Seismic Swarm S20030102.1: Analysis of Activity Near Paxson, Alaska

Seismic swarm S20030102.1 occurred approximately 43 km northwest of Paxson, Alaska, in a region characterized by intense tectonic activity along the Denali Fault system. The swarm initiated at 02:07 on 1 January 2003 and concluded at 02:37 on 6 January 2003, spanning 120 hours and 29 minutes. During this interval, 58 earthquakes were recorded, with magnitudes ranging from 0.9 to 3.6 and focal depths primarily between 0 and 15 km.

The sequence exhibited typical swarm characteristics, featuring numerous small events without a dominant mainshock. On 1 January, activity included 16 events, highlighted by a magnitude 3.6 earthquake at 23:28:41 with a depth of 1 km. Subsequent days showed a gradual decline in frequency, with 19 events on 2 January, fewer on 3–5 January, and a final event on 6 January. Depths remained shallow throughout, consistent with crustal faulting in the upper 15 km. This pattern aligns with fluid migration or stress redistribution along pre-existing fault structures rather than a single rupture event.

Paxson lies within the south-central Alaska region, where the Pacific Plate subducts beneath the North American Plate, generating the Alaska-Aleutian megathrust and associated strike-slip faults. The Denali Fault, a major right-lateral strike-slip structure, passes near the swarm location and accommodates significant lateral motion. The area experienced heightened seismicity following the magnitude 7.9 Denali Fault earthquake of 3 November 2002, which ruptured over 340 km of fault and triggered widespread aftershocks. Swarm S20030102.1 represents the second such cluster since 1 January 2000, following an earlier episode in 2002 that likely formed part of the post-Denali aftershock sequence.

Geological mapping indicates that the swarm epicentral area consists of Paleozoic to Mesozoic metamorphic and sedimentary rocks cut by Quaternary-active faults. Shallow depths of recorded events suggest involvement of near-surface crustal layers influenced by both tectonic loading and possible hydrothermal processes common in the Alaska Range. No surface rupture or significant damage was associated with this swarm, underscoring its low-magnitude, distributed nature.

Such swarms provide valuable data for understanding stress transfer along the Denali Fault. Continued monitoring by regional seismic networks helps distinguish these transient clusters from foreshock sequences that might precede larger events. The 2003 activity reinforces the persistent seismic hazard in this portion of interior Alaska, where historical records document recurrent moderate-to-large earthquakes.

References

SeismoSight internal swarm classification records.
USGS Earthquake Hazards Program regional tectonic summaries for south-central Alaska.
Alaska Earthquake Center Denali Fault aftershock studies.