Seismic Swarm VS20180102.1 Near Amboy, Washington

An earthquake swarm designated VS20180102.1 occurred 38 km NNE of Amboy, Washington, from 08:31 on 1 January 2018 to 07:44 on 2 January 2018. The event sequence lasted 23 hours and 12 minutes and included 24 earthquakes, all of low magnitude and shallow depth.

The swarm unfolded with events clustered in two main phases. The initial activity on 1 January featured six small earthquakes between magnitudes -0.6 and 0.5 at depths ranging from -1 km to 4 km. Activity resumed after midnight on 2 January with 18 additional events, magnitudes between 0.0 and 0.4, and depths mostly between 2 km and 5 km. The final recorded event reached magnitude 0.3 at 3 km depth.

This region lies within the Cascade Range, part of the Cascade Volcanic Arc formed by subduction of the Juan de Fuca Plate beneath the North American Plate. The area experiences frequent low-magnitude seismicity linked to both tectonic strain and magmatic processes associated with nearby volcanoes such as Mount St. Helens. Shallow earthquake swarms are characteristic of the St. Helens Seismic Zone, where fluid migration and crustal stress adjustments commonly produce clusters of events without a single dominant mainshock.

Historical records maintained by SeismoSight indicate eight prior swarms in the same locale since 1 January 2000. These occurred in 2000 (one swarm), 2004 (two swarms), 2005 (one swarm), 2008 (one swarm), 2011 (two swarms), and 2013 (one swarm). Such recurrent swarms reflect ongoing geological dynamics typical of the volcanic arc environment.

The January 2018 swarm remained minor in scale and posed no reported surface impacts. Continued monitoring of similar sequences contributes to improved understanding of precursory patterns in the Cascade subduction zone.

References:
SeismoSight internal swarm classification records VS20180102.1 and historical swarm statistics since 2000.
USGS geological framework reports on the Cascade Volcanic Arc and St. Helens Seismic Zone.