Seismic Analysis of Swarm S20021023.2 Near McKinley Park, Alaska

Earthquake swarm S20021023.2 occurred in a tectonically active zone 58 km east-southeast of McKinley Park, Alaska. The sequence began at 12:17 on 23 October 2002 and concluded at 06:24 on 25 October 2002, spanning 42 hours and 6 minutes. During this interval, 37 earthquakes were recorded.

The events clustered at shallow depths, ranging from 0 km to 17 km. Magnitudes remained modest, with the largest reaching 2.8. Early activity on 23 October featured several events above magnitude 2.0, including a 2.7 at 12:34 and a 2.8 at 19:11. Subsequent days showed lower-frequency, smaller-magnitude occurrences that gradually diminished.

This swarm represents the first documented swarm in the region since systematic recording began in 2000. A second swarm followed later in the observational period, confirming that such clustered seismicity is infrequent but recurrent along this segment of the fault system.

The location lies within the Alaska Range, where ongoing convergence between the Pacific and North American plates drives deformation along major strike-slip structures, notably the Denali Fault. Shallow crustal earthquakes in this setting commonly result from slip on subsidiary faults or distributed fracturing within the fault zone. Depths recorded in the swarm align with typical brittle failure in the upper crust of this region.

Seismic swarms of this type often reflect fluid migration, stress triggering, or aseismic slip episodes rather than a single mainshock-aftershock sequence. The temporal pattern—intense initial activity followed by rapid decay—matches characteristics observed in other intraplate or transform-margin swarms worldwide.

Geological mapping indicates that the Denali Fault accommodates several millimeters of right-lateral motion annually. Historical large-magnitude events along the fault have released accumulated strain over centuries, underscoring the long-term seismic hazard. The 2002 swarm preceded the magnitude 7.9 Denali earthquake by eleven days, highlighting how minor clustered activity can occur within broader tectonic loading cycles.

Continued monitoring of microseismicity remains essential for refining hazard models in central Alaska. Data from S20021023.2 contribute to understanding background rates and the potential for precursory signals ahead of larger ruptures.

References

• SeismoSight internal swarm classification records
• USGS Earthquake Catalog (general regional tectonics)
• Alaska Earthquake Center annual reports (fault zone context)