Seismic Swarm VS20160928.1: Analysis of Activity Near Kokhanok, Alaska

The earthquake swarm VS20160928.1 occurred approximately 93 km southeast of Kokhanok, Alaska, on the Alaska Peninsula. Registered between 10:33 on 27 September 2016 and 14:48 on 28 September 2016, the sequence lasted 28 hours and 15 minutes and included 35 events. All recorded magnitudes remained below 3.0, with the largest reaching 2.1. Focal depths ranged from 1 km to 18 km, indicating shallow crustal activity typical of the regional tectonic setting.

The Alaska Peninsula lies within the tectonically active Pacific Ring of Fire. It forms part of the convergent margin where the Pacific Plate subducts beneath the North American Plate at rates of 6–7 cm per year. This subduction drives both megathrust earthquakes and volcanic arc development. Kokhanok sits near the northeastern shore of Iliamna Lake, placing the swarm epicenters in proximity to the Iliamna volcanic center and associated fault systems. The regional geology features Mesozoic sedimentary and volcanic rocks overlain by Quaternary glacial deposits, with active faults accommodating strain from the subducting slab.

Seismic swarms in this area often reflect stress release along secondary faults or fluid migration within the crust rather than direct magmatic intrusion. The 2016 sequence displayed a rapid onset followed by a gradual decline in event rate, consistent with aftershock-like behavior in a low-magnitude cluster. Depths concentrated between 3 km and 11 km for most events, aligning with the brittle-ductile transition zone in the overriding plate.

Historical records maintained by SeismoSight indicate that only one prior swarm has been documented in the same classification category since 1 January 2000. That earlier episode occurred in 2013, suggesting infrequent swarm-type activity at this specific location despite the high overall seismicity of southern Alaska.

The 35 events of swarm VS20160928.1 provide a clear example of clustered microseismicity that releases accumulated strain without producing damaging ground motion. Continued monitoring remains essential given the proximity to both population centers around Iliamna Lake and active volcanic systems that could interact with tectonic processes.

References

SeismoSight internal swarm classification records
USGS Earthquake Catalog (regional tectonic framework)
Alaska Volcano Observatory geologic summaries (subduction and volcanic context)