Seismic Swarm S20021104.7 Near Paxson, Alaska: Characteristics and Regional Context

Seismic swarm S20021104.7 was recorded 48 km NNW of Paxson in central Alaska. The sequence began at 23:33 on 3 November 2002 and concluded at 22:33 on 30 December 2002, spanning 1367 hours and encompassing 1016 earthquakes. This event represents the sole swarm documented in the region since 1 January 2000. Analysis of the first 100 events reveals predominantly shallow foci, with depths ranging from 0 km to 18 km and a median near 3 km. Magnitudes varied from 0.8 to 4.4, with the largest event (M4.4) occurring at 16:17 on 5 November 2002 at 2 km depth. Early activity included multiple events above M3.0 within the first 24 hours, followed by a gradual decline in both frequency and peak magnitude. The majority of events clustered between M2.0 and M3.5, consistent with typical swarm behavior driven by fluid migration or stress redistribution rather than a single mainshock-aftershock cascade. Central Alaska lies within a tectonically complex zone influenced by the subduction of the Pacific Plate beneath the North American Plate along the Aleutian megathrust and by major strike-slip structures such as the Denali Fault. The swarm epicenter is situated near the western segment of the Denali Fault system, where right-lateral shear accommodates part of the Pacific–North America plate motion. Historical seismicity in the area includes the 2002 Denali Fault earthquake sequence, which produced widespread aftershocks across central Alaska. Swarms in this setting often reflect transient increases in crustal permeability or magmatic processes at depth, though no volcanic activity is associated with the Paxson locale. The 2002 swarm exhibited classic swarm traits: a rapid onset without a dominant mainshock, sustained elevated seismicity over weeks, and a high proportion of events at shallow depths. Such patterns align with observations from other intraplate swarms worldwide, where episodic fluid pressure changes trigger distributed failure on preexisting fractures.

References

USGS Earthquake Hazards Program – Alaska Seismicity Alaska Earthquake Center – Regional Tectonic Summary SeismoSight internal swarm catalog (S20021104.7)