Seismic Swarm S20090622.1: Analysis of Activity Near Skwentna, Alaska

Seismic swarm S20090622.1 occurred 36 km east of Skwentna in south-central Alaska. The sequence began at 19:28 on 22 June 2009 and concluded at 11:54 on 25 June 2009, spanning 64 hours and 26 minutes. During this period, 54 earthquakes were recorded, with magnitudes ranging from 0.8 to 5.4 and focal depths primarily between 53 and 68 km.

The initiating event reached magnitude 5.4 at 64 km depth, followed within minutes by a magnitude 3.8 shock at 60 km. Subsequent activity included multiple events above magnitude 3.0, such as a 4.0 quake at 19:56 on 22 June and a 4.1 event at 14:27 on 23 June, both near 58–60 km depth. Smaller aftershocks dominated the later stages, with the final recorded tremor at magnitude 1.5 on 25 June.

This swarm unfolded in a region shaped by ongoing convergence between the Pacific and North American plates. South-central Alaska experiences distributed deformation linked to the Yakutat terrane collision, which drives crustal shortening and reactivation of inherited faults. Depths of 50–70 km place many events near the base of the crust or within the upper portion of the subducting slab, consistent with the regional tectonic setting where the Pacific plate descends beneath the continent.

Historical records show that the broader Cook Inlet and Susitna basin area has hosted both shallow crustal sequences and deeper intraslab activity. Notable past episodes include clusters associated with the Castle Mountain fault system and distributed seismicity along the Denali fault trend to the north. Such swarms often reflect transient stress perturbations rather than mainshock-aftershock sequences, potentially influenced by fluid migration or aseismic slip at depth.

The 2009 swarm displayed a rapid onset followed by a gradual decay in rate and magnitude, typical of swarm behavior in subduction-influenced crust. No surface rupture or significant damage was reported, reflecting both the moderate maximum magnitude and the substantial focal depths.

Key Events Summary

• 22 Jun 2009 19:28: magnitude 5.4 at 64 km (initial event)
• 22 Jun 2009 19:56: magnitude 4.0 at 60 km
• 23 Jun 2009 00:33: magnitude 3.2 at 57 km
• 23 Jun 2009 14:27: magnitude 4.1 at 58 km
• Numerous events of magnitude 1.0–2.8 continued through 25 June at similar depths.

The sequence underscores the persistent seismic hazard in south-central Alaska, where monitoring networks continue to track both isolated events and clustered activity linked to plate-boundary processes.

References

USGS Earthquake Catalog (ANSS Comprehensive Catalog)
Alaska Earthquake Center, University of Alaska Fairbanks
Alaska Division of Geological & Geophysical Surveys, Regional Tectonic Summaries