Poster of the Loyalty Islands Earthquake Sequence, Decmber 2003 and January 2004

Tectonic Summary

On 25 December 2003, an extended sequence of some 270 earthquakes commenced in the Loyalty Islands, located in the southeastern corner of the French territory of New Caledonia. The sequence included three large shocks, an M 6.8 foreshock on 26 December, an M 7.3 main shock on 27 December, and an M 7.1 earthquake on 3 January 2004 that was part of an aftershock series that essentially terminated on 13 January (see diagram at lower right).

This earthquake sequence occurred within the broader zone of intense regional seismicity of the New Hebrides island arc. The arc gently curves to the SSE 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). The arc's islands, many of which are volcanic, lie parallel to the New Hebrides Trench. Bands of shallow depth (0 - 69 km) and intermediate depth (70 - 299 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 while the aftershocks were located principally under the inner arc (see Epicentral Area map).

The shallow seismicity defines the zone of convergence of the Australian plate with the New Hebrides microplate (a segment of the much larger Pacific plate) and the intermediate seismicity defines the extent of the downgoing slab of Australian plate. The rate of convergence of the Australian plate is estimated to be about 75 mm/yr and the plate motion is ENE relative to the New Hebrides microplate (see Plate Tectonic Setting map). The focal

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