There was a M7.1 in the area of the M8.4 of a month or 2 ago, south of the equator at Sumatra. No Focal mechanism yet. This is at the latitude of the middle of the M8.4 so is a plain vanilla aftershock...large, but probably unsurprising. Focal mechanism will be interesting though, and detailed studies: is it on the exact same fault as the M8.4 with the same slip direction (which means there was anough shear stress/differential stress to break it again), or is it on some out-of-sequence thrust above it, or some other fault?
Chris

• Sumatra 7.1