A magnitude 6.0 earthquake struck 165 kilometers northwest of Port-Olry, Vanuatu, at 11:47 UTC on November 1. It came after a week of repeated shocks in the same patch of ocean, and it was followed within about 20 minutes by two more of magnitude 4.8. No tsunami was flagged, and the damage-estimate rating was the lowest on the scale.
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Vanuatu lies on the Pacific Ring of Fire, and quakes of this size are not unusual there. What stands out is the count of events packed into six days. The islands sit above a subduction zone where one tectonic plate dives beneath another, a setting that produces frequent moderate-to-large shocks at a range of depths, and the feed read for this article includes both shallow and intermediate-depth events in the same week.
The record for the November 1 shock
The U.S. Geological Survey’s reviewed solution for event us7000tupl, available on its event page, puts the 6.0 quake at a depth of 10 kilometers, with a moment-magnitude calculation. The agency’s data record for the event shows a shaking-intensity estimate of 4.1, a significance score of 554, a green alert level from its PAGER impact system and a tsunami flag of zero. Port-Olry is a coastal village on Espiritu Santo, and the epicenter lay out to sea in the northern part of the island chain.
The record was last updated at 13:53 UTC the same day, about two hours after the shaking. The origin time converts to about 10:47 at night in Vanuatu, which keeps UTC+11 time, so the main shock arrived in the middle of the evening rather than at the start of a working day.
Twenty-four shocks in six days
The agency’s weekly list of magnitude 4.5 and larger events, the 4.5 week feed, contains 24 events labelled relative to Port-Olry between October 27 and November 1. Ten fall between 09:18 UTC on October 31 and 12:08 UTC on November 1, all roughly 165 to 236 kilometers northwest of the village. Magnitudes in that run range from 4.7 to 5.4 ahead of the 6.0, and the biggest of them, a 5.4, hit at 06:26 UTC on November 1, about five hours earlier. Two 4.8 aftershocks followed at 11:56 and 12:08 UTC.
The sequence began earlier. A 5.8 on October 27 was listed 196 kilometers northwest, and a separate 6.0 on October 28 at 02:21 UTC was placed 214 kilometers northwest. On October 29 a 6.2 appeared 43 kilometers east-southeast of Port-Olry at 142.8 kilometers depth, a deeper event in a different direction that the feed also lists. Almost every other event in the swarm carries the default depth of 10 kilometers, a figure the agency assigns when data cannot constrain depth tightly.
By UTC date the feed holds four events on October 27, five on October 28, two each on October 29 and 30, five on October 31 and six on November 1. Three reached magnitude 6.0 or higher: the 6.0 on October 28, the 6.2 on October 29 and the 6.0 on November 1. Seismologists generally reserve the word swarm for a run of shocks of similar size with no single dominant event, and reserve aftershock sequence for a decaying string after one large quake. This list shows features of both, which is why the label stays descriptive.
Sorted by size, six of the 24 events reached magnitude 5.5 or above, eight fell between 5.0 and 5.4, and ten were below 5.0. Because each step of one magnitude unit means about 32 times the energy, the six largest account for most of the energy the group released. The weekly list refreshes automatically and events can be revised on review, so the counts here reflect the feed as read on November 1.
Why a swarm this size produced little felt shaking
The feed lists no felt reports for the November 1 quake, and tsunami flags were zero for every event in the list. The 6.2 on October 29 drew five felt reports, the most of any entry, and the 6.0 on October 28 drew one. Distance explains part of that. The USGS explains that a quake has one magnitude but many intensity values, and that intensity depends mostly on distance from the rupture, with local geology and depth also mattering, as set out on its magnitude and intensity page. A magnitude 6.0 releases about 32 times the energy of a magnitude 5.0, so the largest events in the swarm matter far more than the dozens of small ones.
Official tsunami messaging runs through the Pacific Tsunami Warning Center, whose bulletins are posted on tsunami.gov. Whether the sequence is a lingering swarm or a prelude to something bigger cannot be settled from the feeds, and the agency’s reviewed records will show whether further magnitude 6 shocks follow.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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