This animation shows slip on the megathrust at the very southern end of the 2004 earthquake, in the 2005 earthquake, and in the 6th April 2010 earthquake.  According to these slip models, the 2010 slip patches filled in areas of lower slip in 2005, between the patches of highest slip in 2005, although the anti-correlation is not perfect.  The 2004 and 2005 slip distributions are based on both seismology (shaking detected at instruments all over the world) and geodesy (direct measurements of how the earth's surface moved above and near the earthquake rupture); the preliminary 2010 slip distribution is based only on seismology at this point, and is thus less well determined. 

The EOS, along with collaborators at LIPI in Indonesia, will be sending geologists into the field in the coming weeks to make observations of geodesy, which will ultimately improve the accuracy of the slip distribution map for the 6th April 2010 earthquake.  The 2004 slip distribution is from C. Subarya et al., Nature 440, 46 (2006); the 2005 slip distribution is from R. Briggs et al., Science 311, 1897 (2006); the 2010 slip distribution is from G. Hayes, USGS.

On Tuesday, April 06, 2010 at 22:15:02 UTC, an earthquake of M 7.7 struck offshore northern Sumatra, about 20 km north of the Banyaks, a group of small islands situated about halfway between the two larger islands of Simelue and Nias, and also roughly halfway between these islands and the western coast of Sumatra. The depth of the focus is given by USGS to be 35 ± 5 km, and the focal mechanism is consistent with thrust faulting on the shallow north-dipping subduction interface between the India- Australia and Asian plates.

This earthquake can be considered part of the sequence that started with the great, M 9.2 Aceh –Andaman earthquake of 26/12/2004, and continued southwards since with the 2005, M 8.6, Nias and 2007, M 8,4 and 7.9, Mentawai/Bengkulu earthquakes. It appears to have ruptured a small patch of the subduction interface downdip from that which slipped during the 2005 event.

It follows the M 7.6 event of 30 September 2009, destructive in Padang, which was a completely different type of earthquake, deep within the down-going plate, on a steep north-trending plane with a significant strike-slip component of motion.

Offshore Padang, there remains a large un-ruptured patch of the subduction interface, capable of generating an earthquake of M > 8.5, beneath Siberut and the Mentawai islands. This patch  overlaps the sources of the 1797, M 8.4 and 1833, M 9 events.

Sumatran Earthquake of 6 Apr 2010 : PDF (776KB)

Briggs, R., Sieh, K., Meltzner, A., Natawidjaja, D., Galetzka, J., Suwargadi, B., Hsu, Y-j., Simons, M., Hananto, N., Suprihanto, I., Prayudi, D., Avouac, J.-P., Prawriodirdjo, L., Bock, Y., Deformation and slip along the Sunda megthrust in the Great 2005 Nias-Simeulue Earthquake. Science 31 March 2006 311: 1897-1901 [DOI: 10.1126/science.1122602].
[PDF]

Hsu, Y.-J., M. Simons, J.P. Avouac, J. Galetzka, K. Sieh, M. Chlieh, D. Natawidjaja, L. Prawirodirdjo, Y. Bock, Frictional afterslip following the Mw 8.7, 2005 Nias-Simeulue earthquake, Sumatra, Science, 30 June 2006 312: 1921-1926 [DOI: 10.1126/science.1126960].
[PDF] [PDF]