Seismic Swarm VS20041024.1: Early Activity Near Amboy, Washington

The seismic swarm designated VS20041024.1 occurred in a volcanically active region of the Cascade Range in southern Washington. The epicentral area lies approximately 38 km north-northeast of Amboy in Clark County, placing it within the broader volcanic field associated with Mount St. Helens. This portion of the Cascade volcanic arc experiences frequent shallow seismicity driven by magmatic and hydrothermal processes beneath the stratovolcano. Geologically, the region is underlain by Quaternary volcanic deposits, including andesitic to dacitic lavas and pyroclastic materials from repeated eruptive episodes at Mount St. Helens. The volcano’s edifice sits atop older Tertiary bedrock of the Western Cascades, with regional tectonics influenced by the subduction of the Juan de Fuca Plate. Historical records document major eruptive activity at Mount St. Helens in 1980, followed by dome-building eruptions through 1986. Renewed unrest began in September 2004, culminating in the extrusion of a new lava dome that continued until early 2008. Swarm VS20041024.1 began at 12:06 on 23 October 2004 and persisted until 18:43 on 8 July 2005, spanning 6198 hours and 36 minutes. During this interval, 5484 earthquakes were recorded. Historical statistics for the area since 1 January 2000 indicate only two prior swarms, one in 2000 and the present event in 2004. Analysis of the first 100 recorded events reveals a classic volcanic swarm signature. Events were overwhelmingly shallow, with focal depths clustered at 0 km or −1 km. Magnitudes remained low, ranging from −0.1 to 2.5, with the majority falling between 1.4 and 2.2. The initial event on 23 October registered magnitude 1.5 at −1 km depth. Subsequent activity on 24 October included several magnitude 2.0–2.5 events, all at depths of 0 km or −1 km. By 25–28 October, magnitudes continued to hover near 2.0 while depths stayed consistently shallow. The sequence through 1 November maintained this pattern, with no events exceeding magnitude 2.5 and nearly all hypocenters at or above sea level. Such characteristics—high event counts, restricted magnitude range, and extremely shallow depths—align with fluid migration and brittle failure in the shallow crust above an ascending magma body. The timing coincides with the onset of the 2004–2008 eruptive episode at Mount St. Helens, during which new lava reached the surface in early October 2004. References
USGS Volcano Hazards Program – Mount St. Helens
Pacific Northwest Seismic Network (PNSN) catalog summaries
Washington State Department of Natural Resources – Geologic Survey publications on Cascade Range seismicity