Stress and seismicity changes on the Sunda megathrust preceding the 2007 M (sub w) 8.4 earthquake

The Bengkulu M (sub w) 8.4 earthquake on 12 September 2007, close in time and space to the 2004 M (sub w) 9.2 Sumatra-Andaman and 2005 M (sub w) 8.7 Nias megathrust events, suggests that it could be a triggered earthquake. It was located in the southern portion of the historic 1833 M (sub w) 8.9 rupture. However, it appears perplexing that the portion of the Sunda subduction zone between the Nias and Bengkulu rupture patches, which last ruptured in a 1797 M (sub w) 8.7 event, did not recur first. Coulomb failure stress (CFF) modeling of the 2004 and 2005 megathrust earthquakes and subsequent postseismic relaxation processes fails to explain why the 2007 patch ruptured before the northern 1797 segment. Surprisingly, the much smaller 2000 M (sub w) 8.0 Enggano earthquake produced a much larger positive CFF change at the 2007 hypocenter and may help to explain the southern location of the 2007 earthquake. Investigation of changes in seismicity rates in the region following the 2004-2005 events shows that the megathrust earthquakes may have dynamically triggered slip near the northern end of the 2007 rupture zone. A large increase in seismicity levels following the 2000 earthquake may also have influenced the eventual initiation point of the 2007 earthquake. Regrettably, the section of the Sunda megathrust near Siberut that last ruptured in 1797 still poses a great seismic hazard to the region as the only segment not to have ruptured in the sequence of twenty-first century megathrust earthquakes.