A magnitude 6.9 at 1304 UTC and a magnitude 7.3 a half-hour later at 1335 UTC (9:00-9:30 am EDT). The depth of the two quakes ranged from 42-58 km (25-35 miles down). The map below shows the epicenters on New Britain Island by blue boxes.
The red line represents a sea floor trench south of New Britain - the surface expression of a major fault which dips northward below the island.
This area of the world is a complex amalgamation of tectonic microplates.
The Solomon Sea microplate is moving northwards and sliding down beneath the South Bismarck Plate forming a seafloor trench.
As the plate subducts, it melts providing magma for active volcanoes on New Britain Island. It also moves by stick-slip behavior. It sticks, stresses build up as the plate is pulled on by the subducting slab, and then it slips somewhere along its length. Bang - an earthquake.
Nothing unusual for this part of the world.
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