Seismic Swarm S20021104.6: Analysis of Activity Near Paxson, Alaska

Seismic swarm S20021104.6 was recorded 42 km east-northeast of Paxson, Alaska, beginning at 23:48 UTC on 3 November 2002 and concluding at 16:49 UTC on 15 November 2002. Over a duration of 281 hours and 1 minute, the sequence produced 142 earthquakes. Analysis of the first 100 events reveals predominantly shallow focal depths between 0 and 19 km, with magnitudes ranging from 1.2 to 4.9. The initial event reached 4.9 at a depth of 1 km, followed by several events exceeding magnitude 4.0 within the first day, including 4.5, 4.3, and 4.2. Subsequent activity showed a gradual decline in both frequency and maximum magnitude, with most events falling between 1.5 and 3.5.

Depth distribution indicates a concentration of hypocenters in the upper crust, consistent with brittle failure along pre-existing fault structures. Temporal clustering was highest in the first 48 hours, after which event rates decreased steadily while maintaining shallow depths.

The swarm occurred within the central Alaska Range, a region shaped by ongoing convergence between the Pacific and North American plates. The Denali Fault, a major right-lateral strike-slip system, traverses the area and accommodates much of the lateral component of plate motion. This tectonic setting produces frequent moderate seismicity and occasional large earthquakes. The November 2002 timing places the swarm immediately after the Mw 7.9 Denali Fault earthquake of 3 November 2002, whose rupture propagated eastward along the same fault system. Such post-mainshock sequences commonly include both aftershocks on the primary rupture and triggered events on adjacent structures.

Historical records maintained since 1 January 2000 document only two swarms in the broader region, with S20021104.6 representing the first. The second swarm occurred at a later date. This low frequency underscores that swarm-type activity remains episodic rather than persistent in this portion of the Denali Fault zone.

The shallow depths and moderate magnitudes observed align with typical behavior along the Denali Fault, where crustal earthquakes rarely exceed 20 km depth due to the transition to ductile deformation at greater temperatures. Continued monitoring of similar sequences contributes to refined understanding of stress transfer along the fault and improves probabilistic assessments of future seismic hazard in interior Alaska.

References

SeismoSight internal swarm catalog S20021104.6
USGS Earthquake Catalog (Denali Fault event parameters)
Alaska Earthquake Center regional tectonic summaries