Tectonic Summary

The December 5, 2018, M 7.5 earthquake east of New Caledonia in the southwest Pacific Ocean occurred as the result of shallow normal faulting within the oceanic crust of the Australia plate, just west of the South New Hebrides Trench which marks the plate boundary between the Australia and Pacific plates in this region. Focal mechanism solutions indicate faulting occurred on a moderately dipping fault striking either to the northwest or to the southeast. At the location of this earthquake, the Australia plate moves towards the east-northeast with respect to the Pacific at a rate of approximately 78 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 December 5, 2018, earthquake occurred very close to this trench, and just to its west, in the tectonic region sometimes known as the “outer rise” where the subducting plate begins flexing (extending) before sinking into the mantle. The location, depth, and focal mechanism solution of this earthquake are all consistent with the event occurring as a result of intraplate faulting in this outer rise region.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Normal faulting events of the size of the December 5, 2018 earthquake are typically about 75x30 km in size (length x width).

The December 5, 2018 earthquake is sixth M 6+ earthquake to occur in this region over the past three months, and is part of an active sequence of events that began on August 29th, 2018 with a M 7.1 interplate thrust faulting earthquake to the east of the South New Hebrides Trench and about 70 km to the east of the December 5, 2018 earthquake. Over this time period, about 140 M 4+ earthquakes have been recorded in this region by the USGS, most thrust faulting earthquakes to the east of the plate boundary. Today’s earthquake was preceded by 4 minutes by a M 6.8 foreshock, about 13 km further west of the oceanic trench. A similarly active sequence occurred between October-December 2017, just to the north of the 2018 events and predominantly in the Outer Rise region. The 2017 sequence involved upwards of 350 M4+ events, including seven M6+ (and 1 M7+) events.

The Loyalty Islands region is very active seismically, and the region within 250 km of the December 5, 2018 earthquake has hosted 24 other M 7+ earthquakes over the preceding century. The largest was a M 8.1 earthquake in September 1920, which was located about 230 km to the northwest of today’s event, just to the east of the oceanic trench. Five of these M 7+ earthquakes have occurred to the west of the oceanic trench, including a M 7.7 earthquake in May 1995, 125 km to the southeast, a M 7.1 earthquake in January 2004, 40 km to the southeast, and the aforementioned M7.0 earthquake in November 2017, 70km to the northwest. None of these are known to have caused any damage or fatalities. The January 2004 M 7.1 earthquake was also part of an active sequence of about 270 events, beginning in December 2003. That sequence included both interplate thrust faulting earthquakes (the largest event in the sequence was a M 7.3 thrust faulting earthquake on December 27, 2003) and normal faulting earthquakes to the west of the oceanic trench. Between December 25, 2003, and January 3, 2004, 12 earthquakes of M 6+ occurred. The 2003-2004 sequence eventually died down in early-mid February of 2004.

Event Details

Event Map

Tectonic Summary

Tectonic Summary

Historical Magnitude 4.5+ (100km)