Seismic Swarm S20030807.1: Analysis of Earthquake Activity Near McKinley Park, Alaska

Seismic swarm S20030807.1 was recorded from 02:12 on 7 August 2003 to 20:25 on 13 August 2003, centered 85 km east-southeast of McKinley Park, Alaska. Over 162 hours and 12 minutes, the event sequence included 95 earthquakes. This swarm provides a clear example of clustered seismic activity in a tectonically dynamic region.

The swarm began with low-magnitude events on 7 August, including a 1.5 magnitude quake at 3 km depth. Activity intensified later that day, with a notable 4.5 magnitude earthquake at 5 km depth at 06:41:28. Subsequent events remained predominantly below magnitude 3, with depths mostly between 0 and 15 km. A secondary peak occurred on 11 August, featuring a 2.8 magnitude event at 4 km depth. By 13 August, activity tapered off with smaller events around magnitude 1.4–1.5 at shallow depths.

Magnitudes across the swarm ranged from 0.4 to 4.5, with the majority falling between 1.0 and 2.0. Depths were consistently shallow, rarely exceeding 15 km, indicating activity within the upper crust. The temporal distribution showed the highest frequency on the first two days, followed by a gradual decline, consistent with swarm behavior where no single mainshock dominates.

The location lies within the Alaska Range, part of the broader Pacific-North American plate boundary zone. This area experiences ongoing deformation driven by oblique convergence and strike-slip faulting along major structures such as the Denali Fault system. The region has a well-documented history of seismic activity, including both isolated events and episodic swarms.

Since 1 January 2000, 17 swarms have been identified in the area. Earlier sequences occurred in 2002 (12 events) and 2003 (5 events), highlighting recurrent clustered seismicity. Such patterns reflect the influence of local fault networks and fluid migration or stress transfer within the crust.

Geological studies of the Alaska Range confirm its formation through long-term tectonic compression, resulting in high topography and active faulting. The 2002 Denali Fault earthquake sequence, located nearby, demonstrated the capacity for larger events in this setting, though swarm S20030807.1 remained limited to moderate and smaller magnitudes.

Continued monitoring of swarm activity contributes to understanding seismic hazards in central Alaska. The shallow depths observed reinforce the importance of tracking upper-crustal processes for regional risk assessment.

References

SeismoSight internal swarm classification records.
Alaska Earthquake Center historical seismicity data.
USGS geological reports on the Alaska Range and Denali Fault system.