Seismic Swarm S20061005.1: Analysis of Activity Near Paxson, Alaska

Seismic swarm S20061005.1 was recorded 82 km northwest of Paxson, Alaska, from 22:23 on 4 October 2006 to 15:19 on 5 October 2006. Over this 16-hour 56-minute period, 26 earthquakes occurred, with magnitudes ranging from 1.1 to 2.4 and depths predominantly between 0 and 12 km. The events clustered tightly in time, beginning with an initial sequence of four shocks within the first 40 minutes, followed by intermittent activity that peaked again around 02:00–03:00 and 08:00–10:00 on 5 October before tapering off.

Most events were shallow, with 15 occurring at depths of 5 km or less. The largest shocks reached magnitude 2.4 at depths of 1 km and 0 km, respectively. This shallow focus and rapid succession of similar-sized events are characteristic of swarm behavior, in which energy is released through numerous small ruptures rather than a single mainshock-aftershock sequence.

The swarm location lies within south-central Alaska’s tectonically active interior, where ongoing convergence between the Pacific and North American plates produces frequent seismicity along regional fault systems. Paxson sits near the northern flank of the Alaska Range, an area influenced by both the Denali Fault to the south and the broader zone of crustal deformation associated with flat-slab subduction. Historical records indicate that the region experiences episodic clusters of small earthquakes; since 1 January 2000, 21 swarms have been documented in the same general vicinity, with notable activity in 2002 (13 swarms), 2003 (7 swarms), and 2004 (1 swarm).

Such swarms often reflect fluid migration or slow slip on minor faults rather than large-scale tectonic loading. The 2006 sequence fits this pattern, with no single dominant event and a rapid decay in activity after the initial 12 hours. Depths remained consistently crustal, rarely exceeding 12 km, consistent with the brittle upper crust in this part of the plate boundary.

Monitoring of these low-magnitude swarms contributes to understanding background seismicity rates and helps refine hazard assessments for infrastructure corridors near the Richardson Highway and Trans-Alaska Pipeline System. Continued observation of similar clusters will improve models of how stress is accommodated across the complex network of faults in interior Alaska.

References

• Alaska Earthquake Center, University of Alaska Fairbanks – regional seismicity summaries
• USGS Earthquake Catalog – historical event data for south-central Alaska
• SeismoSight internal swarm classification records (S20061005.1 parameters)