Earthquake Swarm S20210915.2: Seismic Activity Near Yakutat, Alaska

An earthquake swarm designated S20210915.2 was recorded northwest of Yakutat, Alaska, beginning at 01:27 on 15 September 2021 and concluding at 15:59 on 21 September 2021. The sequence lasted 158 hours and 31 minutes, during which 77 events were detected at a location 58 km NNW of Yakutat. Magnitudes ranged from 0.9 to 3.4, with the largest event occurring at 01:43 on 15 September. Depths varied between 0 and 14 km, and most activity clustered in the first three days before gradually declining.

The swarm exhibited typical characteristics of clustered seismicity, with numerous small-magnitude events occurring in close succession. Early phases included several events above magnitude 2.0, such as the 2.7 recorded at 01:53 and 02:10 on 15 September. Later activity featured isolated events up to magnitude 2.6 on 17, 18, and 20 September. Depths remained predominantly shallow, consistent with crustal deformation in the upper 10 km.

Yakutat lies within the tectonically complex transition zone between the strike-slip Queen Charlotte–Fairweather fault system and the eastern end of the Alaska-Aleutian subduction zone. The region experiences ongoing collision of the Yakutat terrane with the North American plate, producing rapid uplift in the St. Elias Mountains and elevated seismic hazard. Historical large earthquakes, including the magnitude 8 events of September 1899, underscore the area’s capacity for significant seismic release.

Since 2000, only three swarms have been documented in the immediate vicinity: one each in 2011, 2019, and 2021. The 2021 sequence fits this sparse pattern, suggesting episodic rather than continuous swarm behavior. Such clusters may reflect localized stress adjustments along secondary faults or fluid involvement within the accretionary complex, though definitive mechanisms require further geophysical investigation.

Monitoring data indicate that the swarm remained below levels that would trigger widespread felt reports or structural concern. Continued surveillance of the Yakutat area remains essential given its position at a major plate-boundary transition.

References

Alaska Earthquake Center, University of Alaska Fairbanks
U.S. Geological Survey Earthquake Hazards Program
SeismoSight internal swarm classification records