Seismic Swarm S20211106.1 Near Salcha, Alaska

SeismoSight recorded swarm S20211106.1 beginning at 22:12 on 5 November 2021 and concluding at 05:29 on 10 November 2021. The sequence occurred 28 km west-southwest of Salcha in interior Alaska and comprised 51 earthquakes over 103 hours and 17 minutes.

The initial events included a magnitude 4.1 earthquake at 8 km depth at 22:12:50 on 5 November, followed rapidly by a magnitude 3.1 event at the same depth. Subsequent activity featured numerous smaller shocks, with magnitudes ranging from 0.0 to 4.1 and focal depths between 0 km and 14 km. The largest event marked the peak of the sequence, after which activity declined gradually through 10 November.

Interior Alaska experiences distributed crustal seismicity driven by far-field stresses from the convergent Pacific-North American plate boundary. The Salcha area lies within the Yukon-Tanana terrane, where active faults accommodate right-lateral shear transferred from the Denali Fault system approximately 100 km to the south. Shallow focal depths recorded during the swarm are consistent with brittle failure in the upper 15 km of crust typical of this intraplate setting.

Historical records since 2000 indicate only three prior swarms in the immediate region: one in 2007, one in 2011, and one in 2020. These infrequent clusters suggest episodic release of accumulated strain along local fault segments rather than continuous background seismicity.

No damage or felt reports were associated with the 2021 swarm beyond the initial magnitude 4.1 event. Such low-magnitude sequences contribute to ongoing strain monitoring and help refine seismic hazard assessments for the Fairbanks-Salcha corridor.

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