The potential apocalypse of the 21st of December may have passed without incident for much of the planet, but the earth did shift significantly in the western Pacific.
The date had shifted to the 22nd of December, local time, however, before the United States Geological Survey reported an earthquake of magnitude 6.8 (M6.8) to the north of the Pacific island of Vanuatu.
The earthquake is the latest in a continuing series of tremors to affect the highly seismically-active boundary between the Australian and Pacific tectonic plates.
Despite its magnitude, no damage had been reported at the time of writing and no tsunami warning was issued.
The Tectonic Setting of Vanuatu
The boundary between the two major tectonic plates is complex along its length, changing in both nature and direction from New Zealand to Sumatra: the latter area was the source region for the notorious Boxing Day earthquake of 2004. High rates of convergence between the two plates further increase its vulnerability to regular earthquake activity.
Earthquake activity in the Vanuatu region is largely controlled by the eastwards movement of the Australian plate which, at this point, is subducted beneath the Pacific plate (in contrast to the pattern further west, where the direction of subduction is reversed). The situation is further complicated by the existence of two significant and near-parallel thrust belts within the Pacific plate to the west. The location of the December 22nd tremor, some way to the west of the subduction zone, suggests that it is associated with these thrust zones.
Historic Seismicity: the Earthquake and Tsunami of 1999
As is to be expected, major earthquakes are by no means unusual in the whole of this section of the plate boundary, as the map of historic seismicity shows. There have been four earthquakes in Vanuatu so far in 2012 measuring at least M6.2, and the largest of these was M7.1 (February 2nd). The largest, and one of the most notable to be recorded in the country, was the M7.5 earthquake which struck in broadly the same area as the most recent tremor in November 1999.
The magnitude scale is logarithmic; though only 0.7 larger in magnitude terms, the 1999 earthquake appeared 5 times bigger on a seismogram, and released over 11 times as much energy. As seems likely with the December 2012 event, the 1999 quake resulted from thrusting along the fault segments within the Pacific plate. That quake, however, generated a tsunami which was widely felt and cost the lives of ten people in the area.
Regnier, M., Calmant, S., Pelletier, B., Lagabrielle, Y. and Cabioch, G. The Mw 7.5 1999 Ambrym earthquake, Vanuatu: A back arc intraplate thrust event. in Tectonics July 2003. Accessed December 22, 2012.
USGS. Seismicity of the Earth 1900–2010 Eastern Margin of the Australia Plate. Accessed December 22, 2012.
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