Sunday, July 18, 2010

PNG earthquakes

Couple of big earthquakes today in Papua New Guinea (PNG) and numerous aftershocks.

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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