Location:

Period:

23 Jan 2018 09:55:36 - 1 Feb 2018 13:11:16 (9 days 3 hours 15 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

140

3 swarms found nearby.

2018

PS20180123.1(13.4km)

23 Jan

1 day 12 hours

9 earthquakes

S20180124.6(18.4km)

23 Jan

2 days 8 hours

55 earthquakes

S20180123.3(21.4km)

23 Jan

5 days 2 hours

78 earthquakes

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Seismic Swarm S20180123.1 Near Chiniak, Alaska: Geological Context and Event Analysis

Seismic swarm S20180123.1 occurred in a tectonically active region approximately 247 km southeast of Chiniak on Kodiak Island, Alaska. This area lies within the Alaska-Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of about 6-7 cm per year. The resulting compressional forces generate frequent earthquakes, including megathrust events such as the magnitude 9.2 Great Alaska Earthquake of 1964, whose epicenter was located near Prince William Sound but produced widespread effects across the Kodiak region.

Kodiak Island and its offshore environs feature complex geology shaped by subduction, including accreted terranes, volcanic arcs, and active fault systems. The swarm's location aligns with the broader seismicity of the Aleutian megathrust, where intermediate-depth events often occur along the plate interface or within the subducting slab. Depths recorded during the swarm, predominantly between 7 and 24 km, are consistent with activity in the upper portion of the subduction zone.

The swarm began at 09:55 on 23 January 2018 and concluded at 13:11 on 1 February 2018, spanning 219 hours and 15 minutes. A total of 140 earthquakes were registered. Analysis of the first 100 events reveals a sequence dominated by moderate magnitudes, with the largest reaching 4.8. Most events clustered between magnitudes 3.0 and 4.0, and focal depths showed a concentration around 10-22 km. Early activity included several events above magnitude 4.0 within the first hours, followed by a gradual decline in both frequency and peak magnitudes over subsequent days. Later events in this subset maintained similar depth ranges while exhibiting slightly lower average magnitudes, suggesting a typical swarm decay pattern without a single dominant mainshock.

This swarm represents the sole such episode recorded in the region since 2000, highlighting the episodic nature of clustered seismicity amid the area's persistent background tectonic activity. No significant surface rupture or volcanic association has been linked to the sequence.

References

Alaska Earthquake Center, University of Alaska Fairbanks (updated regional tectonic summaries).
USGS Earthquake Hazards Program (subduction zone parameters and historical Alaskan seismicity).