Seismic Swarm S20191222.1: Analysis of Earthquake Activity Southeast of Old Harbor, Alaska

A seismic swarm designated S20191222.1 was recorded southeast of Old Harbor on Kodiak Island, Alaska. The sequence began at 17:41 on 21 December 2019 and concluded at 21:53 on 22 December 2019, encompassing 26 earthquakes over a period of 28 hours and 11 minutes. The epicentral region lies approximately 106 km southeast of Old Harbor within the tectonically active Aleutian subduction zone.

Kodiak Island occupies a position above the convergent boundary where the Pacific Plate subducts northwestward beneath the North American Plate. Convergence rates average 6–7 cm per year, generating frequent seismicity across a range of magnitudes. Shallow crustal earthquakes and intermediate-depth events both occur regularly, reflecting slip along the megathrust interface and within the overriding plate. The broader region experienced the 1964 Great Alaska Earthquake (Mw 9.2), one of the largest instrumentally recorded events, which produced extensive rupture along the subduction interface and triggered widespread aftershock sequences.

The 2019 swarm displayed typical characteristics of clustered activity in a subduction setting. Magnitudes ranged from 2.5 to 4.7, with the largest events (4.6 and 4.7) occurring within minutes of each other on 21 December at depths of 7 km and 1 km, respectively. Most hypocenters were concentrated between 0 km and 15 km depth, consistent with upper-plate or near-interface faulting. Activity initiated with a 2.6 event at 9 km depth, built to a peak on the evening of 21 December, and gradually declined through the following day, ending with a 3.1 event at 5 km depth.

Such swarms commonly arise from localized stress perturbations, including fluid migration along fractures or slow slip on adjacent fault segments. The short duration and moderate magnitudes suggest a confined release of accumulated strain rather than a foreshock sequence preceding a larger mainshock. Historical records maintained since 1 January 2000 indicate only one prior swarm in the immediate vicinity, which occurred in 2009, underscoring the relative infrequency of clustered events in this specific sector.

Ongoing monitoring by regional seismic networks remains essential for distinguishing swarm behavior from potential precursory activity. Continued observation will refine understanding of strain accumulation along this portion of the Alaska-Aleutian megathrust and improve hazard assessments for nearby communities on Kodiak Island.

References

• USGS Earthquake Catalog
• Alaska Earthquake Center
• SeismoSight internal swarm classification records