OK, correction: I was forgetting that USA longitudes are negative and -93 is west of -90. In Sumatra longitudes are positive and 93 is east of 90. So, where I was saying that aftershocks were to the ENE, it is actually WNW, which matches the nodal planes well.

I talked to Nano and he reminded me to look at the map than just the list of quakes. The southern quake, the smaller one, makes an alignment that looks like 500 km-long, while the northern one makes a cluster that is less than 200 km across. But, what is probably actually happening is that the NW cluster of quakes belongs to the northern quake, not the southern quake; in fact some of these occured before the southern quake. If you connect the NE cluster to the NW cluster this is 500 or 550 km, which makes the magnitude a bit less mysterious. Maybe you can make a M8.6 across 500 km if it breaks to 30 km or more depth to the sea floor, and has up to on the order of 10 m or more of slip in parts.

Chris