Location:

Period:

23 Jan 2018 11:43:46 - 27 Jan 2018 07:58:32 (3 days 20 hours 14 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

3 swarms found nearby.

2018

PS20180123.1(76.1km)

23 Jan

1 day 12 hours

9 earthquakes

S20180123.2(27.7km)

23 Jan

10 days 6 hours

191 earthquakes

S20180208.1(9.8km)

7 Feb

1 day 15 hours

32 earthquakes

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Seismic Swarm S20180124.3: January 2018 Earthquake Sequence Southeast of Chiniak, Alaska

Seismic swarm S20180124.3 occurred 222 km southeast of Chiniak, Alaska, from 11:43 UTC on 23 January 2018 to 07:58 UTC on 27 January 2018. Over 92 hours and 14 minutes, the sequence produced 85 earthquakes. This event represents the first swarm recorded in the region since 2000 according to internal SeismoSight classifications.

The swarm initiated with a magnitude 4.1 event at 7 km depth. Subsequent activity included multiple events exceeding magnitude 4.0, with the largest reaching 4.3 at 16 km depth on 23 January and another 4.3 at 6 km depth on 25 January. Depths ranged primarily between 4 km and 30 km, with many clustered around 10–23 km. Later phases on 26–27 January featured events at shallower depths near 5–10 km before the sequence concluded with a magnitude 3.6 at 8 km depth.

This offshore location lies within the tectonically active margin of southern Alaska. The area forms part of the Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–7 cm per year. The resulting compressional stress generates frequent earthquakes along the megathrust interface and within the overriding and subducting plates. Regional geology features the Kodiak accretionary complex and the adjacent Aleutian Trench, contributing to elevated seismic hazard.

Historical records document major earthquakes in the vicinity, including the 1964 Great Alaska Earthquake of magnitude 9.2, which ruptured a segment extending from Prince William Sound toward Kodiak Island. That event produced widespread uplift, subsidence, and tsunamis. Smaller magnitude sequences occur regularly, but swarm-type clustering with rapid onset and decay remains less common in the catalog since 2000.

Insight from the 2018 swarm shows a typical pattern of mainshock-aftershock decay interspersed with secondary bursts. Early high-magnitude events (4.1–4.3) on 23 January transitioned to sustained activity around magnitude 3.0–3.9 through 24–25 January. Activity then diminished, with isolated larger events on 26 January before termination. Depths remained consistent with the seismogenic zone of the subduction interface, suggesting possible triggering along a localized fault segment or fluid-related processes common in subduction settings.

The sequence highlights the value of dense monitoring for distinguishing swarms from typical aftershock sequences. No significant damage or tsunami was reported, consistent with the moderate magnitudes involved. Ongoing subduction dynamics indicate that similar clusters may recur, underscoring the importance of continued seismic surveillance in the Kodiak region.

References

SeismoSight internal swarm classification data for S20180124.3
USGS Earthquake Catalog (historical regional events)
Alaska Earthquake Center tectonic summaries (subduction zone parameters)