Earthquake Swarm S20070715.1: Seismic Activity Near Nikolski, Alaska

An earthquake swarm designated S20070715.1 occurred southeast of Nikolski, Alaska, beginning at 13:12 on 15 July 2007 and concluding at 01:55 on 23 July 2007. Over approximately 180 hours and 42 minutes, the swarm produced 182 events. The first 100 recorded earthquakes ranged in magnitude from 1.8 to 5.9, with the majority occurring at depths between 4 and 15 km. Notable larger events included a magnitude 5.9 earthquake at 13:26 on 15 July, a magnitude 5.1 at 14:09 the same day, and a magnitude 4.8 at 13:15. Subsequent activity featured repeated events in the 2.0–3.3 range, clustered in the initial 48 hours before tapering.

The swarm's location, 92 km southeast of Nikolski on Umnak Island, places it within the central Aleutian Islands. This region lies along the Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–7 cm per year. The resulting compressional stress generates frequent seismic sequences, including both mainshock-aftershock patterns and swarm-like activity without a dominant mainshock.

Geologically, the central Aleutians feature a chain of volcanic islands formed by this subduction process. Umnak Island hosts several stratovolcanoes, such as Mount Vsevidof and Mount Recheshnoi, whose magma systems interact with regional faulting. Depths of the swarm events align with the upper crustal levels above the subducting slab, where brittle failure occurs along thrust and strike-slip faults. Historical records document recurrent moderate-to-large earthquakes in the area, including the 1957 magnitude 8.6 event and the 1946 magnitude 8.1 quake, both of which produced significant tsunamis affecting the Aleutians.

Seismic swarms in this tectonic setting often reflect fluid migration or stress transfer along the plate interface and overlying crust. The 2007 sequence fits this pattern, with events distributed across a compact area and lacking a clear single rupture plane. Updated monitoring by regional networks confirms ongoing background seismicity in the central Aleutians, underscoring the persistent hazard from both earthquakes and associated volcanic unrest.

Analysis of the initial 100 events shows a rapid onset followed by a gradual decline in rate and magnitude, consistent with swarm behavior driven by localized crustal adjustments rather than a classic aftershock sequence. Depths remained shallow throughout, rarely exceeding 15 km, reflecting the brittle-ductile transition in this arc environment.

References

USGS Earthquake Hazards Program – Aleutian Islands Seismicity Alaska Earthquake Center – Regional Tectonic Summary NOAA National Centers for Environmental Information – Historical Earthquake Database