Seismic Swarm VS20001105.1: Analysis of Microearthquake Activity Near Amboy, Washington

Seismic swarm VS20001105.1 occurred approximately 37 km north-northeast of Amboy in southern Washington. The sequence began at 09:01 on 5 November 2000 and concluded at 18:56 on 7 November 2000, spanning 57 hours and 54 minutes. During this interval, 45 earthquakes were recorded, with magnitudes ranging from -1.1 to 1.4 and depths predominantly at or near 1 km.

The events clustered tightly in time, with the majority occurring on 5 November. Peak activity included multiple events between 22:13 and 22:26, featuring magnitudes up to 1.4. Later events on 6 and 7 November were fewer and included the sequence’s deepest event at 9 km on 7 November. All but one event registered magnitudes below 1.0, classifying them as microearthquakes detectable only by sensitive instruments.

This swarm took place within the southern Washington Cascades, a region shaped by ongoing subduction of the Juan de Fuca Plate beneath the North American Plate. The area lies along the Cascade volcanic arc, where magma movement and hydrothermal processes can generate shallow seismic swarms. Amboy is situated near the western margin of the arc, approximately 60 km southeast of Mount St. Helens.

Mount St. Helens remains the most active volcano in the contiguous United States. Its catastrophic 1980 eruption and subsequent dome-building episodes through 2008 demonstrate persistent magmatic and hydrothermal systems capable of producing earthquake swarms. Regional faults, including the St. Helens Seismic Zone, accommodate both volcanic and tectonic strain, contributing to background seismicity.

Swarm VS20001105.1 exhibited characteristics typical of volcanic or hydrothermal swarms: high event rates over short durations, very shallow focal depths, and low magnitudes. Such sequences often reflect fluid migration or minor stress changes rather than large tectonic ruptures. The predominance of depths at -1 km suggests activity within near-surface volcanic or sedimentary layers.

No damage or felt reports were associated with the swarm, consistent with its microseismic scale. Modern monitoring by the Pacific Northwest Seismic Network continues to track similar activity, providing data that refine models of Cascade volcanism and seismic hazard.

References

USGS Earthquake Hazards Program – Pacific Northwest Seismicity
Pacific Northwest Seismic Network – Mount St. Helens Monitoring Reports
Washington State Department of Natural Resources – Geologic History of the Cascade Range