Testing the intraplate origin of mega-earthquakes at subduction margins

Abstract

The disastrous Mw 9.3 (seismic moment 1.0×1030dyn/cm) earthquake that struck northwest Sumatra on 26 December 2004 and triggered ∼30m high tsunami has rejuvenated the quest for identifying the forcing behind subduction related earthquakes around the world. Studies reveal that the strongest part (elastic core) of the oceanic lithosphere lie between 20and 60km depth beneath the upper (∼7km thick) crustal layer, and compressive stress of GPa order is required to fail the rock-layers within the core zone. Here we present evidences in favor of an intraplate origin of mega-earthquakes right within the strong core part (at the interface of semi-brittle and brittle zone), and propose an alternate model exploring the flexing zone of the descending lithosphere as the nodal area for major stress accumulation. We believe that at high confining pressure and elevated temperature, unidirectional cyclic compressive stress loading in the flexing zone results in an increase of material yield strength through strain hardening, which transforms the rheology of the layer from semi-brittle to near-brittle state. The increased compressive stress field coupled with upward migration of the neutral surface (of zero stress fields) under non-coaxial deformation triggers shear crack. The growth of the shear crack is initially confined in the near-brittle domain, and propagates later through the more brittle crustal part of the descending oceanic lithosphere in the form of cataclastic failure.