Teleseismic relocation and assessment of seismicity (1918-2005) in the region of the 2004 M-w 9.0 Sumatra-Andaman and 2005 M-w 8.6 Nias Island great earthquakes

The M-w 9.0 2004 Sumatra-Andaman Islands and M-w 8.6 Nias Island great earthquake sequences have generated over 5000 catalog-reported earthquakes along similar to 1700 km of the Sumatra-Andaman and western Sunda regions. studies of prior regional seismicity have been limited to global catalog locations that often have poorly constrained epicenters and depths. Approximately 3650 teleseisinically wellrecorded earthquakes occurring in this region during the period 1918-2005 are relocated with special attention to focal depth. Reduced uncertainties of epicenters and depths in the region (on the order of 15 and 10 km, respectively) foster interpretation of focal mechanism data and provide additional details about the subducting Indian and Australian plates. The revised earthquake dataset reveals a sharp delineation between aftershocks of the 2004 and 2005 earthquakes near Simeulue Island and a steepening in slab dip from south to north. The downdip width of the aftershock zone of the 2004 M-w 9.0 earthquake varies from similar to 200 km at its northern end to similar to 275 km at its southern end, and events located between 35 and 70 km focal depth occur more frequently in the southernmost section of this aftershock zone. Outerrise and near-trench normal and strike-slip faulting earthquakes also increase in frequency following the 2004 and 2005 earthquakes. Earthquake swarins triggered along the Andaman backarc spreading center both north of Sumatra and near Siberut Island, 100 km south of the Nias Island aftershock sequence, illustrate the complex and variable nature of seismicity following these great earthquakes.