Where the Pacific plate collides with the Indo-Australian plate, it is forced downwards into the trench, a subductionzone, and the volcanoes are carried with it.
Instead of occurring at a plate boundary along an area called a subductionzone, 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.