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.
Big Kerry Sieh fan here!