Delayed triggering of the 2012 Mw 8.6 Wharton Basin earthquake
Thursday, 18 December 2014
The April 2012 Mw 8.6 Wharton Basin earthquake, followed by a magnitude 8.2 aftershock, is the largest ever instrumentally recorded strike-slip earthquake. The large magnitude event culminated a sustained strike-slip seismic activity in the diffuse boundary between the Indian and Australian plates and it reactivated some fracture zones of the Wharton Basin, previously simple structural discontinuities, into active faults. Previous studies (Delescluse et al., 2012; Wiseman and Burgmann et al., 2012) have shown the importance of the static Coulomb stress change due to the 2004 Mw 9.2 Aceh-Andaman and the 2005 Mw 8.6 Nias earthquakes in the triggering of the 2012 main shock. However, the mechanism responsible for the 7-year delay remains unclear. Here, we investigate the role of viscoelastic relaxation in the oceanic asthenosphere and aseismic slip on the N-S oriented reactivated fracture zones in the Wharton basin to explain the triggering mechanism. The deformation in the oceanic plate transitions from localized frictional sliding in the lithosphere to more distributed flow in the asthenosphere. There, the time-dependent deformation following large stress perturbations may induce significant stress changes at larger distances than coseismically. Similarly, if some fracture zones in the Wharton Basin have been reactivated into active faults, they may serve as efficient stress guides between the Sunda megathrust and the epicentral area of the 2012 event. To test these hypotheses, we use Relax (Barbot & Fialko, 2010), a numerical method that can evaluate coseismic and postseismic stress evolution using a realistic representation of the Sumatra subduction zone geometry and the Wharton Basin fracture zones, and rheological parameters inferred from laboratory experiments. Using typical parameters for power-law flow (Brace & Kohlstedt, 1980) and rate-strengthening friction (Marone et al, 1991), we infer the changes in confining pressure, shear stress and Coulomb stress near the 2012 Wharton Basin hypocenter as a function of time since the 2004 and 2005 megathrust earthquakes. Our study highlights the importance of time-dependent mechanisms of stress transfer in the triggering of large earthquakes at regional distance.