Seismic Swarm S20130105.1: Analysis of the January 2013 Event Near Port Alexander, Alaska

Southeastern Alaska lies within a tectonically active margin where the Pacific Plate interacts with the North American Plate along major strike-slip faults, including segments of the Queen Charlotte–Fairweather fault system. This region experiences frequent seismicity due to oblique convergence and transform motion, with earthquakes commonly occurring at crustal depths of 5–25 km. Port Alexander, situated on Baranof Island in the Alexander Archipelago, is approximately 78 km from the epicentral area of Swarm S20130105.1, placing the events in a zone of distributed crustal deformation typical of the broader southeastern Alaska seismic belt.

Swarm S20130105.1 began at 10:18 UTC on 5 January 2013 and concluded at 10:04 UTC on 6 January 2013, spanning 23 hours and 45 minutes. During this interval, 32 earthquakes were recorded. Magnitudes ranged from 2.3 to 4.0, with the largest events reaching 4.0 at depths of 20 km. Depths across the sequence clustered between 5 km and 23 km, showing no systematic migration. The temporal distribution indicated clustered activity, with multiple events exceeding magnitude 3.0 occurring within the first 12 hours, followed by a gradual decline.

Key events included a magnitude 3.2 earthquake at 10:18:42 on 5 January at 20 km depth, a pair of magnitude 4.0 events at 11:39:36 and 15:32:13, and a magnitude 3.9 shock at 19:53:22 on the same day at 5 km depth. Subsequent activity on 6 January featured a magnitude 3.5 event at 06:22:54 before the sequence tapered off with a final magnitude 2.8 shock at 10:04:03. The absence of a single dominant mainshock and the relatively uniform magnitude distribution align with characteristics of seismic swarms rather than typical mainshock-aftershock sequences.

Historical records since 1 January 2000 indicate that only one swarm has been identified in this locale, making S20130105.1 the sole documented swarm event. This rarity underscores the episodic nature of clustered seismicity in the area, where background tectonic loading occasionally produces short-lived bursts without associated volcanic or magmatic indicators.

The swarm provides insight into localized stress release along pre-existing crustal fractures. Events at varying depths suggest activation of multiple fault strands within a narrow volume, consistent with distributed shear in the transform-dominated tectonic setting. Such sequences can illuminate fault connectivity and stress heterogeneity without producing surface rupture, given the modest magnitudes involved.

References

SeismoSight internal swarm classification database USGS Earthquake Catalog (regional southeastern Alaska tectonics)