Seismic Swarm S20030107.1 Near Denali National Park, Alaska

Seismic swarm S20030107.1 occurred in a tectonically active region of central Alaska, approximately 65 km east-southeast of McKinley Park. The sequence began at 12:11 on 6 January 2003 and concluded at 09:58 on 11 January 2003, spanning 117 hours and 47 minutes. During this period, 72 earthquakes were recorded, with magnitudes ranging from 1.0 to 2.1 and focal depths predominantly between 0 and 26 km.

The swarm exhibited typical characteristics of clustered microseismicity, featuring frequent low-magnitude events without a dominant mainshock. Early activity on 6 January included multiple events of magnitude 1.4–1.8 at shallow depths, followed by a gradual decline in rate over subsequent days. Peak magnitudes reached 2.1 on 7, 8, and 11 January, often at depths of 1–3 km. This pattern suggests fluid migration or stress redistribution along pre-existing fractures rather than a single rupture.

The location lies within the influence of the Denali Fault system, a major right-lateral strike-slip structure marking the boundary between the Pacific and North American plates. The fault accommodates significant lateral motion and has produced large historical earthquakes. The swarm occurred roughly two months after the magnitude 7.9 Denali Fault earthquake of 3 November 2002, whose epicenter was only 12 km from the swarm centroid. Post-mainshock stress changes likely contributed to the elevated seismicity rate.

Regional geology features complex fault networks, including subsidiary strands of the Denali system and nearby thrust faults associated with the Alaska Range. Shallow crustal earthquakes dominate due to the transpressional regime. Historical records since 2000 indicate 15 swarms in the broader area, with 14 occurring in 2002 and one in 2003, highlighting episodic swarm activity following the major 2002 event.

Analysis of event timing shows clustering in the first 48 hours, with 18 events on 6 January alone. Depths remained consistently shallow, consistent with brittle failure in the upper crust. No events exceeded magnitude 3, underscoring the swarm’s low-energy nature compared with aftershock sequences of larger quakes.

This sequence provides insight into post-seismic relaxation processes along the Denali Fault. Continued monitoring of similar swarms aids in understanding triggered seismicity and long-term hazard assessment in central Alaska.

References

USGS Earthquake Hazards Program – Denali Fault, Alaska Earthquake (2002)
Alaska Earthquake Center – Regional Seismicity Catalog
SeismoSight internal swarm classification records