6.9 is no aftershock. The one yesterday was about 13 miles off shore, this one is on land right on the fault line.
The plate boundary in that location is complex; the Indo-Australian Plate isn’t subducting under the Eurasian plate at right angles - it’s actually moving obliquely down under it to the northeast. So in addition to the offshore subduction zone, there’s the Sumatran Fault which runs the whole length of Sumatra, where the plates slide past each other like the San Andreas Fault.
Just glancing at the location and preliminary this probably is on the strike-slip fault rather than on the subduction zone.
Whether something is an “aftershock” is open to debate. It’s highly likely the previous earthquake triggered this one by increasing the strike-slip strain.
Oh, someone here posted it was a 5.2 so I assumed it was an after shock