Seismic Swarm S20210713.1 Near Aleneva, Alaska: Geological Context and Event Analysis

Seismic swarm S20210713.1 was recorded 81 km NNW of Aleneva, Alaska, on Afognak Island within the Kodiak archipelago. The sequence began at 17:17 on 12 July 2021 and concluded at 13:07 on 14 July 2021, spanning 43 hours and 50 minutes. During this interval, 28 earthquakes were detected, with magnitudes ranging from 0.4 to 2.4 and focal depths between 0 and 15 km. The largest event reached magnitude 2.4 at a depth of 6 km on 12 July at 18:17. Subsequent notable shocks included a magnitude 1.9 event at 8 km depth shortly afterward and additional events clustered in the early morning hours of 13 July, with the sequence tapering to smaller magnitudes by 14 July.

This swarm occurred in a region dominated by the convergent boundary between the Pacific and North American plates. The Pacific Plate subducts beneath the North American Plate along the Aleutian megathrust at rates of approximately 6–7 cm per year, generating frequent seismic activity across the Kodiak segment of the subduction zone. Shallow crustal events, such as those in swarm S20210713.1, commonly arise from stress adjustments within the overriding plate or fluid migration along fractures. Depths recorded in this sequence align with typical upper-crustal seismicity in the area, where the megathrust interface lies at greater depths offshore.

The Kodiak region has experienced significant historical seismicity. The 1964 Great Alaska Earthquake, magnitude 9.2, originated nearby and produced extensive uplift and subsidence across Afognak and Kodiak islands. Post-1964 aftershock patterns and subsequent swarms reflect ongoing strain accumulation and release along the locked portion of the megathrust. Since 2000, four prior swarms have been documented in the immediate vicinity: one each in 2013, 2016, 2017, and 2020. These episodes share characteristics with S20210713.1, featuring low-magnitude events over short durations without a dominant mainshock, consistent with swarm behavior driven by aseismic slip or hydrothermal processes rather than classic foreshock-mainshock-aftershock sequences.

Analysis of the 2021 swarm reveals a rapid onset followed by a gradual decline in event rate and magnitude. Early events on 12 July clustered at depths of 4–8 km, while later activity on 13 July extended to slightly greater depths up to 15 km before returning to shallower levels. This temporal and spatial distribution suggests localized stress redistribution within a small crustal volume. No damage or felt reports were associated with these microearthquakes, underscoring their minor energetic release compared with larger regional events.

Ongoing monitoring by regional seismic networks continues to track activity in this high-hazard zone. The recurrence of swarms highlights the dynamic nature of the subduction margin and the value of dense instrumentation for distinguishing swarm patterns from potential precursors to larger earthquakes.

References

USGS Earthquake Hazards Program – Alaska Earthquake Catalog
Alaska Earthquake Center – Regional Seismicity Reports
NOAA National Centers for Environmental Information – Historical Earthquake Database