Seismic Swarm VS20090920.1: Analysis of Activity Near Ashford, Washington

A seismic swarm designated VS20090920.1 occurred 24 km east-northeast of Ashford, Washington, from 12:41 on 20 September 2009 to 15:28 on 22 September 2009. In 50 hours and 46 minutes, the swarm produced 121 earthquakes. This event represents the second swarm recorded in the region since 1 January 2000, following one in 2006.

The first 100 events displayed consistently shallow focal depths, predominantly between -2 km and 2 km. Magnitudes remained low, with the majority below 1.0 and a peak of 2.5 recorded at 16:45:27 on 20 September. Early activity featured frequent events spaced minutes apart, including multiple magnitude 0.9–1.2 quakes between 16:32 and 16:51. Activity then moderated through the evening and into 21 September, with events becoming less frequent after midnight. Depths clustered near 0–1 km throughout, indicating a near-surface source consistent with fluid migration or minor fracturing in the upper crust.

Ashford lies on the western flank of Mount Rainier, an active stratovolcano within the Cascade Range. The area experiences background seismicity driven by volcanic processes, including hydrothermal fluid movement and glacial loading. Mount Rainier’s edifice hosts extensive glaciers and a well-developed hydrothermal system that can trigger shallow earthquake swarms without accompanying eruptive activity. Historical records document similar swarms in 2006 and earlier episodes in the 1990s, all characterized by low-magnitude, shallow events clustered over days rather than single large shocks.

Regional tectonics involve oblique subduction of the Juan de Fuca plate beneath North America, producing the Cascade volcanic arc. Mount Rainier itself last erupted in the 19th century, with the most recent confirmed activity around 1894–1895. Modern monitoring by the Pacific Northwest Seismic Network shows that swarms near the volcano typically last hours to several days and rarely exceed magnitude 3. The 2009 swarm followed this pattern exactly, remaining well below thresholds that would indicate magmatic intrusion.

No damage or felt reports were associated with VS20090920.1, reflecting both the small magnitudes and the remote location relative to populated areas. Continued monitoring of the Mount Rainier region remains essential for distinguishing benign swarm activity from any precursory signals of renewed volcanic unrest.

References
Pacific Northwest Seismic Network (PNSN) catalog data, 2009.
USGS Volcano Hazards Program, Mount Rainier geologic history.
Cascades Volcano Observatory, swarm monitoring summaries.