Wednesday, October 27, 2010 4:02:39 PM
The earthquake occurred as a result of thrust faulting on or near the subduction interface plate boundary between the Australia and Sunda plates. At the location of this earthquake, the Australia Plate move north-northeast with respect to the Sunda plate at a velocity of approximately 57-69 mm/yr. On the basis of the currently available fault mechanism information (see “beach ball”) and earthquake depth it is likely that this earthquake occurred along the plate interface. The subduction zone adjacent to the region of this event last slipped during the Mw 8.5 and 7.9 earthquakes of September 2007, and the new event appears to have occurred near the rupture zones of those earthquakes. Today's earthquake is the latest in a sequence of large ruptures along the Sunda megathrust, including a M 9.1 earthquake that ruptured to within 800 km north of this earthquake in 2004; a M 8.6 700 km to the north between Nias and Simeulue in 2005; and a M 7.5 300 km to the north near Padang in 2009. The new earthquake occurred near the southern edge of a Mw 8.7-8.9 rupture in 1797 and within the rupture area of a Mw 8.9-9.1 earthquake in 1833.
This NASA image above shows the region where the earthquake and numerous aftershocks occurred on October 25 and 26, 2010. Bathymetry appears in shades of blue, and topography appears in shades of brown. Thin black lines delineate coastlines, and a thick black line marks the fault line in this region. The epicenter of the 7.7-magnitude earthquake appears as a red star. Aftershocks appear as red circles, with bigger circles indicating stronger aftershocks.
On my diagram below the area of the recent larger earthquakes is (approximately) shown in blue.
Further information at Dave’s Landslide Blog: http://daveslandslideblog.blogspot.com/2010/10/mantawai-islands-tsunami-and-eruption.html
If you read Italian Aldo Piombino has written a good account here: http://aldopiombino.blogspot.com/2010/10/il-terremoto-e-lo-tsunami-del-25.html