Seismic Analysis of Earthquake Swarm S20200318.3 Near Shungnak, Alaska

Earthquake swarm S20200318.3 occurred approximately 70 km south of Shungnak in northern Alaska. The sequence began at 19:06 on 17 March 2020 and concluded at 02:00 on 26 March 2020, spanning 198 hours and 54 minutes. During this period, 107 earthquakes were recorded.

The first 100 events provide key insight into swarm dynamics. Magnitudes ranged from 0.5 to 5.0, with the largest event (magnitude 5.0) occurring on 18 March at 15:31:33 UTC at a depth of 8 km. Depths were predominantly shallow, between 0 and 14 km, consistent with crustal activity. Most events clustered between magnitudes 0.7 and 2.5, indicating a typical swarm pattern of numerous smaller tremors accompanying a few moderate shocks. Notable activity included multiple events above magnitude 2.0 on 18 March, followed by a gradual decline in both frequency and intensity through 24 March.

This swarm represents one of six documented sequences in the region since 1 January 2000. The initial swarm occurred in 2019, establishing a recent pattern of episodic seismic clustering in this part of northern Alaska.

The Shungnak area lies within the Arctic Alaska tectonic province, specifically in the southern foothills of the Brooks Range. This region forms part of the Arctic Alaska terrane, shaped by Mesozoic accretion and later Cenozoic deformation. Seismicity here is generally intraplate in character, influenced by far-field stresses from the Pacific-North American plate boundary to the south. Shallow crustal faulting, possibly along reactivated structures associated with the Kobuk fault system or related shear zones, provides a plausible mechanism for swarm activity. Depths recorded in the swarm align with brittle failure in the upper crust, where permafrost and sedimentary cover overlie metamorphic basement rocks.

Northern Alaska experiences lower overall seismicity rates than southern coastal zones dominated by subduction, yet localized swarms can arise from fluid migration, glacial isostatic adjustment, or minor tectonic strain accumulation. Historical records from the Alaska Earthquake Center confirm that such sequences remain infrequent but recurrent in the interior Brooks Range transition zone.

The 2020 swarm underscores the value of continuous monitoring for understanding low-level seismic hazards in remote northern communities. No significant damage or felt reports were associated with these events, reflecting their modest magnitudes and remote epicentral location.

References

• Alaska Earthquake Center, University of Alaska Fairbanks (earthquake data archive)
• USGS Earthquake Catalog (regional seismicity records)
• Geological Survey of Alaska, Brooks Range tectonic framework reports