A scientist on the radio last week predicted that, due to the pressure built up from the December quake, a large one was coming soon to other faults in that area. Not good...
per the Weather Channel it was the same fault line
This one looks to be in almost the same spot, maybe a little south of the tsunami quake.
The epicentral location is just where the quake started; the epicenter is at the southern limit of the December quake rupture; if the rupture propagated southwards, it's quite possible this is a rupture of the section that went in 1833 and had been heavily studied by Kerry Sieh at Caltech (and had been expected to rupture in a large quake even before the December quake.
And yes, it is one of the areas that stress actually increased after the December quake; the other area is on land in Northern Sumatra, a San Andreas like strike-slip fault.
Depth is hard to estimate, this was a bit deeper if the NEIC depth is accurate. It may or may not have produced a tsunami.
Also the magnitude, as always, might turn out to be bigger after more study. It's actually impossible to estimate the mag of a really big quake till after hours of data have been recorded.