Seismic Swarm S20080925.1: Shallow Activity Northwest of Yakutat, Alaska

Seismic swarm S20080925.1 was recorded 111 km northwest of Yakutat, Alaska, beginning at 07:38 on 24 September 2008 and concluding at 14:36 on 27 September 2008. Over the 78-hour-and-58-minute period, 43 earthquakes were detected, all of low magnitude and predominantly shallow depth. Magnitudes ranged from 0.5 to 1.7, with the majority falling between 1.0 and 1.6. Depths were overwhelmingly at or near the surface, although two events reached 11 km and 15 km. The sequence lacked a distinct mainshock, consistent with swarm behavior driven by fluid migration or minor crustal adjustment rather than a single large rupture.

The temporal distribution showed peak activity on 24 and 25 September, with events occurring at intervals of minutes to hours before tapering on 26 and 27 September. All but two events registered depths of 0–2 km, indicating very shallow sources within the upper crust. Such shallow swarms are common in tectonically active margins where small fractures respond to local stress changes without producing significant surface deformation.

Yakutat lies within one of North America’s most rapidly deforming regions. The area marks the ongoing collision and partial subduction of the Yakutat microplate with the North American plate. This interaction drives uplift of the St. Elias Mountains, the highest coastal range on Earth, and produces a complex network of thrust and strike-slip faults. The Fairweather Fault system, located immediately east of the swarm epicenters, accommodates much of the Pacific–North America relative motion through right-lateral strike-slip motion at rates exceeding 4 cm per year. Additional contraction is absorbed by the fold-and-thrust belt that underlies the coastal plain and adjacent continental margin.

Historical seismicity in the Yakutat region includes both large plate-boundary events and frequent small-magnitude sequences. The 1899 Yakutat Bay earthquakes (magnitudes approximately 8.0 and 8.2) produced extensive coastal uplift and are among the largest known events in the area. More recent moderate earthquakes, such as the 1979 St. Elias event (magnitude 7.2), illustrate continued strain accumulation along the transition from strike-slip to convergent regimes. Microseismic swarms like S20080925.1 occur regularly and are interpreted as background release of accumulated strain within the shallow crust above the deeper plate interface.

Current monitoring by the Alaska Earthquake Center and the USGS confirms that the Yakutat block continues to converge at several centimeters per year, sustaining elevated seismic hazard. Although the 2008 swarm produced no reported damage or felt shaking beyond instrumental detection, it exemplifies the persistent low-level seismicity that characterizes this dynamic margin. Ongoing geodetic measurements indicate that elastic strain continues to build, underscoring the importance of sustained seismic and geodetic networks for understanding both swarm processes and larger seismic potential in southeastern Alaska.

References

Alaska Earthquake Center, University of Alaska Fairbanks.
U.S. Geological Survey Earthquake Hazards Program.
Plafker, G., et al., 1994, Geology of the southern Alaska margin.