Seismic Swarm S20230828.1 Near Ashford, Washington

A seismic swarm designated S20230828.1 was recorded 6 km east-northeast of Ashford, Washington, between 05:36 on 27 August 2023 and 15:58 on 30 August 2023. The sequence lasted 82 hours and 22 minutes and comprised 43 earthquakes. All events occurred at shallow depths, predominantly between 7 km and 10 km, with the majority clustered around 8–9 km. Magnitudes ranged from 0.0 to a peak of 2.6, indicating low-energy release consistent with swarm-type activity rather than a mainshock-aftershock sequence.

The largest event, magnitude 2.6, occurred at 10:10 on 27 August at 9 km depth. Subsequent activity remained below magnitude 2.0, with only a few events reaching magnitude 1.2–1.8. Depths stayed remarkably consistent throughout the swarm, suggesting a localized source volume. The sequence tapered gradually, with the final recorded event of magnitude 0.6 at 15:58 on 30 August.

Ashford lies on the western flank of Mount Rainier, an active stratovolcano within the Cascade Volcanic Arc. The region experiences background seismicity driven by the subduction of the Juan de Fuca Plate beneath the North American Plate. Shallow crustal earthquakes in this area commonly result from hydrothermal fluid migration, glacial loading and unloading, or minor magmatic movement beneath the volcano. Depths of 7–10 km align with the brittle-ductile transition zone beneath Mount Rainier, where such processes frequently generate swarms.

Mount Rainier is monitored continuously by the Pacific Northwest Seismic Network and the USGS Cascades Volcano Observatory. Historical records show that seismic swarms have occurred intermittently. Since 1 January 2000, only two prior swarms have been documented in the immediate vicinity: one in 2006 and another in 2009. Both earlier episodes were similarly brief and low-magnitude, reinforcing the episodic but infrequent nature of swarm activity at this location.

The August 2023 swarm did not produce any felt reports or surface deformation detectable by regional instrumentation. Such events are typical of Mount Rainier’s background behavior and do not indicate an increased likelihood of eruptive activity in the near term. Continued monitoring remains essential given the volcano’s history of Holocene eruptions and its proximity to populated areas in the Puget Sound region.

References

USGS Cascades Volcano Observatory – Mount Rainier monitoring reports
Pacific Northwest Seismic Network – regional earthquake catalog
SeismoSight internal swarm classification S20230828.1