Seismic Swarm S20130106.1 Near Port Alexander, Alaska

The seismic swarm designated S20130106.1 occurred approximately 67 km south-southwest of Port Alexander, Alaska, in a tectonically active zone of southeastern Alaska. The sequence began at 09:27 on 5 January 2013 and concluded at 15:07 on 7 January 2013, spanning 53 hours and 40 minutes. During this interval, 34 earthquakes were recorded, with magnitudes ranging from 2.3 to 4.7 and focal depths predominantly between 10 and 20 km.

The swarm initiated with a magnitude 4.7 event at 20 km depth, followed within minutes by a magnitude 4.1 shock at 10 km. Subsequent activity included multiple events of magnitude 3.0–3.7 clustered in the first 12 hours, such as the 3.7 at 12:24 and the 4.0 at 15:41. On 6 January, activity persisted with a notable magnitude 4.5 event at 08:37 (20 km depth) amid a series of smaller shocks at similar depths. The sequence tapered on 7 January, ending with a magnitude 4.0 event at 15:07. Depths remained consistent around 20 km for most later events, suggesting a stable source volume.

Southeastern Alaska experiences frequent seismicity due to its position along the transform boundary between the Pacific and North American plates, primarily accommodated by the Queen Charlotte–Fairweather fault system. This right-lateral strike-slip regime produces shallow to moderate-depth earthquakes, consistent with the observed swarm characteristics. The broader region forms part of the Pacific Ring of Fire, where oblique convergence and transform motion have shaped the coastal geology over millions of years, including the uplift of the Alexander Archipelago and adjacent continental margin.

Historically, southeastern Alaska has recorded numerous significant earthquakes. The 1949 Queen Charlotte Islands event (magnitude 8.1) and the 1958 Fairweather fault rupture (magnitude 7.8) demonstrate the capacity for large-magnitude releases along nearby structures. Smaller swarms and aftershock sequences are common, often linked to stress transfer within the fault network. The 2013 swarm fits this pattern of clustered activity without a single dominant mainshock.

Analysis of the temporal distribution shows peak rates on 5 January, with 13 events in the first 12 hours, declining steadily thereafter. Magnitudes generally decreased after the initial larger shocks, though isolated magnitude 4.0+ events occurred later. Depths clustered tightly around 20 km after the first day, indicating possible migration or stabilization within a mid-crustal volume.

This sequence provides insight into swarm dynamics in transform settings, where fluid migration or aseismic slip may trigger distributed failure without culminating in a larger rupture. The event parameters align with regional monitoring records from the period.

References

• Alaska Earthquake Center, University of Alaska Fairbanks
• USGS Earthquake Hazards Program
• SeismoSight internal swarm classification records