Earthquake Swarm S20180103.1: Seismic Activity in Washington State

Earthquake swarm S20180103.1 occurred in Washington from 08:36 on 3 January 2018 to 21:11 on 5 January 2018. Over this 60-hour period, 81 events were recorded, providing insight into localized seismic sequences typical of the region.

The swarm began with a magnitude 3.8 earthquake at 10 km depth. Subsequent events ranged in magnitude from -0.5 to 2.7, with the majority clustered between 0.3 and 1.4. Depths predominantly fell between 7 km and 10 km, though a few shallower or deeper outliers appeared, including events at 2 km and 3 km. Activity peaked early on 3 January before gradually declining through 5 January, when the final event registered magnitude 0.9 at 9 km depth.

Washington's seismicity stems from its position along the Cascadia subduction zone, where the Juan de Fuca plate descends beneath the North American plate. This tectonic setting generates both megathrust earthquakes and smaller swarms along crustal faults or near volcanic centers such as Mount Rainier and Mount St. Helens. Swarms differ from mainshock-aftershock sequences by lacking a dominant event followed by decaying aftershocks; instead, they feature numerous events of similar size over hours to days, often linked to fluid migration or slow slip.

Historical records indicate nine swarms in Washington since 1 January 2000. These occurred in 2000 (one swarm), 2004 (two), 2005 (one), 2008 (one), 2011 (two), 2013 (one), and 2018 (one). Such episodes underscore recurring patterns of clustered seismicity without progression to larger destructive quakes.

Monitoring by regional networks allows precise tracking of swarm parameters, aiding differentiation from tectonic mainshocks. Depths around 9 km align with typical crustal activity in the Pacific Northwest, where brittle failure occurs above the ductile transition zone.

References

• USGS Earthquake Catalog
• Pacific Northwest Seismic Network historical data
• Cascadia subduction zone tectonic summaries from USGS and NOAA