Seismic Swarm S20180124.6: Earthquake Activity Southeast of Chiniak, Alaska

Seismic swarm S20180124.6 occurred southeast of Chiniak, Alaska, in a tectonically active region shaped by the subduction of the Pacific Plate beneath the North American Plate along the Aleutian megathrust. The swarm began at 10:30 on 23 January 2018 and concluded at 19:10 on 25 January 2018, lasting 56 hours and 40 minutes. During this period, 55 earthquakes were recorded at a location 229 km southeast of Chiniak on Kodiak Island. This event represents the first swarm documented in the area since 1 January 2000, highlighting episodic seismic behavior in an otherwise stable segment of the subduction zone.

The swarm exhibited a characteristic pattern of clustered seismicity with magnitudes predominantly between 3.0 and 4.6. Early activity on 23 January included events of 3.6, 3.8, and a peak of 4.4 within the first few hours, followed by sustained moderate shaking through the evening. Depths ranged from 10 km to 24 km, with many initial shocks occurring around 20 km before shifting shallower later in the sequence. On 24 January, activity continued at a steady rate with events such as 3.8 and 3.6, maintaining similar depth distributions. The swarm culminated on 25 January with the largest event of magnitude 4.6 at 10 km depth, accompanied by several smaller aftershocks at shallow levels.

Analysis of the temporal distribution reveals two main phases: an initial high-frequency burst on the first day followed by more dispersed events over the subsequent 48 hours. Depths showed a slight shallowing trend toward the end of the swarm, consistent with fluid migration or stress transfer within the overriding plate. No events exceeded magnitude 5.0, indicating a swarm rather than a mainshock-aftershock sequence. This pattern aligns with known behavior in subduction-related settings where aseismic slip or pore-pressure changes can trigger dense clusters without a dominant rupture.

The broader geological context of the Kodiak region features repeated megathrust earthquakes due to the locked interface between the subducting Pacific Plate and the North American Plate. Historical records document major events, including the 1964 Great Alaska Earthquake of magnitude 9.2, which ruptured a vast portion of the Aleutian subduction zone extending through the Kodiak segment. Modern monitoring confirms ongoing strain accumulation, with smaller swarms serving as indicators of localized stress release along the plate boundary and within the forearc crust.

Updated assessments of Alaska's seismic hazard emphasize the potential for future large events in this area, informed by continuous GPS and seismic networks operated by regional agencies. The 2018 swarm provides valuable data for refining models of episodic tremor and slip, contributing to improved forecasting of seismic hazards in the Aleutian arc.

References
USGS Earthquake Hazards Program: Alaska Earthquake Information
Alaska Earthquake Center: Regional Tectonic Overview
NOAA National Centers for Environmental Information: Historical Earthquake Database