A magnitude 6.6 quake hit just 10 kilometres deep off New Caledonia’s Loyalty Islands

A magnitude 6.6 earthquake struck 10 kilometres deep beneath the sea off New Caledonia’s Loyalty Islands at 21:23 UTC on September 25, 2026, about 80 kilometres east-northeast of the town of Tadine on Maré Island. It was the strongest quake in the U.S. Geological Survey’s significant-events feed for that week, and reported shaking near the source was strong even though few people were there to report it.

The Pacific Tsunami Warning Center found no wave threat. Early reports had briefly called the quake a 7.0.

What a depth of 10 kilometres does, and does not, establish

The USGS weekly significant-events feed lists the event at magnitude 6.6 and depth 10 kilometres, with a modeled intensity of 5.67 and a community-reported intensity of 6.9 from 11 felt reports. Its alert level is green, meaning no significant casualties or economic losses were expected. The European-Mediterranean Seismological Centre gave the same magnitude and depth, according to a Watchers news report that put the epicentre 248 kilometres from Nouméa, the capital.

A caution applies to that depth. USGS states in its public FAQ that “sometimes data are too poor to compute a reliable depth for an earthquake,” and that in such cases the depth is assigned to be 10 km. The agency chose the value because reliable depths in many regions average near it, and it used 33 kilometres before that. The event feed does not say whether the 10 kilometres for this quake was computed or assigned, so the number should be read as the agency’s reported depth, and a shallow source is the working assumption rather than a proven fact.

Eleven reports and an intensity of 6.9

An intensity of 6.9 on the 10-point community scale describes strong shaking, and it stands out for a quake with so few respondents. The small count reflects geography. Maré, the second-largest of the Loyalty Islands, is 42 kilometres long and holds roughly 5,800 to 6,900 residents in tribal communities. Eleven answers to a web form are a small sample of a small population, and an average built on so few responses can swing on a single nearby respondent. The modeled figure, 5.67, is the steadier one. It comes from ground-motion estimates rather than from who happened to submit a report, and the two values bracket a range of moderate to strong shaking near the coast.

The subduction zone that keeps producing large quakes

Maré and its neighbours sit on the Loyalty Ridge, part of the Australian plate, which is being driven into the New Hebrides subduction zone. The New Hebrides Trench absorbs plate convergence at rates from 40 millimetres a year in its central section to 120 millimetres a year in the south, and its southern end, nearest the Loyalty Ridge, is comparatively quiet by regional standards.

Quiet is relative. A magnitude 7.7 shallow thrust earthquake struck the Loyalty Islands on February 11, 2021 and sent a tsunami to Vanuatu, where the gauge at Lenakel recorded 1.35 metres. It was reported as much stronger than the usual seismicity on that part of the trench, and no casualties were reported.

The ground under Maré carries the same history. Maré is a raised coral atoll, a former ring of reef that has been lifted about 120 metres, a reminder that the Loyalty Ridge has been bending and rising for a long time as it presses toward the trench.

The September 25 event was smaller by 1.1 magnitude units, which is roughly 45 times less energy. No source read this week ties it to the 2021 sequence. Placed beside its neighbours in the USGS monthly significant-events feed, it also ranks at the top: the monthly list shows two magnitude 6.5 quakes, at Nikolski in Alaska and off Indonesia, and a 6.4 in Papua New Guinea, all below the New Caledonia event.

The tsunami call and the 7.0 that became 6.6

Maui Now, reporting from Hawaii at 11:13 a.m. local time on September 25, relayed an early magnitude of 7.0 that was later reduced to 6.6, and quoted the Pacific Tsunami Warning Center saying there was no threat to Hawaii. The Watchers report carried the same center’s fuller wording: “Based on all available data, there is no tsunami threat from this earthquake.”

Depth and rupture style decide whether such a call holds. The USGS says tsunamis are generated when the seafloor moves vertically by “shallow faults that slip during large earthquakes.” A shallow magnitude 6.6 is smaller than the events that usually do that, and the USGS feed carries no tsunami flag for the quake, unlike the Aleutian event of September 17. Depth remains the open question. A revised solution would either confirm the 10 kilometres or replace it, and the feeds read on Tuesday still showed the original figure.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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