Partial rupture of a locked patch of the Sumatra megathrust during the 2007 earthquake sequence

The great Sumatra-Andaman earthquake and tsunami of 2004 was a dramatic reminder of the importance of understanding the seismic and tsunami hazards of subduction zones1-4. In March 2005, the Sunda megathrust ruptured again, producing an Mw 8.6 event5 south of the 2004 rupture area, the site of a similar event in 18616. Concern was focused on Mentawai area, where large earthquakes had occurred in 1797 (Mw 8.8) and 1833 (Mw 9.0) 6, 7. A magnitude 8.4 and twelve hours later, a magnitude 7.9 earthquake indeed occurred on September 12, 2007. Here we show that these earthquakes ruptured only a fraction o