Instead of occurring at a plate boundary along an area called a subduction zone, where one tectonic plate is diving beneath another, this earthquake occurred in the middle of an oceanic plate, where the faults in the crust essentially moved from side to side instead of up and down.

MSN: Why this Indonesia quake didn't spark a monster tsunami

The Australian plate borders the Pacific plate to the East (the Samoan quake occurred along this boundary), while on the Western side, it borders the Eurasian plate (where the Indonesia quake occurred).

CNN: Two quakes in the same region in a day - coincidence?