Seismic Swarm VS20180507.1: Analysis of Activity Near Amboy, Washington

A notable seismic swarm, designated VS20180507.1, occurred in the region 38 km north-northeast of Amboy, Washington, between 6 May and 14 May 2018. The sequence began at 20:56 UTC on 6 May and concluded at 20:19 UTC on 14 May, spanning 191 hours and 23 minutes. During this period, 91 earthquakes were recorded, providing valuable data on local seismicity patterns.

The events exhibited predominantly low magnitudes, with the majority falling between -0.8 and 1.1. Several entries registered at -5.0, consistent with the detection thresholds of sensitive monitoring networks for microseismicity. Depths ranged from negative values, likely reflecting location uncertainties near the surface, to a maximum of 13 km, though most clustered between 1 km and 5 km. This shallow distribution suggests activity within the upper crust, potentially linked to fluid migration or minor fault adjustments rather than deeper tectonic processes.

Temporal analysis reveals peak activity on 7 May and 8 May, with multiple events per hour during certain intervals. Magnitudes showed no clear escalation trend, indicating a typical swarm behavior where energy release remains distributed without a dominant mainshock. The sequence tapered off after 12 May, with only isolated events until termination.

Regional Geological Context

The swarm location lies within the southern Cascade Range of Washington state, part of the broader Pacific Northwest subduction zone. Here, the Juan de Fuca plate descends beneath the North American plate, generating volcanic arcs and associated seismicity. Amboy and surrounding areas sit near the western margin of the Gifford Pinchot National Forest, where Quaternary volcanism and fault systems influence crustal dynamics. Historical records indicate episodic swarms in this sector, often tied to hydrothermal activity or stress changes along regional faults.

Historical Swarm Patterns

Since 1 January 2000, ten seismic swarms have been documented in the vicinity. These occurred in the following years with associated event counts: 2000 (1), 2004 (2), 2005 (1), 2008 (1), 2011 (2), 2013 (1), and 2018 (2). The recurrence highlights persistent low-level seismic unrest, underscoring the value of continuous monitoring for distinguishing background activity from potential precursors to larger events.

This 2018 swarm aligns with prior episodes in its modest energy release and shallow focus, reinforcing the area's characteristic microseismic regime. Ongoing observation remains essential for understanding long-term crustal behavior in this tectonically active setting.

References

USGS Earthquake Catalog Pacific Northwest Seismic Network reports Cascades Volcano Observatory geological summaries