M 7.7 - southeast of the Loyalty Islands

Tectonic Summary

The February 10, 2021, M 7.7 earthquake located about 415 km to the east of the island of New Caledonia in the southwest Pacific Ocean occurred as the result of low angle thrust faulting on or near the plate boundary interface between the Australia and Pacific plates. Focal mechanism solutions indicate the earthquake occurred on either a shallow fault striking west and dipping to the north, or on a steep fault striking east. The earthquake was preceeded by two foreshocks of M 6.1 and M 6.0 in the hour before the earthquake. At the location of this earthquake, the Australia plate moves towards the east-northeast with respect to the Pacific at a rate of approximately 75 mm/yr. At the South New Hebrides Trench, Australia lithosphere converges with and sinks beneath the Pacific plate, descending into the mantle and forming the New Hebrides/Vanuatu subduction zone, stretching from New Caledonia in the south to the Santa Cruz Islands in the north, a distance of about 1,600 km. The February 10, 2021 earthquake occurred in close vicinity to the New Hebrides trench, and its location, depth and focal mechanism solution are all consistent with it occurring on the subduction zone interface between the two plates.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Reverse (thrust) faulting events of the size of the February 10, 2021 earthquake are typically about 90 km long by 50 km wide in size.

The Loyalty Islands region is very active seismically, and the region within 250 km of the February 10, 2021 earthquake has hosted 15 other M 7+ earthquakes over the preceding century. The largest prior event was a M 7.7 earthquake on May 16, 1995, which was located about 165 km west of today’s event, and to the southwest of the oceanic trench in the region commonly referred to as the outer rise. One of the most proximate of these historic earthquakes was a M 7.0 thrust faulting event in August 1926, about 68 km to the west of today’s earthquake. Several earthquake sequences in this region entail spatially and temporally complex seismicity characteristics. A M 7.3 earthquake in December 2003 located 220 km northwest of the February 2021 earthquake was part of an active sequence of about 250 M 4+ events, beginning in December 2003 and continuing into February 2004. That sequence included both interplate thrust faulting earthquakes (the largest event in the sequence was the M 7.3 earthquake) and normal faulting earthquakes to the west of the oceanic trench (the largest being a M 7.1 earthquake in January 2004). A similar sequence of interplate thrust and outer rise normal faulting earthquakes occurred in October-December 2017, just to the north of the 2003-04 sequence. The 2017 sequence began with a M 6.7 thrust faulting earthquake on October 31, 2017. The largest event in the sequence was a M 7.1 outer rise earthquake on November 19, 2017. Because of their remote locations far from land, these earthquakes do not cause significant shaking in populated areas and are not known to have resulted in damage or fatalities.