Stress switching in subduction forearcs: Implications for overpressure containment and strength cycling on megathrusts

Seismogenic megathrusts contained within subduction interface shear zones (SISZ) appear generally to be overpressured to near-lithostatic values (lambda(v) > 0.9) below forearc hanging-walls. Solution transfer within fine-grained material along the deeper interface (150 < 350 degrees C) contributes to hydrothermal sealing of fractures lowering bulk permeability. Down-dip variations in overpressuring likely affect the depth of the peak in frictional shear resistance which may serve as the prime asperity affecting megathrust rupture. To account for postseismic changes in the velocity structure of the fore-arc hanging-wall following the 1995 Antofagasta, Chile, M(w)8.0 megathrust rupture, Husen and Kissling (2001) proposed massive trans-megathrust discharge of fluids across the interface. Such discharges are a form of 'fault-valve' action where the megathrust itself acts as a seal to overpressured fluids derived from within the SISZ and from dehydration of the descending slab. Brittle failure or fault reactivation limits fluid overpressure which is highest at low differential stress under a compressional stress regime. Over much of the forearc hanging-wall of the 2011 M(w)9.0 Tohoku-Oki megathrust rupture, focal mechanisms show that the stress-state switched from compressional reverse-slip faulting prefailure to extensional normalslip faulting postfailure. Mean stress and fault-normal stress thus changed from being greater than vertical stress prefailure, to less than vertical stress postfailure. Reductions in overpressure are expected from a combination of poroelastic effects and fluid loss through fault-fracture networks enhancing postfailure permeability in the changing stress field. Local drainage across the subduction interface increases frictional strength significantly, giving rise to a postfailure distribution of strength asperities. The amplitude of strength variations from such fluid discharge is potentially large (