Seismic Swarm VS20050720.1: Geological Context and Event Analysis Near Amboy, Washington

The seismic swarm designated VS20050720.1 occurred in a tectonically active region of southwestern Washington, approximately 37 km north-northeast of Amboy. This area lies within the Cascade Range volcanic arc, influenced by ongoing subduction of the Juan de Fuca Plate beneath the North American Plate. The subduction zone drives both volcanic and seismic activity across the Pacific Northwest, with regional faults and crustal stresses contributing to earthquake swarms.

The swarm initiated at 09:37 on 19 July 2005 and concluded at 14:32 on 10 September 2005, spanning 1276 hours and 55 minutes. During this period, 548 earthquakes were recorded. Depths were predominantly shallow, clustered between 0 and -2 km, with one outlier at 9 km. Magnitudes ranged from 0.6 to 3.3, consistent with low-to-moderate energy release typical of swarm sequences rather than a single mainshock-aftershock pattern.

Analysis of the first 100 events reveals an initial burst of activity on 19–20 July, featuring several events above magnitude 2.0, including peaks of 3.3 and 2.6. Subsequent days through 28 July showed sustained but declining frequency, with magnitudes mostly between 1.4 and 2.9. Negative depth values likely reflect near-surface or slightly adjusted hypocenters common in local network processing.

Historical records since 2000 indicate this was the third swarm in the vicinity, following one event in 2000 and two in 2004. Such recurrent swarms suggest episodic stress release along local structures, potentially linked to fluid migration or magmatic influences in the broader volcanic setting.

The Amboy region’s geology features Quaternary volcanic deposits and fault systems associated with the Mount St. Helens seismic zone to the north. These conditions support swarm-type seismicity, where events occur in clusters without a dominant rupture.

References

SeismoSight internal swarm classification VS20050720.1 data records.