Modeling Stress Changes Following the 2004-2005 Sumatra Earthquake Sequence: Exploring Triggering of the 2007 Rupture and its Relationship to the 1797 and 1833 Events

The September 2007 Sumatra M8.4 earthquake initiated in the southern section of the 1833 rupture segment. Twelve hours later a deeper M7.9 aftershock ruptured further to the north. Their occurrence, close in time and space to the 2004 M9.2 and 2005 M8.7 Sumatra earthquakes, suggest the possibility of these being triggered events. Earlier studies of vertical motion, derived from coral growth histories, suggest that interseismic strain accumulated along the 1833 segment has approached levels relieved in the historic earthquake. Along the 1797 segment the accumulated interseismic strain appears to have exceeded previously relieved levels. Following the 2004 and 2005 events, investigations of coseismic and viscoelastic deformation show that the 1797 segment had higher Coulomb failure stress changes than the 1833 segment. Therefore, it is puzzling that the 1797 and northern 1833 rupture zones did not recur before the southern 1833 segment. We model additional postseismic processes, including afterslip and poroelastic rebound, in an attempt to find scenarios that can explain why the southern 1833 segment was triggered prior to the 1797 segment.