Seismic Swarm S20240502.1 Near Nikolski, Alaska: Geological Context and Event Analysis

A seismic swarm designated S20240502.1 was recorded 110 km south-southwest of Nikolski, Alaska, from 07:01 on 2 May 2024 to 09:51 on 4 May 2024. Over 50 hours and 50 minutes, the swarm comprised 93 earthquakes, providing a concentrated episode of activity in the central Aleutian region.

The swarm initiated with several events exceeding magnitude 5.0, including a 5.1 quake at 10 km depth, followed rapidly by a 5.3 at 12 km and a 5.4 at 18 km. Subsequent activity featured a broad distribution of magnitudes, with the majority falling between 2.0 and 4.0. Depths clustered predominantly around 10 km, though a subset reached 30–41 km, indicating both shallow crustal and slightly deeper processes. Notable later events included a 4.7 at 10 km depth on 2 May at 11:05 and a 4.0 at 10 km on the same day at 20:23. The sequence tapered gradually, concluding with smaller events below magnitude 3.0.

This swarm occurred within the tectonically active Aleutian subduction zone, where the Pacific Plate descends beneath the North American Plate. The central Aleutians, including areas near Umnak Island and Nikolski, experience frequent seismicity due to this convergent margin. Historical records since 2000 document seven prior swarms in the immediate region: one each in 2007, 2008, and 2022, plus four in 2015. These episodes reflect episodic stress release along the plate interface and associated crustal faults.

Geological studies of the Aleutian arc highlight its role in the Pacific Ring of Fire, with volcanic and seismic hazards driven by subduction dynamics. The 2024 swarm aligns with patterns of clustered, moderate-magnitude activity typical of the zone, where shallow events often link to fluid migration or fault slip, while deeper ones may involve slab-related processes.

Analysis of the listed events shows a high initial rate of larger shocks followed by a prolonged tail of smaller aftershocks. Depths near 10 km dominated early phases, consistent with upper-plate or interface seismicity, whereas deeper occurrences appeared later. This temporal evolution offers insight into swarm mechanics without indicating a single mainshock-aftershock sequence.

References

• SeismoSight internal swarm classification S20240502.1 (2024)
• USGS Earthquake Catalog (Aleutian region data, accessed 2024)
• Tectonic summaries from the Alaska Earthquake Center (University of Alaska Fairbanks)