At 10:34 a.m. Alaska time on Oct. 6, a magnitude 5.5 earthquake broke roughly 88 kilometers beneath the Alaska Peninsula, and by Oct. 8 the U.S. Geological Survey had logged 342 first-hand accounts of it from people who felt the ground move. The USGS locates the rupture 92 km north-northwest of Aleneva, a small community on the Kodiak Island side of Shelikof Strait.
The count is large for a region with few residents. A shock this deep spreads its energy over a wide footprint, so a modest magnitude can be noticed across a long stretch of thinly populated coast, mountains and water.
USGS parameters for the Aleneva-referenced event
The agency’s event record lists the shock as a magnitude 5.5 earthquake at 18:34 UTC, with a USGS alert level of green and a tsunami flag of zero. The same entry was also carried in the agency’s significant-earthquakes-this-week feed, which is where the event first drew attention as a 5.5 with 342 felt reports.
The place string is a reference point, not an address. Aleneva is the named place USGS used for this shock, while the epicenter itself sits on the mainland peninsula at about 58.80 degrees north and 153.61 degrees west.
The record carries more than one solution. The magnitude type on the Alaska network’s reviewed solution is ML, and the USGS entry also attaches moment-tensor solutions of Mww 5.5 and Mwb 5.6, the latter preliminary. The national center’s own ShakeMap places the depth near 88.2 km, against 87.6 km on the event’s primary entry. Differences of that size are routine for events this deep.
Fourpeaked Mountain and an 88-kilometer depth
The Alaska Earthquake Center at the University of Alaska Fairbanks describes the same shock in local terms: about 3 miles northeast of Fourpeaked Mountain, 83 miles northwest of Kodiak and 210 miles southwest of Anchorage, at a depth of 54.6 miles. The center’s review status for the event is reviewed by a seismologist, meaning an analyst checked the automatic solution.
Fourpeaked is a volcano, and that raises the obvious question about whether the quake was volcanic. The Alaska Volcano Observatory lists Fourpeaked Mountain at 2,104 meters elevation, with a color code and alert level both listed as unassigned, and states that the volcano is not seismically monitored. The depth makes a volcanic cause unlikely on its face. Magma-related shaking is typically shallow, while a shock 88 km down belongs to the intermediate-depth class.
The Alaska Earthquake Center’s general note for the Alaska Peninsula explains where such events originate: below about 32 km, earthquakes occur in the Wadati-Benioff zone, the slab of Pacific Plate descending beneath North America. The page states that most of these deep earthquakes are too small and too deep to be felt. The Oct. 6 shock was neither, which is why it stands out in the record. The center’s page does not itself assign this event to the slab, so that attribution rests on the depth and the regional setting.
The DYFI felt-report tally of 342
The 342 figure comes from the USGS DYFI program, a citizen-science project that collects information from people who felt an earthquake and turns it into intensity maps. The event’s detail feed lists 342 responses in the DYFI product, updated Oct. 8 at 18:44 UTC. An earlier cached read of the same event showed 293, so the tally had grown by about 49 reports as submissions continued in the days after the shock.
The aggregated community intensity is 4.6, with the DYFI maximum also at 4.6. On the Modified Mercalli scale, which USGS says has no mathematical basis and ranks observed effects, the lower numbers describe mainly how people feel the shaking. The structural-damage numbers sit far higher, since engineers help assign values of VIII and above. The modeled ShakeMap values agree with the low reading: a maximum intensity around 4.2. Fox Weather, quoting the USGS, reported that many people felt it indoors and few outdoors, and that dishes, windows and doors were disturbed. No injuries or damage were confirmed in the coverage available at the time of writing.
Tsunami status and what the shaking model shows
The event’s tsunami flag reads zero, and the national tsunami warning center status page for Alaska, British Columbia and the West Coast carried no warning, advisory, watch or threat when it was read on Oct. 9, listing only a magnitude 6.6 shock near Vanuatu from Oct. 8.
The ShakeMap numbers explain the modest felt effects. USGS lists a peak ground acceleration of 0.017 in the national solution and 0.028 in the Alaska solution, both small fractions of gravity. Both are well short of the intensities tied to structural damage.
One item stays open in the record: the USGS entry carries both a reviewed Mww 5.5 and a preliminary Mwb 5.6, so the magnitude could still be revised, though the 342-report tally and the 92 km place string are the figures USGS had published as of its Oct. 8 update.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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