A two-week swarm of strong earthquakes has rattled the seas off Vanuatu

Seven earthquakes of magnitude 5.5 or stronger struck within about six and a half hours on Oct. 24 in the sea northwest of Vanuatu’s Espiritu Santo island, and the day was only the middle of a longer run. The USGS earthquake catalog lists eight quakes of magnitude 6.0 or higher labeled as near the town of Port-Olry between Oct. 19 and Nov. 1, a stretch of 14 days.

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The biggest was a magnitude 6.8 on Oct. 24 (UTC). No shaking report or damage account turned up in the sources read for this piece, and the catalog records ground motion, not consequences.

Eight magnitude-6 quakes in 14 days

The query that anchors this count is the USGS event service, asked for every magnitude 6.0 or stronger quake worldwide since Oct. 13. Of the events it returned, eight carry Port-Olry in their location labels. In order of size: a magnitude 6.8 on Oct. 24 placed 203 kilometers northwest of Port-Olry at a depth of 16 kilometers; a magnitude 6.3 on Oct. 20, 202 kilometers northwest, also 16 kilometers deep; a magnitude 6.2 on Oct. 29, 43 kilometers east-southeast of the town; two magnitude 6.1 events, on Oct. 24 and Oct. 25; and three magnitude 6.0 events on Oct. 19, Oct. 28 and Nov. 1.

That is the sequence in the catalog, and the dates are in UTC, so some fall a day earlier in Vanuatu’s own time zone.

Shallow clusters and two deeper outliers

Most of the strong events share a signature. The sequence is a catalog reading, not a forecast, and the dates are USGS origin times. Six of the eight sit between roughly 160 and 215 kilometers northwest of Port-Olry, and the catalog lists them at depths of 10 to 16 kilometers; the band between the nearest and farthest of them is only about 51 kilometers wide, which points to a single compact source region rather than scattered shocks. The Oct. 24 pair is the tightest: a magnitude 6.8 at 02:31 UTC followed by a magnitude 6.1 at 02:35, about four and a half minutes later, with several smaller shocks of magnitude 5.7 to 5.9 over the following hours.

Two events do not fit the pattern. The magnitude 6.1 on Oct. 25 sits 25 kilometers east-northeast of Port-Olry at a depth of 116 kilometers, and the magnitude 6.2 on Oct. 29 was at about 143 kilometers deep. Those two are far deeper and much nearer the town than the northwest cluster, so the catalog is recording at least two different pockets of rock failing during the same fortnight.

A second pull of the USGS service, this time covering a box around Vanuatu from Sept. 1 and dropping the threshold to magnitude 5.5, returned 20 events. Nineteen were labeled Port-Olry, 11 of them between magnitude 5.5 and 5.9. The twentieth was a magnitude 6.3 on Oct. 8, labeled 117 kilometers northeast of Norsup, which sits outside the Port-Olry cluster and 11 days before it began.

Other days carried their own bursts. On Oct. 28 the box recorded a magnitude 5.5 at 01:39 UTC, the magnitude 6.0 at 02:21 and a magnitude 5.6 at 02:59, three shocks inside 80 minutes. Oct. 19 opened the run with the magnitude 6.0 at 07:49, then added magnitude 5.5 at 10:40 and a pair of 5.7 and 5.5 events at 18:52 and 18:55. The same pull started on Sept. 1 and found nothing at magnitude 5.5 or higher before Oct. 8, so the activity arrived out of a quiet background.

Reading the swarm through the magnitude scale

Magnitude is logarithmic. The USGS explains that each whole-number step means a tenfold increase in measured wave amplitude and about 32 times more energy, so by that rule the magnitude 6.8 released roughly 16 times the energy of each magnitude 6.0 in the series. The agency describes quakes around magnitude 6.3 as “strong,” and cites the 2014 magnitude 6.0 Napa, California earthquake as an example of a magnitude 6 event.

The 5.5 cutoff matters for reading the counts. Smaller quakes certainly occurred in the same stretch and fall below the threshold of both queries, so the eight and the nineteen are floors for the strongest activity, not tallies of everything the seafloor produced.

Whether this deserves the word swarm depends on definitions the catalog does not settle. Some sequences have one dominant mainshock followed by decaying aftershocks. This one produced a 6.3 on Oct. 20, a 6.8 on Oct. 24, and later 6.0 to 6.2 events up to Nov. 1, none clearly an aftershock of the one before.

The catalog pull for this article showed no event of magnitude 5.5 or higher in the Vanuatu box after Nov. 1. USGS did not attribute the sequence to a named cause in the records read, and no Vanuatu agency statement was found, so the open question is whether the run has ended or only paused.

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