I checked for the focal mechanisms of the aftershocks to see what was going on in the north...near the Andaman Islands (13-14 deg N). The large northern aftershocks on the Harvard page (linked at bottom) seem to be normal slip, oblique normal slip, and strike-slip
(some are strike-slip and normal farther south also). The link to the Harvard page is a search...just do a start date (Dec 25) and number of days (say, if today, 7). First...partial explanations: the arrivals of the seismic waves at a bunch of seismometers distributed around the region or globe allow the fault orientation to be deteremined in 3D....plus the plane that is perpendicular to the slip. The only problem is that you don't know which of the 2 planes is the actual fault plane just by looking at it. The circles represent the lower half of a sphere...and the curves on that circle are the intersections of the planes with that sphere. The white areas represent where the 3D volume is being squeezed, while the dark (or, in the case of USGS, yellow) areas are being stretched. If you ask, someone else might link to a page that explains this, and how to read these.

the main earthquake was a thrust...where the earth is being contracted. These northern aftershocks are showing stretching, or translation...where blocks slide past each other horizontally.

Perhaps (I'm guessing) the large thrust motion in the south caused a twist in the hanging-wall...like a vertical axis rotation, that stretched the area to the north. Also, the farther southeast along the plate boundary the more nearly thrust the plate motion is, while the farther north you go, the more nearly strike-slip it is. Tectonic maps on the USGS page even show a spreading center in the north....east of the oblique subduction plate boundary.

Chris
USGS slip solutions:
http://neic.usgs.gov/neis/FM/

• Havard slip solutions