Seismic Swarm S20040129.1: Analysis of Activity Near Paxson, Alaska
Seismic swarm S20040129.1 occurred in a tectonically active region of central Alaska, approximately 83 km northwest of Paxson. This area lies within the broader influence of the Pacific Plate's subduction beneath the North American Plate along the Aleutian megathrust, with additional contributions from strike-slip motion along the Denali Fault system. The Denali Fault, a major right-lateral structure extending across much of interior Alaska, has shaped the regional geology through repeated episodes of displacement, contributing to elevated seismicity in the surrounding crust.
The swarm initiated at 21:34 on 28 January 2004 and concluded at 06:10 on 1 February 2004, spanning 80 hours and 35 minutes. During this interval, 47 earthquakes were recorded. Event depths ranged primarily from 0 to 18 km, with the majority occurring at shallow levels under 10 km, consistent with brittle failure in the upper crust. Magnitudes varied from 1.0 to 2.9, indicating low-to-moderate energy release without progression to larger events.
Temporal distribution showed clustering, with initial activity on 28 January featuring a magnitude 2.6 event at 5 km depth. Subsequent days exhibited repeated sequences of smaller events, including multiple instances near 1.3–1.5 magnitude at depths of 1–9 km. Peak activity included a magnitude 2.9 event at 3 km depth on 29 January. Later phases on 30 and 31 January, as well as early 1 February, maintained similar shallow characteristics before tapering off.
This pattern aligns with swarm behavior observed in the region, where fluid migration or localized stress adjustments can trigger prolonged sequences without a dominant mainshock. Historical records since 1 January 2000 document 20 such swarms in the vicinity. Earlier episodes include 13 swarms in 2002 and 7 in 2003, underscoring recurring seismic unrest northwest of Paxson.
The geological setting features Paleozoic to Mesozoic sedimentary and volcanic rocks deformed by Cenozoic tectonics, with Quaternary glacial deposits overlying much of the surface. Ongoing convergence continues to drive microseismicity, providing insights into fault interactions along secondary structures subsidiary to the Denali system.
References
- Alaska Earthquake Center swarm catalog (internal classification S20040129.1).
- USGS Earthquake Hazards Program tectonic summaries for central Alaska.