Five quakes up to magnitude 3.8 struck off Alaska’s Nikolski within 49 minutes

The first shock came at 08:59 UTC on Nov. 5, and the last arrived 49 minutes later at 09:48. In between, three more followed, so that five earthquakes of magnitude 3.2 to 3.8 were logged by the U.S. Geological Survey in roughly 50 minutes in the same patch of ocean, 115 to 120 kilometers southwest of Nikolski, a village on Umnak Island in the Aleutians.

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That is a compact burst, and a plausible one for a part of the world where the Pacific plate dives under the North American plate.

Minute-by-minute: the five Nikolski shocks

The opener was a magnitude 3.6 at 08:59 UTC, 115 kilometers southwest of Nikolski and the deepest of the group at about 43 kilometers. The strongest followed 15 minutes afterward: a magnitude 3.8 at 09:14 UTC, also 115 kilometers out, at 24.5 kilometers deep. Then a 30-minute pause.

The cluster closed with a rapid trio inside four minutes. At 09:44 UTC the catalog shows a magnitude 3.2, 120 kilometers southwest at 20 kilometers deep. One minute later, at 09:45 UTC, came a magnitude 3.5 at 117 kilometers and 32 kilometers deep. At 09:48 UTC the magnitude 3.4 arrived, 120 kilometers out at 19 kilometers, closing the 49-minute window.

Listed by size, the sequence is 3.8, 3.6, 3.5, 3.4 and 3.2, with the biggest in the middle rather than at the start, which is why seismologists often call this a swarm rather than a mainshock and its aftershocks.

In sequence by size the five break down as follows: 3.2 at the low end, then 3.4, 3.5, 3.6 and the 3.8 on top. The spread between the smallest and largest is only 0.6 of a magnitude unit, an unusually narrow band, and no event outranked the others by enough to read as a clear mainshock. Such tight groupings fit the swarm pattern seismologists describe, in which no single rupture dominates.

Timing adds to that picture. Gaps between events were 15 minutes, 30 minutes, one minute and three minutes, a pattern of one early pair and a later trio, and the 30-minute lull between them would be unusual for an aftershock sequence, where rates usually fall steadily after the first large event.

Swarm geometry on the Aleutian arc

All five locations fall within a 5-kilometer band of distance from Nikolski, and their depths run from 19 to 43 kilometers, a spread typical of the plate boundary region south of the island chain. The USGS event records report locations to the nearest kilometer and depths with uncertainties that are often larger than the gaps among these events, so the cluster should be read as one general zone rather than a single fault patch.

Nikolski sits at the southwestern end of Umnak,, a very small village. At 115 kilometers away, shaking from a 3.8 would not be felt there,. The Alaska Earthquake Center at the University of Alaska Fairbanks, which operates the regional seismic network, posts its own maps and event lists and often catalogs smaller shocks in the Aleutians that global networks miss.

Few sensors sit near these epicenters. The Aleutian arc stretches for more than 1,000 miles, and offshore events 115 to 120 kilometers from the closest village depend on stations scattered along the islands. That is why location and depth estimates for such events carry wider error bars than a comparable quake near a city, and why the USGS catalog often shows several revisions for a single shock in the hours after it occurs.

What came after the burst

Activity did not stop completely. The same seven-day USGS feed of magnitude 2.5 and larger events lists a magnitude 2.6 at 00:34 UTC on Nov. 8, located 91 kilometers south-southwest of Nikolski at a depth of 4.7 kilometers, shallower and closer in than the cluster. The agency does not link it to the earlier five, and given the gap of nearly three days and a different depth it may be unrelated.

Nikolski is also a useful reference point because it is one of the few named places near the cluster, not because the quakes struck it. The catalog uses the nearest populated place as an address, which is the standard USGS convention, so a location phrase like 115 kilometers southwest of Nikolski describes a spot in open water well offshore.

The open question for the Aleutians is whether a quiet interval follows or the zone steps up again. A swarm of magnitude 3 shocks usually decays within days, and the feed would show a larger event within minutes if the pattern changed. For now the USGS counts five events in 49 minutes and nothing larger in the feed excerpt read for this article, although the excerpt was cut off before the end of the listing, so a final tally of the week would need the full feed. Anyone tracking the region can refresh that same USGS list, which updates within minutes of each new solution.

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

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