Seismic Swarm S20221001.1: Analysis of Activity Near Ninilchik, Alaska

Seismic swarm S20221001.1 occurred 71 km northwest of Ninilchik on Alaska’s Kenai Peninsula. The sequence began at 23:23 on 30 September 2022 and concluded at 12:35 on 4 October 2022, spanning 85 hours and 11 minutes. During this interval, 74 earthquakes were recorded.

The swarm initiated with a magnitude 4.1 event at 13 km depth. Subsequent activity consisted predominantly of microearthquakes with magnitudes below 1.0, although several events reached magnitudes between 1.1 and 2.6. Depths clustered tightly between 9 km and 14 km, with the majority occurring near 11 km. A secondary cluster of slightly shallower events (5–7 km) appeared on 1 and 3 October. The largest aftershock within the swarm was a magnitude 2.6 event recorded minutes after the initial shock.

This pattern—brief, intense microseismicity without a single dominant mainshock—is characteristic of swarm behavior rather than a classic foreshock–mainshock–aftershock sequence. The rapid onset and gradual decay over four days align with fluid-migration or stress-transfer mechanisms commonly observed in subduction-related crust.

The Kenai Peninsula lies above the Aleutian subduction zone, where the Pacific Plate descends beneath the North American Plate at rates of approximately 6 cm per year. Ninilchik sits within the Cook Inlet forearc basin, a region that experiences frequent moderate earthquakes and occasional swarms. Historical records maintained since 2000 indicate only one prior swarm in the immediate vicinity, occurring in 2009. No comparable sequences have been documented in the intervening years, underscoring the relative rarity of swarm activity at this specific location.

The 2022 swarm’s depth range corresponds to the seismogenic zone within the overriding plate, above the deeper megathrust interface. Events at 5–7 km may reflect brittle failure in shallower crustal layers, while the dominant 11 km cluster likely occurred near the base of the seismogenic zone. Such depth distributions are consistent with regional velocity models for the Cook Inlet area.

No damage or felt reports beyond the immediate epicentral zone were associated with the swarm. Continued monitoring by the Alaska Earthquake Center remains essential for distinguishing isolated swarms from potential precursors to larger subduction-zone events.

References

• Alaska Earthquake Center, University of Alaska Fairbanks – regional seismicity catalog.
• USGS Earthquake Hazards Program – tectonic framework of the Aleutian subduction zone.