Location:

Period:

23 Jan 2018 10:55:21 - 28 Jan 2018 13:52:00 (5 days 2 hours 56 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

3 swarms found nearby.

2018

PS20180123.1(34.6km)

23 Jan

1 day 12 hours

9 earthquakes

S20180123.2(14.0km)

23 Jan

10 days 6 hours

191 earthquakes

S20180123.1(21.4km)

23 Jan

9 days 3 hours

140 earthquakes

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Seismic Swarm S20180123.3: Analysis of Earthquake Activity Southeast of Chiniak, Alaska

Seismic swarm S20180123.3 occurred in a tectonically active region 225 km south-southeast of Chiniak, Alaska, within the Aleutian subduction zone. This area marks the convergence of the Pacific and North American plates, where the Pacific plate subducts northwestward at rates of approximately 6–7 cm per year. The resulting compressional stresses generate frequent seismic events, including both isolated earthquakes and clustered swarms.

The swarm initiated at 10:55 on 23 January 2018 and concluded at 13:52 on 28 January 2018, spanning 122 hours and 56 minutes. During this interval, 78 earthquakes were recorded. Magnitudes ranged from 2.5 to 5.0, with the largest event occurring on 26 January at 16:42:04 (magnitude 5.0 at 9 km depth). Depths varied between 2 km and 33 km, reflecting activity across multiple crustal levels consistent with subduction-related faulting.

Early phases of the swarm featured several events above magnitude 4.0 on 23 January, including a magnitude 4.3 at 20 km depth and another at 14 km depth. Activity persisted with moderate events through 24 and 25 January, shifting toward shallower depths initially before transitioning to deeper occurrences around 26–27 km on 25 January. The peak magnitude event on 26 January was followed by after-sequence activity, including a magnitude 4.6 at 6 km depth on 27 January. Later events on 28 January remained predominantly in the 3.0–3.9 range at depths of 7–25 km.

This swarm represents the first such clustered sequence in the region since 1 January 2000, according to historical records maintained through 2018. No prior swarms were identified in the available dataset for the preceding 18 years, underscoring the episodic nature of seismic clustering in this portion of the subduction zone.

The distribution of depths and magnitudes suggests a complex interplay of brittle failure along the plate interface and within the overriding crust. Shallow events (under 10 km) may relate to upper-plate deformation, while deeper events align with the subducting slab. Such patterns are typical in Alaska’s convergent margin, where the 1964 Great Alaska Earthquake (magnitude 9.2) demonstrated the potential for large-magnitude release along similar structures.

Ongoing monitoring by regional seismic networks continues to track background seismicity in this high-hazard area. The 2018 swarm provides valuable data for refining models of stress accumulation and release in subduction environments.

References

Alaska Earthquake Center, University of Alaska Fairbanks (updated seismic catalogs through 2023)
USGS Earthquake Hazards Program (tectonic setting and historical events for the Aleutian arc)
SeismoSight internal swarm classification records