Seismic Swarm S20021105.1: Analysis of Central Alaska Earthquake Sequence

Central Alaska occupies a tectonically complex region at the northern edge of the Pacific Ring of Fire. The area experiences frequent seismic activity driven by the subduction of the Pacific Plate beneath the North American Plate along the Aleutian megathrust, combined with right-lateral strike-slip motion along major crustal faults such as the Denali and Tintina systems. These interactions produce both deep subduction-related events and shallower crustal earthquakes. Seismic swarms in this setting often reflect fluid migration, stress triggering, or aftershock sequences following larger ruptures. The November 2002 Denali Fault earthquake (magnitude 7.9) occurred one day before the onset of Swarm S20021105.1, highlighting the potential for dynamic triggering across the regional fault network. Swarm S20021105.1 began at 05:12 on 4 November 2002 and concluded at 14:53 on 14 November 2002, spanning 249 hours and 41 minutes. During this interval, 157 earthquakes were recorded. Analysis of the first 100 events reveals a compact spatiotemporal cluster. The sequence initiated with a magnitude 3.1 event at 1 km depth, followed within hours by additional events of magnitude 2.8–3.0. The largest event, magnitude 4.6, occurred at 10:15 on 4 November at 1 km depth, after which activity transitioned to smaller magnitudes predominantly between 1.1 and 3.6. Depth distribution among the initial 100 events shows strong concentration in the upper crust: 78 events occurred at depths of 0–5 km, 17 at 6–10 km, and only five exceeded 10 km, with a maximum recorded depth of 17 km. This shallow focus is consistent with brittle failure within the seismogenic zone above the brittle-ductile transition in central Alaska. Temporal patterns indicate an initial energetic phase on 4–5 November, featuring the highest magnitudes and event rates, followed by a gradual decline. Magnitudes decreased overall, with only four events exceeding magnitude 3.0 after 5 November. The sequence displayed typical swarm characteristics: no single dominant mainshock-aftershock pairing, but rather a diffuse energy release distributed across numerous small faults or fractures. Such shallow swarms contribute to the long-term seismic hazard assessment of central Alaska by illuminating active fault segments and stress conditions. Continued monitoring by regional networks remains essential for distinguishing swarm behavior from potential precursors to larger events.

References

• Alaska Earthquake Center, University of Alaska Fairbanks
• U.S. Geological Survey Earthquake Hazards Program
• Denali Fault Earthquake sequence reports (2002)