2021 Loyalty Island Earthquake and Tsunami

The 7.9 Magnitude Quakefeed Notification on the iPad

The **2021 Loyalty Islands earthquake** was a 7.7 magnitude earthquake that struck offshore between New Caledonia and Vanuatu. It is currently the 3rd largest earthquake of 2021, behind the 2021 Kermadec Islands earthquake and the 2021 Chignik earthquake.

## Tectonic setting

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 Indo-Australian 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 preceded by two foreshocks of M 6.1 and M 6.0 in the hour before the earthquake. At the location of this earthquake, the Indo-Australian 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 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 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 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 the 2021 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 the 2021 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 7.0 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.

## Earthquake

The earthquake was initially reported as 7.9 Mw before it was downgraded to 7.5 then revised to 7.7. It is the largest earthquake in New Caledonia since 1995.

The earthquake occurred due to shallow thrust faulting along the New Hebrides subduction zone, where the Australian Plate subducts beneath the New Hebrides Plate. Bases on inferring the finite fault model by the United States Geological Survey, slip along the rupture was distributed mainly on the shallow subduction zone interface, near the trench. A maximum slip of 4.1 meters and uplift of 2.1 meters was estimated.

## Tsunami

A tsunami warning was issued, but later cancelled. The Pacific Tsunami Warning Centre reported waves up to 0.78 m (2.6 ft) high in Lenakel, Vanuatu. Further analysis found the tsunami had a height of 1.35 m (4.4 ft) in Vanuatu.

## New Zealand tsunami warning

A National Advisory: Tsunami activity has been cancelled for New Zealand following the magnitude 7.7 earthquake that occurred southeast of Loyalty Islands at 2021-02-11 2:20 AM

NEMA (National Emergency Management Agency) issue official tsunami warnings for New Zealand.

Early Thursday morning NEMA (The National Emergency Management Agency) and GNS Science assessed that a magnitude 7.7 Loyalty Islands earthquake had created a tsunami that could affect beach and marine areas in parts of Northern New Zealand. The tsunami threat has now passed for New Zealand, although there could still be large unexpected currents and the public are advised to continue to take precautions in coastal zones for the rest of today.

The M7.7 earthquake was located southeast of the Loyalty Islands, North of New Zealand and produced a tsunami that was recorded at tide gauges around the pacific.

On our own network of tsunami gauges the arrival of the waves can be seen at a number of sites around New Zealand. We recorded 60cm wave height at Aotea Great Barrier Island and around 20-25cm at North Cape and East Cape.

Although 50-60cm may not sound like much, tsunami are nothing like normal wind-generated waves, that small tsunami is a river of water surging faster than you can run (30km/h at the coast and 600km/h in the open ocean). Surges come in non-stop for around 5 minutes or more, and then recede just as fast for a similar period of time. Acting more like a very rapid tide than a wave. This type of back-and-forth pattern continues for several hours (if the tsunami was generated close by) to days (for tsunami that has travelled across the Pacific). For someone fishing on the rocks, that relentless surge of water might be enough to knock them off their feet and not let them back up.

As our Seismic Duty Officer John explained "50-60cm is like trying to stand in a fast flowing river that is about knee height, and really hard to stand in"

For more information of Aotearoa's history of tsunami and what causes them check out our story here and for how we know a tsunami is coming and what to do, check this story out. 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.