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Andrew Alden

Big Quakes in Vanuatu

By October 7, 2009

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A pair of major earthquakes has struck the South Pacific nation of Vanuatu. The first event (at 9:03 am Thursday local time) had a tentative magnitude of 7.8, according to its dedicated USGS page. It was a typical subduction-related thrust event, unlike last week's Samoa earthquake, which was in the bending slab outboard from the subduction zone. Also, it was on a different plate boundary, so they were not related beyond the tenuous, unpredictable sense in which distant large events can slightly tweak a fault zone and speed up its inevitable failure.

A tsunami about half a meter high was reported in Vanuatu, but alerts around the Pacific were canceled.

A nearly identical aftershock of magnitude 7.7 occurred 15 minutes later. There was confusion for a few hours as seismologists sorted out the signals—the first report of a 7.3 aftershock was canceled, then reissued with the larger magnitude. A better name for this pair of events is an earthquake doublet. As more data comes in, we will be able to map the rupture zones and see to what extent they overlap.

A particularly large aftershock of magnitude 7.1 followed at 10:13, with the same mechanism. When you look at the maps of the mainshock and this event, keep in mind that the subducting plate is moving eastward, opposite to the case of the Samoan quake.

Aftershocks have a small but finite chance of being bigger than the preceding event. In that case, they might well be called the mainshock and the first event renamed a foreshock.

Background:
About aftershocks
Introduction to earthquakes
Earthquake triggering
Teleseismic triggering (2008)
Basics of Plate Tectonics

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