Seismic Swarm S20021104.4: Analysis of Early Events Near Paxson, Alaska

A significant seismic swarm, designated S20021104.4, occurred 25 km north-northeast of Paxson, Alaska. It initiated at 22:44 on 3 November 2002 and concluded at 08:58 on 4 February 2003, encompassing 2412 earthquakes over 2218 hours and 13 minutes. This activity unfolded in a tectonically dynamic region shaped by the ongoing subduction of the Pacific Plate beneath the North American Plate along the Aleutian megathrust, coupled with right-lateral strike-slip motion on the Denali Fault system.

The swarm's timing aligns closely with the magnitude 7.9 Denali Fault earthquake of 3 November 2002, whose epicenter lay approximately 90 km to the east. Post-mainshock stress redistribution likely triggered the swarm through a combination of static and dynamic triggering mechanisms. The area's geology features Paleozoic to Mesozoic sedimentary and volcanic rocks intruded by granitic plutons, overlying a complex basement of accreted terranes. Crustal thickness averages 35–40 km, with shallow seismicity concentrated above 15 km depth due to the brittle-ductile transition.

Examination of the first 100 events reveals a rapid onset dominated by moderate-magnitude shocks. The initial event registered magnitude 4.0 at 8 km depth, followed within minutes by magnitudes 4.4 and 4.9. Depths clustered predominantly between 0 and 10 km, with only isolated events exceeding 15 km. Magnitudes declined progressively after the first day, transitioning from frequent events above 4.0 to a predominance of 2.0–3.5 shocks. Temporal spacing averaged several minutes during peak activity on 3–4 November, lengthening thereafter. This pattern indicates fluid migration or aseismic slip facilitating aftershock triggering along fault segments parallel to the Denali system.

Geodetic and seismic monitoring in south-central Alaska documents elevated background seismicity rates since the early 2000s, consistent with post-Denali relaxation. No prior swarms have been recorded in the instrumental catalog since 1 January 2000, underscoring the exceptional nature of this sequence. Updated regional hazard assessments from the Alaska Earthquake Center continue to highlight the Denali Fault's potential for future large events, informed by paleoseismic trenching that reveals recurrence intervals of 300–900 years for magnitude 7+ ruptures.

Continued analysis of swarm statistics aids in refining models of fault interaction and induced seismicity in convergent margins. Such insights support improved real-time monitoring protocols for similar tectonic settings worldwide.

References

• Alaska Earthquake Center, University of Alaska Fairbanks – Catalog and swarm metadata.
• U.S. Geological Survey – Denali Fault earthquake reports and regional tectonics summaries.
• Geological Society of America – Publications on Denali Fault paleoseismology and crustal structure.