Co-seismic and post-seismic viscoelastic Coulomb stress transfer after the 2004 M9.2 Sumatra-Andaman earthquake: Effects on the 2012 M8.6 Sumatran strike-slip earthquake

Abstract:

The 11 April 2012 M8.6 Sumatran earthquake off the northwestern coast of Sumatra is the largest recorded strike-slip earthquake and is thought to be associated with complex oblique subduction of the Sumatra subduction system. The 2012 M8.6 earthquake was predominantly a right-lateral strike-slip earthquake within the subducting Indo-Australian oceanic plate and the mainshock consisted of two noticeable subevents: Subevent 1 had its strike roughly in the northeast direction and the northern end of its seismicity zone approached the Sumatra trench; Subevent 2 was located at the southern end of Subevent 1, had a strike roughly in the northeast direction, and its seismicity zone extended westward reaching the Ninetyeast Ridge. In this study we investigate the co-seismic and post-seismic stress interaction between the 2012 M8.6 Sumatran strike-slip quake and the 2004 M9.2 Sumatra-Andaman earthquake on the Sumatra subduction zone. We first calculated co-seismic coulomb stress change on the rupture planes of the 2012 earthquake caused by the 2004 M9.2 Sumatra-Andaman earthquake. The calculated Coulomb stress increase is largest (~9 bar) on the northern end of the rupture plane of the 2012 Subevent 1 approaching the the Sumatra subduction zone. The calculated Coulomb stress was positive on most of the Subevent 1 rupture plane but with values decreasing significantly towards the southwestern end of the rupture zone. The rupture plane of Subevent 2 is roughly parallel to the Sumatra subduction zone; the 2004 M9.2 earthquake was calculated to have decreased confining normal stresses on much of the rupture plane of Subevent 2. We also modeled the oblique subduction of the Sumatra subduction system and calculated time-dependent post-seismic viscoelastic stress transfer after the 2004 M9.2 quake. Overall, the modeling results suggest multiple-scales of stress interaction between the 2012 M8.6 Sumatran earthquake and the 2004 M9.2 Sumatra-Andaman earthquake in a complex oblique subduction system.