A magnitude 5.6 earthquake struck interior Alaska at 11:07 UTC on Thursday, October 15, which was 3:07 a.m. local time, and 106 people who felt it sent reports to the U.S. Geological Survey. The shaking was centered 43 kilometers north-northwest of Lake Minchumina, in country with almost no one living in it.
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No tsunami was generated. The count of 106 is modest for a quake of that size, and the reason sits in the geography of the epicenter rather than in the shaking itself. The quake began at 3:07 a.m., so most of the residents who noticed it were woken by it, and the nearest borough has a population density of about one person for every ten square miles.
The USGS record for the Lake Minchumina quake
The USGS lists the event on its own event page as an ML 5.6 earthquake, 43 kilometers north-northwest of Lake Minchumina, Alaska, with a status of reviewed. The origin came from the Alaska network, which put the depth at 4.5 kilometers. That is shallow, and the page carries other estimates: the ShakeMap model used 0.8 kilometers, and moment-tensor solutions placed the centroid at 10 and 17.5 kilometers. Differences of that kind are normal in the hours after a large quake and are the reason the numbers on a reviewed page can move for days.
The machine-readable detail record for the event shows the page was last updated at 11:17 UTC on October 18, three days after the shaking. It lists a community intensity of 4.8 on the zero-to-10 scale built from the felt reports, a tsunami flag of zero, and a ShakeMap description that places the epicenter 54 kilometers north-northwest of Lake Minchumina, a different distance from the 43 kilometers given for the catalog epicenter. The two figures describe different solutions and both belong to the same event.
The origin of the 106 felt reports
The felt-report number comes from the USGS felt-report program, a citizen-science project that collects information from people who felt an earthquake and builds maps of what they experienced. Anyone can find the event in a list and file a report, so the total reflects who was awake, who was near a phone and who knew the form existed. A quake at 3:07 a.m. catches many people asleep. The detail record lists 106 responses for the program’s product and a felt value of 106 for the event, so the two counts agree.
The same record shows how far apart two kinds of intensity can sit. Alongside the community intensity of 4.8, which averages what respondents described, it carries an instrumental intensity value of about 7.0 from the ShakeMap, the model of ground shaking built from seismometer data and the quake’s rupture. The gap is expected when the strongest shaking falls on ground with nobody standing on it and the people who file reports are scattered across villages, cabins and highway pull-offs at a distance. A shallow source of a few kilometers depth keeps the strongest motion close to the epicenter and lets it fade quickly with distance, which narrows the crowd that felt anything at all.
That matters because the count is a measure of people, not of ground motion. A magnitude 5.6 shock released far more energy than the smaller events the USGS feed lists in the same week, yet the number of respondents depends mostly on how many humans are within range of the shaking.
Denali Borough and the empty ground around Lake Minchumina
The nearest local government is Denali Borough, which the Census Bureau’s QuickFacts table puts at 1,616 residents as of July 1, 2025, with 0.1 persons per square mile at the 2020 Census. At that density a quake can shake thousands of square miles and still be felt by only a few hundred people, which makes 106 responses a reasonable sample of an audience that barely exists. The community intensity of 4.8 suggests the shaking people did report was widely noticeable.
A smaller shock followed in the same area. The feed lists a magnitude 3.0 earthquake 40 kilometers north-northwest of Lake Minchumina at 13:05 UTC the same morning, at a depth of 8.2 kilometers, which drew two felt reports. Its position next to the main shock is consistent with an aftershock, though the USGS record read for this article does not label it as one.
The weekly summary feed, the USGS magnitude 2.5 and larger feed for the past seven days, is where this event surfaced as the largest quake in the United States for the week it covered, and the USGS notes in the same data that no tsunami was generated. Anyone wanting the raw numbers can pull the event identifier aka2026ukynnb from that feed and open the page directly, and the same identifier works in the USGS detail service, which returns the origin, magnitude type, review status and felt counts in a single file. Because the record was updated three days after the shaking, the figures here reflect the reviewed state rather than the first automatic solution.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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