Seismic Swarm S20191018.1 Near Shungnak, Alaska

The seismic swarm S20191018.1 began at 03:48 on 17 October 2019 and ended at 08:59 on 13 November 2019. Located 60 km south of Shungnak in northwest Alaska, the sequence lasted 653 hours and 10 minutes and produced 1,214 earthquakes. This activity represents the first swarm recorded in the region since systematic monitoring began in 2000, with a total of three swarms identified through the present. Shungnak lies within the Kobuk River valley of the Arctic Alaska province. The local geology consists of Paleozoic and Mesozoic sedimentary and metamorphic rocks of the Arctic Alaska terrane, deformed during the Brookian orogeny. Regional seismicity is driven by far-field stresses transmitted from the Pacific–North America plate boundary, approximately 800 km to the south, combined with possible reactivation of inherited crustal faults. Earthquake depths in the swarm were predominantly shallow (0–15 km), consistent with brittle failure in the upper crust of this intraplate setting. Analysis of the first 100 events reveals a rapid onset followed by a decaying rate of activity. The initial event at 03:48 on 17 October measured magnitude 1.5 at 0 km depth. Magnitudes ranged from 0.9 to a peak of 5.4, with the largest shock occurring at 00:58 on 18 October at 6 km depth. Subsequent events included magnitudes of 3.9, 3.7, 3.4, 2.9, and 2.8, all within the first 36 hours. Depths remained concentrated between 1 km and 15 km, with the majority clustered around 5–9 km. After the mainshock, the sequence transitioned to smaller events (mostly 1.1–2.2) that persisted at a lower rate through mid-November. The swarm exhibited classic characteristics of fluid-driven or stress-triggered sequences rather than a single mainshock-aftershock pattern. No surface rupture or significant damage was reported, reflecting the moderate magnitudes and remote location. Continued monitoring is recommended given Alaska’s elevated seismic hazard and the occurrence of multiple swarms in the broader region since 2000.

References

SeismoSight internal swarm classification S20191018.1
USGS Earthquake Catalog (preliminary locations)