A strong earthquake hit the seafloor southeast of the Loyalty Islands on Sunday at 14:45 UTC, and nobody was hurt. No injuries, damage or tsunami threat were reported.
The U.S. Geological Survey and the European-Mediterranean Seismological Centre each put the quake at magnitude 6.3 and a depth of 10 kilometers, within minutes of each other. Early wire reports from Xinhua and Anadolu Agency said 6.7. That figure came from the USGS automatic preliminary solution, the computer-generated first pass, and it was revised down to 6.3 after seismologists reviewed the data.
Why the alert was green
The USGS issued a green alert through PAGER, its automated impact system. Green does not mean the shaking was mild. It means few people were exposed to damaging shaking. Think of it like a thunderclap in an empty field: loud, but nobody close enough to be harmed.
The epicenter lay about 400 kilometers east-southeast of Tadine, the main settlement on Maré island in New Caledonia, and 429 kilometers south-southeast of Isangel in Vanuatu. Both have fewer than 10,000 residents. The same rupture directly beneath Tadine would almost certainly have drawn a yellow or orange alert.
PAGER sets green at fewer than one estimated fatality and less than $1 million in estimated economic losses. Yellow starts at one fatality or $1 million, orange at 100 fatalities or $100 million, and red at 1,000 fatalities or $1 billion.
The Pacific Tsunami Warning Center issued a Tsunami Information Statement within minutes. It reported that no destructive Pacific-wide tsunami was expected and no public action was required. The U.S. Tsunami Warning System confirmed there was no threat to Guam, American Samoa or Hawaii.
Why depth matters
At 10 kilometers, the quake was shallow, which matters because seismic waves lose strength as they travel through rock. A deep earthquake spends much of its energy on the way up. A shallow one has little rock to cross, so the shaking arrives with more force. The PAGER building data for the area lists mostly informal structures of metal, timber and galvanized iron, which collapse easily in strong shaking. A magnitude 6.3 at this depth can damage poorly built buildings near the epicenter.
A busy corner of the Pacific
The quake fits the region’s pattern. At the South New Hebrides Trench, Australian lithosphere sinks beneath the Pacific plate in the Vanuatu subduction zone, a structure about 1,600 kilometers long. Convergence in the southern section runs at around 120 millimeters per year. In the 100 years before the February 2021 magnitude 7.7 Loyalty Islands quake, the area within 250 kilometers of Sunday’s epicenter produced 15 other earthquakes of magnitude 7 or higher.
The USGS had issued no elevated aftershock probability statement for the event.
