There was a M6.3 and 6.1 on or very close to the Great Sumatra fault, which is the right-lateral fault that takes up the strike-slip component of the oblique plate motions (the other part is subduction). The first, the 6.3, has a focal mechanism and is consistent with right-lateral parallel to GSF. The fault system is certainly capable of larger, but like Parkfield, does not mean it will happen any time soon. Might be interesting to know something of the paleo-seismology of that fault: when was the last large (M7+) quake nearby, and what is the history over the last 500 or 2000 years...
Chris