Seismic Swarm VS20040923.1: Analysis of Activity near Morton, Washington

The seismic swarm VS20040923.1 was recorded 28 km south-southeast of Morton in Lewis County, Washington. Activity began at 15:07 on 22 September 2004 and concluded at 06:57 on 17 October 2004, spanning 591 hours and 50 minutes. During this interval, 2267 earthquakes were detected, characteristic of a swarm sequence with no dominant mainshock.

The region lies within the Cascade volcanic arc, formed by subduction of the Juan de Fuca plate beneath the North American plate. This tectonic setting produces frequent shallow crustal seismicity, often linked to fluid migration or magmatic processes. Morton sits near the southern Washington Cascades, where historical swarms have been associated with both tectonic strain release and hydrothermal activity. The local geology features Quaternary volcanic deposits overlying older sedimentary and metamorphic basement rocks, with the nearest major volcanic center being Mount St. Helens approximately 50 km to the southwest.

Analysis of the first 100 events reveals predominantly microseismic activity. Magnitudes ranged from -1.1 to 0.9, with the majority falling between -0.8 and 0.5. Depths were shallow, concentrated between 0 and 2 km, although a few events reached 15 km. Negative depth values in the catalog likely reflect location uncertainties common in sparse networks. Event timing showed clustering in the initial 24 hours, followed by a steady decline in rate. These patterns align with typical swarm behavior driven by pore-pressure changes rather than brittle failure from a single fault rupture.

Since 1 January 2000, only one prior swarm has been documented in the immediate area, occurring in 2000. This low recurrence rate underscores the episodic nature of seismicity in the region, where swarms may reflect transient fluid involvement within the volcanic arc crust.

The swarm's shallow focus and low magnitudes suggest limited surface impact, consistent with background activity levels in the Cascades. Ongoing monitoring by regional networks continues to track such sequences for potential links to deeper magmatic unrest.

References

SeismoSight internal swarm classification records.
USGS Earthquake Catalog (regional Pacific Northwest data).
Cascades Volcano Observatory geological summaries.