Seismic Swarm S20190306.1 Near Shungnak, Alaska: Geological Context and Event Analysis

Seismic swarm S20190306.1 occurred 63 km south of Shungnak in northwestern Alaska, a remote area within the Kobuk River valley and adjacent upland terrain. The swarm initiated at 00:58 UTC on 6 March 2019 and concluded at 07:32 UTC on 2 August 2019, spanning 3582 hours and registering 3870 earthquakes. This represents the sole swarm recorded in the region since 1 January 2000.

Northwestern Alaska lies within a tectonically complex zone influenced by the interaction between the North American Plate and distant plate boundaries. The area features Paleozoic to Mesozoic sedimentary and metamorphic rocks of the Arctic Alaska terrane, deformed during the Brooks Range orogeny. Seismicity here is generally moderate compared with southern coastal zones, often linked to intraplate stress release along reactivated faults rather than active subduction. Depths of events in the swarm ranged primarily from 0 to 18 km, consistent with shallow crustal faulting in this continental interior setting.

The sequence opened with low-magnitude events (M1.0–M1.4) at depths near 7–10 km. Activity escalated rapidly, culminating in a magnitude 5.2 mainshock at 21:33 UTC on 6 March at 9 km depth. This event was followed by an immediate aftershock sequence dominated by magnitudes below 3.0, with depths clustering between 5 and 14 km. Over the subsequent hours, more than 80 events occurred, including multiple M2.0–M2.8 shocks at depths of 3–13 km. The first 100 events exhibited a typical swarm pattern: an initial foreshock phase, a distinct peak with the M5.2 shock, and a prolonged decay of smaller aftershocks. Magnitudes remained modest after the mainshock, rarely exceeding M2.7, while depths showed minor variation without clear migration.

The swarm’s extended duration and high event count reflect sustained stress adjustment within a localized fault network. No larger regional events preceded or followed the sequence, underscoring its isolated character relative to Alaska’s broader seismic catalog. Updated monitoring by regional networks confirms the absence of additional swarms in this sector through the present.

References
USGS Earthquake Catalog
Alaska Earthquake Center annual reports
Geological Survey of Alaska tectonic maps