Location:

Period:

23 Jan 2018 09:44:12 - 2 Feb 2018 15:45:44 (10 days 6 hours 1 minute)

Volcanoes in 100km radius:

None

Earthquakes:

191

4 swarms found nearby.

2018

PS20180123.1(48.5km)

23 Jan

1 day 12 hours

9 earthquakes

S20180123.3(14.0km)

23 Jan

5 days 2 hours

78 earthquakes

S20180124.3(27.7km)

23 Jan

3 days 20 hours

85 earthquakes

S20180208.1(18.2km)

7 Feb

1 day 15 hours

32 earthquakes

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Seismic Swarm S20180123.2: Activity in the Kodiak Subduction Zone

An earthquake swarm designated S20180123.2 was recorded southeast of Chiniak on Kodiak Island, Alaska. The sequence began at 09:44 on 23 January 2018 and concluded at 15:45 on 2 February 2018. Over 246 hours and one minute, 191 earthquakes were detected at a location 219 km southeast of Chiniak.

The region lies within the Alaska-Aleutian subduction zone, where the Pacific Plate descends beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent seismicity, including both thrust and strike-slip events at depths typically ranging from 10 to 40 km. Kodiak Island itself sits above the shallow portion of the megathrust, making it susceptible to both large interface earthquakes and smaller intraslab activity.

The swarm exhibited magnitudes between 2.5 and 4.7, with the majority of events clustered between 4 and 34 km depth. Early activity included several events above magnitude 4.0 within the first 24 hours, followed by a steady decline in both rate and size. Depths remained predominantly in the upper 30 km, consistent with seismicity along the plate interface and within the overriding crust.

Analysis of the first 100 recorded events reveals a typical swarm pattern: an initial energetic phase with multiple felt shocks, followed by aftershock-like decay without a single dominant mainshock. The largest event reached magnitude 4.7 at 13 km depth on 23 January. Subsequent events showed no clear migration, suggesting distributed failure on a network of small faults rather than a single propagating rupture.

Historically, such swarms are uncommon in this sector of the subduction zone. Records since 1 January 2000 indicate only one prior swarm, which is the present sequence itself. The 1964 magnitude-9.2 Great Alaska Earthquake, centered farther east near Prince William Sound, remains the dominant historical benchmark for regional hazard. That event produced widespread uplift and subsidence across Kodiak and triggered numerous aftershocks that continued for years.

Current monitoring by the Alaska Earthquake Center and the U.S. Geological Survey continues to track background seismicity in the area. While the 2018 swarm did not culminate in a larger mainshock, it underscores the persistent strain accumulation along the megathrust and the value of dense seismic networks for distinguishing swarm behavior from foreshock sequences.

References

USGS Earthquake Catalog (ANSS Comprehensive Catalog)
Alaska Earthquake Center regional reports
Alaska-Aleutian subduction zone tectonic summaries (USGS Professional Papers)