Seismic Swarm S20021125.1 Near Cantwell, Alaska: Geological Context and Event Analysis

Seismic swarm S20021125.1 occurred approximately 63 km east of Cantwell in central Alaska, within the tectonically active Alaska Range. The sequence began at 06:30 on 24 November 2002 and concluded at 03:28 on 28 November 2002, spanning 92 hours and 57 minutes. During this period, 81 earthquakes were recorded, with magnitudes ranging from 0.5 to 2.9 and focal depths predominantly between 0 and 24 km.

Central Alaska lies at the boundary between the Pacific and North American plates, where oblique convergence drives right-lateral strike-slip motion along the Denali Fault system. This major intracontinental fault extends over 2,000 km and accommodates significant strain accumulation. The Cantwell area sits near the fault's central segment, where crustal deformation produces frequent shallow seismicity. Depths observed in the swarm align with typical brittle failure in the upper 15–20 km of the crust, consistent with the regional fault architecture.

The swarm commenced three weeks after the 3 November 2002 magnitude 7.9 Denali Fault earthquake, which ruptured a 340 km section of the fault and triggered widespread aftershocks. While the swarm events remained small, their timing and clustered spatial distribution suggest possible dynamic triggering or post-seismic stress redistribution. Event depths clustered between 1 and 7 km during peak activity on 25–27 November, indicating shallow slip on subsidiary structures.

Analysis of the sequence reveals a gradual increase in event rate after the initial hours, with several magnitude 2+ events on 25 November, including peaks of 2.8 and 2.6. Activity persisted through 27 November, culminating in a magnitude 2.9 event at shallow depth on that date. The final events on 28 November were of lower magnitude and marked the swarm's decline. Overall, 22 events exceeded magnitude 1.5, illustrating a classic swarm pattern of numerous small earthquakes without a single dominant mainshock.

This swarm represents the earliest recorded since systematic monitoring began in 2000, highlighting episodic clustered seismicity along the Denali system. Subsequent swarms have occurred in the region, underscoring its persistent seismic productivity.

References
Alaska Earthquake Center, University of Alaska Fairbanks
USGS Earthquake Hazards Program – Denali Fault 2002 sequence reports
SeismoSight internal swarm classification database