There were just two M5.1 quakes offshore Sumatra...at the equator...at the southern end of the March 2005 M8.7 rupture. The focal mechanism on the first one (thrust with NE-dipping plane being one choice) and the depth permit it being on the subduction zone. The 2nd does not have focal mechanism yet. As I've been saying over the last year, it is the area south of the equator which should fire off some Great earthquake(s)...sometime in the next 100 years...probably sooner more likely than, say, 99 years from now. There is some finite chance that it will take more than 100 years...
Chris