varies over geological timescales,affecting the stability of the outer wedge and its response to earthquake rupture. [ABSTRACT FROM AUTHOR]