Seismic Swarm S20191118.1: Analysis of Activity South of Shungnak, Alaska

Seismic swarm S20191118.1 was recorded 66 km south of Shungnak, Alaska, beginning at 22:48 on 17 November 2019 and concluding at 23:57 on 27 November 2019. Over 241 hours and 8 minutes, the swarm comprised 177 earthquakes. This event marks the first of four documented swarms in the region since 1 January 2000. Analysis of the initial 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from 0.8 to 3.9, with the majority falling between 1.0 and 1.6. Depths were shallow, concentrated between 1 km and 12 km. Notable events include a magnitude 3.9 earthquake at 19:51 on 18 November at 12 km depth, followed by a 3.2 event on 20 November at 9 km depth. Subsequent larger shocks reached 2.3 and 2.2, while the sequence featured frequent smaller tremors clustered in time, indicative of swarm behavior rather than a classic mainshock-aftershock pattern. The temporal distribution shows peak activity on 18 and 19 November, with events occurring at intervals of minutes to hours. Depths remained consistent across the early phase, suggesting a localized crustal source. No events exceeded magnitude 4.0 in the examined subset, consistent with the swarm's overall moderate energy release. The Shungnak region lies within northern Alaska's Brooks Range province, where tectonic deformation arises from ongoing convergence between the North American and Pacific plates transmitted through the interior. The area features Paleozoic to Mesozoic sedimentary and metamorphic rocks folded and thrust during Cretaceous orogenesis, with later Cenozoic extension influencing fault networks. Seismicity here is generally diffuse and lower than in southern Alaska, often linked to reactivated basement structures and minor strike-slip or normal faulting. Historical records indicate sparse but recurrent low-level activity, with swarms potentially reflecting fluid migration or stress perturbations along these features. This swarm provides insight into the subtle seismic regime of the region, highlighting the value of dense monitoring for detecting clustered events that might otherwise go unnoticed. Continued observation supports improved characterization of northern Alaska's intraplate dynamics.

References

USGS Earthquake Hazards Program catalog data Alaska Earthquake Center regional reports SeismoSight internal swarm classification records