Seismic Swarm S20130824.1: Analysis of Activity Near Mossyrock, Washington

Seismic swarm S20130824.1 was recorded approximately 24 km south-southeast of Mossyrock in Lewis County, Washington. The sequence began at 21:38 on 23 August 2013 and concluded at 19:37 on 26 August 2013, spanning 69 hours and 59 minutes. During this interval, 56 earthquakes were detected, with magnitudes ranging from 0.0 to 3.7 and focal depths primarily between 9 km and 16 km.

The largest event reached magnitude 3.7 at a depth of 13 km shortly after initiation. Subsequent notable shocks included magnitudes of 3.4 and 3.1, both at depths near 13 km, occurring within the first several hours. The majority of events registered below magnitude 1.0, indicating a typical swarm pattern dominated by smaller tremors clustered in time and space. Depths remained consistent throughout, suggesting activity along a localized fault segment within the mid-crust.

This swarm fits within a broader pattern of seismic episodes in the region. Since 1 January 2000, seven swarms have occurred near Mossyrock, distributed across 2000 (one swarm), 2004 (two swarms), 2005 (one swarm), 2008 (one swarm), and 2011 (two swarms). Such recurrent activity highlights the area's ongoing susceptibility to clustered seismicity rather than isolated mainshock-aftershock sequences.

The geological setting of southwestern Washington lies within the forearc of the Cascadia subduction zone, where the Juan de Fuca plate descends beneath the North American plate. This tectonic framework produces both deep intraslab earthquakes and shallower crustal events along regional faults. The Mossyrock vicinity sits south of the Southern Washington Cascades conductor, an area influenced by volcanic processes associated with the Cascade Range. Historical seismicity here reflects interactions between subduction-driven compression and local crustal structures, including possible influences from the St. Helens Seismic Zone to the east.

Depths observed in the 2013 swarm align with typical crustal seismicity in the Pacific Northwest, where events commonly nucleate between 5 km and 20 km. No surface rupture or significant ground deformation was associated with this sequence, consistent with the modest magnitudes involved.

References

USGS Earthquake Hazards Program catalog (earthquake.usgs.gov)
Pacific Northwest Seismic Network reports on Cascadia tectonics
Washington State Department of Natural Resources geological summaries