Seismic Swarm S20200812.1 Near White Mountain, Alaska

The seismic swarm S20200812.1 occurred 68 km north-northeast of White Mountain, Alaska. It began at 04:43 on 11 August 2020 and ended at 10:17 on 14 August 2020, lasting 77 hours and 34 minutes. A total of 46 earthquakes were recorded during this interval.

The events displayed typical swarm characteristics, with no single dominant mainshock. Magnitudes ranged from 0.9 to 3.5, and focal depths remained shallow, between 0 and 13 km. Activity initiated with low-magnitude events on 11 August, intensified through the afternoon and evening, and included the largest earthquake of magnitude 3.5 at 20:23 that day. Subsequent events on 12 and 13 August consisted primarily of magnitudes between 1.0 and 2.4, gradually declining in frequency before the swarm concluded on 14 August.

This swarm aligns with patterns observed in western Alaska, where clusters of small earthquakes often occur without clear mainshock-aftershock sequences. Such activity may reflect localized stress release along minor faults or fluid movement within the crust.

White Mountain lies on the Seward Peninsula in western Alaska, a region underlain by Precambrian to Paleozoic metamorphic rocks intruded by Mesozoic granites. The broader tectonic setting involves distributed deformation between the Pacific Plate and the North American Plate, with the Aleutian subduction zone located several hundred kilometers to the south. Although major plate-boundary earthquakes dominate southern Alaska, interior and western areas experience moderate intraplate seismicity driven by far-field stresses and possible mantle upwelling. Updated seismic monitoring by the Alaska Earthquake Center confirms ongoing low-level activity across the Seward Peninsula, consistent with its position in a transitional tectonic domain.

Historical records since 2000 indicate only one prior swarm in the immediate vicinity, occurring in 2019. This low frequency suggests that swarm-type sequences are infrequent but recurrent features of the local seismic regime.

References
SeismoSight internal swarm classification S20200812.1
Alaska Earthquake Center regional seismicity reports
USGS Earthquake Hazards Program – Alaska tectonic framework