The Far Reach of Megathrust Earthquakes: Evolution of Stress, Deformation and Seismicity Following the 2004 Sumatra-Andaman Rupture

Starting with the 2004 Mw 9.2 megathrust event, Southeast Asia has been home to an exceptional amount of seismic activity over the past eight years. The series of megathrust earthquakes have been imperfect dominoes, rupturing the northernmost section of the Sunda subduction zone in 2004, then the Nias segment next in line to the south in 2005, followed by the Bengkulu earthquake ∼750 km further south in 2007. The Bengkulu earthquake skipped over the northern Mentawai segment, which has not ruptured in a great event since 1797. However, the subduction zone has not been silent is this section. Analysis of focal mechanisms and geodetic data reveals the reactivation of the Mentawai backthrust system in the overriding plate, and a large, deep earthquake near the city of Padang in 2009 is shown through finite fault inversions and aftershock analysis to have obliquely ruptured the subducting slab. At the same time, the entire region spanning from the Indian Ocean, through the trench and forearc islands, and throughout Thailand has been aseismically deforming in response to the stress changes in the mantle following the megathrust earthquakes. Geodetic observations of postseismic deformation during the first five years following the 2004 earthquake have shown that the far-field regions of Thailand and the Malay Peninsula have moved more postseismically than coseismically, peaking at ∼0.4 m of horizontal displacement in Phuket. In 2012, the stress changes associated with this continued postseismic deformation, along with the initial push from the megathrust earthquakes, appears to have triggered the largest instrumentally recorded strike-slip earthquake. This was a complex earthquake, consisting of four conjugate fault segments, that ruptured the diffuse India-Australia plate boundary zone. Understanding how the faults interact throughout the subduction system, from the incoming plate, to the slab, to the megathrust interface, and overriding plate is an essential part of determining the future seismic hazard for Southeast Asia