Seismic Swarm S20030910.1: Analysis of Earthquake Activity Near McKinley Park, Alaska

Seismic swarm S20030910.1 was recorded 88 km east-southeast of McKinley Park, Alaska, beginning at 12:39 on 9 September 2003 and concluding at 21:36 on 11 September 2003. Over 56 hours and 56 minutes, the event sequence comprised 35 earthquakes, providing a clear example of clustered seismic activity in central Alaska.

The swarm exhibited a range of magnitudes from 0.8 to 4.3, with the majority of events below magnitude 2.0. Notable larger shocks included a magnitude 4.3 earthquake at 07:34 on 10 September at a depth of 4 km, followed closely by a magnitude 3.5 event at 07:38. Depths throughout the sequence varied from surface levels to 16 km, indicating activity within the shallow to mid-crustal layers typical of the region. The temporal distribution showed peak activity on 10 September, with events spaced closely in time during the morning hours before tapering off.

This swarm forms part of broader historical patterns in the area. Since 1 January 2000, 18 swarms have been documented in the vicinity. Earlier episodes occurred in 2002 (13 swarms) and 2003 (5 swarms), underscoring recurring clustered seismicity near the Denali Fault system.

Central Alaska lies at the boundary between the Pacific and North American plates, where oblique convergence drives significant deformation. The Denali Fault, a major right-lateral strike-slip structure extending across the region, accommodates much of this motion and has produced large historical earthquakes. The 2002 magnitude 7.9 Denali event, located nearby, released substantial strain and influenced subsequent seismic patterns through stress transfer. Shallow crustal earthquakes in this setting often occur along subsidiary faults and fractures associated with the main trace of the Denali system, consistent with the depths observed in swarm S20030910.1.

Such swarms typically reflect fluid migration, aseismic slip, or localized stress adjustments rather than mainshock-aftershock sequences. In the McKinley Park area, the combination of high background seismicity and complex fault geometry supports frequent low-to-moderate magnitude clusters. Depths between 0 and 16 km align with the brittle-ductile transition zone in this tectonically active corridor.

Ongoing monitoring by regional seismic networks continues to track activity along the Denali Fault, contributing to improved understanding of strain accumulation and release in one of North America’s most dynamic seismic zones.

References

• Alaska Earthquake Center, University of Alaska Fairbanks (seismic catalog data)
• U.S. Geological Survey Earthquake Hazards Program (regional tectonics reports)
• SeismoSight internal swarm classification records