Seismic Swarm S20190818.1: Analysis of Events Near Shungnak, Alaska

Seismic swarm S20190818.1 was recorded 68 km south of Shungnak, Alaska, commencing at 17:01 on 17 August 2019 and concluding at 00:56 on 22 August 2019. Over 103 hours and 54 minutes, the swarm comprised 51 earthquakes, with magnitudes ranging from 0.5 to 3.2 and focal depths between 0 and 13 km. This sequence represents one of only two swarms documented in the region since 1 January 2000, marking the initial such event in 2019.

The swarm initiated with a magnitude 0.9 event at 10 km depth. Subsequent activity included a magnitude 1.4 shock at 7 km depth later that evening. Peak intensity occurred on 18 August, featuring the largest event of magnitude 3.2 at 6 km depth at 02:48, followed closely by a magnitude 3.1 event at 10 km depth at 03:31. Additional notable shocks on that day included magnitudes of 2.1 at 3 km depth and 1.9 at 4 km depth. Depths remained predominantly shallow, clustering between 2 and 10 km for most events, consistent with upper-crustal processes.

Activity continued at lower intensity through 19–21 August, with events such as a magnitude 1.5 at 8 km depth on 20 August and a final magnitude 0.8 shock at 5 km depth closing the sequence. The temporal distribution showed highest frequency on 18 August, tapering thereafter, with no aftershock sequence exceeding the main swarm parameters.

Northwest Alaska lies within a tectonically active margin influenced by the interaction of the Pacific and North American plates. The Brooks Range and adjacent Kobuk River valley host a network of faults accommodating regional compression and strike-slip motion. Shungnak sits near the southern flank of the Brooks Range, where Paleozoic and Mesozoic sedimentary and metamorphic rocks overlie older basement units deformed during Cretaceous orogenesis. Seismicity in this area often reflects reactivation of inherited structures rather than direct subduction-related thrusting, which is more pronounced farther south along the Aleutian arc.

Historical records indicate sparse swarm-type activity in this specific locale prior to 2019, underscoring the unusual nature of the 2019 sequence relative to background seismicity. The shallow depths observed align with faulting within the upper 15 km of crust, where brittle deformation predominates in the cold, stable interior of Alaska.

Further monitoring by regional seismic networks continues to refine understanding of strain accumulation along local fault systems. Such swarms provide valuable data on stress transfer and fluid involvement in triggering clustered events within intraplate settings.

References

Alaska Earthquake Center, University of Alaska Fairbanks – Regional seismic catalog and swarm documentation.
United States Geological Survey – Tectonic framework of northwest Alaska and fault mapping.
SeismoSight internal classification records for swarm parameters and statistics since 2000.