Seismic Swarm S20210905.1 Near Yakutat, Alaska

Seismic swarm S20210905.1 was recorded beginning at 00:33 on 5 September 2021 and concluding at 19:56 on 13 September 2021. The activity was centered 55 km NNW of Yakutat, Alaska, and comprised 413 earthquakes over 211 hours and 23 minutes. This event represents the third swarm documented in the region since 2000, following isolated episodes in 2011 and 2019. Analysis of the first 100 events reveals a rapid onset with magnitudes ranging from 0.9 to 4.0. Depths were predominantly shallow, spanning 0–19 km, consistent with crustal deformation. Early activity included a magnitude 4.0 event at 13 km depth within the first hour, accompanied by several magnitude 3+ shocks at depths of 3–16 km. Subsequent events clustered between 1.0 and 2.5 in magnitude, with the majority occurring at 5–12 km depth. The sequence showed no single dominant mainshock but rather sustained low-to-moderate energy release, typical of swarm behavior driven by fluid migration or stress transfer along local faults. The Yakutat area lies within the St. Elias orogen at the Pacific–North American plate boundary. Here the Yakutat terrane undergoes oblique collision and partial subduction, producing a network of thrust and strike-slip faults. This tectonic regime generates frequent seismicity and rapid uplift of the adjacent Saint Elias Mountains. Historical records indicate that the broader coastal zone has hosted large earthquakes, including the 1958 Fairweather Fault rupture, underscoring the persistent seismic hazard. The 2021 swarm aligns with known patterns of episodic cluster activity in the region. Such swarms often occur without a clear foreshock–mainshock–aftershock progression and may reflect transient changes in pore-fluid pressure within the shallow crust. Depths recorded during the swarm fall within the seismogenic zone documented for the Yakutat block, where brittle failure predominates above approximately 20 km. Continued monitoring of this sector remains essential given the proximity to critical infrastructure and the potential for larger events. The low overall magnitudes observed in 2021 suggest limited surface rupture risk, yet the cumulative energy release warrants integration into regional hazard assessments.

References

SeismoSight internal swarm classification S20210905.1
USGS Earthquake Catalog (Yakutat, Alaska regional seismicity)
Alaska Earthquake Center tectonic summaries