December 27, 2003 Earthquake: Loyalty Islands, New Caledonia

Tectonic Summary

The December 27, 2003, M 7.3 earthquake was part of an extended sequence of about 270 earthquakes that commenced in December 2003 in the Loyalty Islands, located in the southeastern corner of the French territory of New Caledonia. The sequence included three large earthquakes: a M 6.8 foreshock on December 26, this M 7.3 mainshock on December 27, and a M 7.1 earthquake, which was part of an aftershock series that essentially terminated on January 13, 2004.

This earthquake sequence occurred within the broader zone of intense regional seismicity of the New Hebrides (Vanuatu) Island Arc. The arc gently curves to the south-southeast from the Santa Cruz Islands, across the island nation of Vanuatu, and terminates where the seismic zone curves sharply to the east near the Loyalty Islands (see Seismicity of the New Hebrides Arc map in the related Earthquake Summary Poster). The arc’s islands, many of which are volcanic, lie parallel to the New Hebrides Trench southwest of the December 27th earthquake. Bands of shallow (0–70 km) and intermediate-depth (70–300 km) earthquakes are also parallel to the axes of the trench and island chain. A band of shallow seismicity clearly defines the extent of the outer ridge of the New Hebrides Trench. The foreshocks were located principally in this outer ridge band, but the aftershocks were located principally under the inner arc.

The shallow seismicity defines the zone of convergence of the Australia plate with the New Hebrides microplate (a segment of the much larger Pacific plate) and the intermediate seismicity defines the extent of the down-going slab of Australia plate. The rate of convergence of the Australia plate is estimated to be about 75 mm/yr, and the plate motion is east-northeast relative to the New Hebrides microplate. Focal mechanism solutions of the foreshock sequence were typical of normal faulting in a tensional horizontal stress regime, but mechanisms of the December 27th mainshock and the many aftershocks were characteristic of shallow thrust faulting and northeastward-oriented subduction.

The region surrounding the Loyalty Islands and Vanuatu frequently hosts large earthquakes. Within 400 km of the December 27th event, 11 other earthquakes of M 7 or larger have occurred over the preceding 40 years. The largest of these events was a M 7.7 earthquake in May 1995 about 130 km to the south of the December 27th earthquake. This large event had no associated casualties but did generate a local tsunami that was recorded along the coast of New South Wales, Australia.

(reference #1053)