Seismic Swarm S20151006.1: Analysis of Earthquake Activity Near Yakutat, Alaska

Seismic swarm S20151006.1 occurred approximately 119 km northwest of Yakutat, Alaska, registering 30 earthquakes between 21:39 on 5 October 2015 and 09:56 on 7 October 2015. The sequence lasted 36 hours and 17 minutes, with events exhibiting magnitudes between 0.6 and 1.4 and all recorded at a depth of 0 km. This shallow-depth clustering distinguishes the swarm from typical tectonic mainshock-aftershock sequences in the region.

The swarm initiated with a magnitude 1.0 event at 21:39 on 5 October, followed by additional low-magnitude tremors throughout the night. Activity continued steadily on 6 October, peaking with events such as magnitude 1.4 at 07:01 and magnitude 1.3 at 19:11. The final event, magnitude 0.8, occurred at 09:56 on 7 October. Magnitudes remained consistently below 2.0, indicating a diffuse release of energy without a dominant mainshock.

Yakutat lies within the tectonically complex transition zone of southeastern Alaska, where the Yakutat microplate collides with the North American Plate. This interaction drives ongoing deformation, including thrust faulting and strike-slip motion along the Fairweather Fault system. The area's geology features accreted terranes, extensive glaciation, and proximity to the St. Elias Mountains, contributing to elevated seismic hazard. Historical records show persistent low-to-moderate seismicity influenced by both plate boundary forces and glacial processes.

Since 2000, only two prior swarms have been documented in the immediate vicinity: one in 2008 and another in 2014. These infrequent episodes suggest episodic rather than continuous swarm behavior, potentially linked to fluid migration or stress adjustments along local fault structures. The 2015 swarm aligns with this pattern of clustered, low-magnitude activity.

Insights from the event data reveal a rapid onset and gradual decay in frequency, typical of swarm dynamics where seismicity migrates without clear aftershock decay laws. All events at 0 km depth point to very shallow sources, which may reflect interactions with near-surface materials such as sediments or ice in this glaciated coastal environment. No larger events accompanied the sequence, underscoring its isolated character within the regional seismic framework.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center (aeic.alaska.edu)
USGS Professional Paper on Yakutat Terrane Tectonics