Seismic Swarm S20021023.1: Analysis of Activity near Cantwell, Alaska

The earthquake swarm designated S20021023.1 began at 11:27 on 23 October 2002 and concluded at 18:15 on 4 November 2002. Centered 44 km east-northeast of Cantwell, Alaska, the sequence lasted 294 hours and 47 minutes and produced 727 recorded events. This episode occurred within the tectonically active Alaska Range, a region shaped by ongoing convergence between the Pacific and North American plates.

Cantwell lies near the Denali Fault, a major right-lateral strike-slip structure that accommodates much of the lateral motion associated with subduction along the Aleutian megathrust. The local crust consists of accreted terranes intruded by Mesozoic and Cenozoic plutons, with overlying sedimentary basins that record uplift and erosion linked to fault movement. Shallow crustal depths in the area typically range from 0 to 20 km, consistent with the observed event depths during the swarm.

The sequence opened with a magnitude 6.6 earthquake at 4 km depth. Subsequent events showed a rapid decline in magnitude, with the majority falling between 1.0 and 3.3. Depths remained predominantly shallow, clustering between 1 and 15 km. The first 100 events illustrate this pattern: an initial energetic phase lasting several hours gave way to a sustained sequence of smaller shocks, many occurring at depths of 1–9 km. Notable follow-up events included magnitudes of 3.8, 3.6, 3.3, and 3.2, all within the first day. Activity tapered gradually, yet maintained elevated rates until early November.

Such swarms are characteristic of the Denali Fault zone, where fluid migration and stress transfer along subsidiary fractures can trigger prolonged clusters without a single dominant mainshock-aftershock decay. Historical records document similar episodes in the central Alaska Range, including swarms in the 1980s and 1990s that preceded larger regional events. The 2002 sequence occurred weeks before the magnitude 7.9 Denali Fault earthquake of 3 November 2002, highlighting the fault system's capacity for both clustered microseismicity and great earthquakes.

Seismic monitoring in the region relies on the Alaska Earthquake Center network, which provides precise locations and magnitudes for events above magnitude 1.0. Depths reported for the swarm align with the brittle upper crust, where temperatures permit stick-slip behavior on pre-existing faults.

The swarm underscores the persistent seismic hazard along the Denali Fault. Ground shaking from the opening magnitude 6.6 event was felt across central Alaska, though damage remained limited due to the remote location. Continued monitoring helps refine models of fault segmentation and strain accumulation that inform regional hazard assessments.

References

SeismoSight internal swarm classification S20021023.1
Alaska Earthquake Center regional seismicity reports
USGS Earthquake Hazards Program tectonic framework summaries for the Denali Fault system