Seismic Swarm S20070418.2 Near Potter Valley, California: Characteristics and Regional Context

An earthquake swarm designated S20070418.2 occurred 17 km north of Potter Valley in Mendocino County, California. The sequence initiated at 18:00 UTC on 17 April 2007 and concluded at 19:22 UTC on 25 April 2007, spanning 193 hours and 22 minutes. During this interval, 107 earthquakes were recorded.

The swarm exhibited predominantly low to moderate magnitudes. Among the first 100 events, the largest was a magnitude 4.7 earthquake at 08:42 UTC on 18 April at a depth of 4 km. Other notable events included magnitudes of 3.5, 2.9, 2.8, and 2.6, with the majority falling between 1.0 and 2.2. Focal depths remained shallow, clustering between 0 and 5 km, consistent with activity along upper-crustal structures. A single event reached 8 km depth. Temporal distribution showed peak activity on 18 April, followed by a gradual decline through 24 April.

The Potter Valley region lies within the northern Coast Ranges of California, where the Pacific and North American plates interact along the San Andreas Fault system. This tectonic setting produces right-lateral strike-slip motion accommodated by subsidiary faults such as the Maacama and Bartlett Springs faults. Historical seismicity in the area reflects ongoing strain accumulation and release, with the 1906 San Francisco earthquake and subsequent events underscoring the region's long-term hazard potential.

Since 2000, only one prior swarm has been documented in the immediate vicinity, occurring in 2000. The 2007 sequence therefore represents a relatively rare clustered episode within an otherwise steady background of distributed seismicity.

References
USGS Earthquake Catalog
California Geological Survey Fault Database
SeismoSight internal swarm classification records