Analysis of the October 2006 Earthquake Swarm Near Packwood, Washington

The earthquake swarm designated S20061008.1 occurred 27 km NNE of Packwood, Washington, from 17:16 on 7 October 2006 to 18:08 on 12 October 2006. Over this 120-hour and 52-minute period, 103 events were recorded. This sequence represents a classic example of swarm behavior, characterized by a rapid onset of activity without a single dominant mainshock, followed by a gradual decline.

The first 100 events provide clear insight into the swarm's dynamics. Magnitudes ranged from -0.8 to 4.5, with the large majority below 1.0 and only one event exceeding magnitude 3. Depths were predominantly shallow, concentrated between 0 and 5 km, although a smaller number extended to 12 km. The initial event on 7 October registered magnitude 0.9 at a depth of -2 km. Activity intensified rapidly on 8 October, including the peak magnitude 4.5 event at 02:48:26 at 1 km depth. Subsequent events showed a mix of positive and negative magnitudes, reflecting the detection of both small tectonic events and microseismicity near the surface. Many events clustered temporally within the first 48 hours, after which the rate decreased steadily through 12 October.

Geologically, the swarm occurred within the Cascade Range of southern Washington, a region shaped by ongoing subduction of the Juan de Fuca plate beneath the North American plate along the Cascadia Subduction Zone. This tectonic setting produces frequent volcanic and seismic activity, with the study area lying near the southern flank of Mount Rainier. The shallow depths observed are consistent with upper-crustal faulting and fluid migration commonly associated with volcanic-arc environments. Historical records indicate that similar swarms have occurred in the broader Mount Rainier region, often linked to hydrothermal processes or minor slip on local faults rather than large-magnitude tectonic ruptures.

The 2006 swarm did not produce any reported damage or felt shaking beyond the immediate vicinity, consistent with the modest magnitudes involved. The predominance of events at depths less than 5 km suggests possible involvement of near-surface structures or pressurized fluids within the volcanic edifice. Such sequences contribute to the understanding of background seismicity rates in the Cascade arc and help refine models of strain accumulation along the subduction interface.

References
SeismoSight internal classification data for swarm S20061008.1.
USGS Earthquake Hazards Program regional tectonic summaries for the Cascade Range.
Pacific Northwest Seismic Network historical earthquake reports.