Seismic Swarm S20000518.1 Near Potter Valley, California: Geological Context and Event Analysis

The seismic swarm designated S20000518.1 occurred 8 km north-northeast of Potter Valley in Mendocino County, California. It began at 21:49 on 17 May 2000 and concluded at 02:15 on 22 May 2000, registering 56 earthquakes over 100 hours and 25 minutes. This sequence exemplifies swarm behavior, characterized by clustered events without a dominant mainshock followed by decaying aftershocks.

Potter Valley lies within the northern California Coast Ranges, a region shaped by ongoing transform tectonics at the Pacific-North American plate boundary. The area is influenced by the Maacama Fault, a right-lateral strike-slip structure that forms part of the broader San Andreas Fault system. Shallow crustal deformation here results from distributed shear and minor extension associated with the Mendocino Triple Junction to the northwest. Historical records document recurrent moderate seismicity along this fault zone, including events in the late 19th and 20th centuries that reflect persistent strain accumulation.

The swarm's temporal and magnitude distribution reveals episodic energy release. The largest event reached magnitude 4.3 at 22:32 on 17 May at 6 km depth. Subsequent activity included three magnitude 3.2 shocks on 18, 21, and 21 May, alongside multiple magnitude 3.0 events. Depths remained predominantly shallow, ranging from 0 to 9 km, consistent with brittle failure in the upper crust. Early phases on 17 May featured the highest initial magnitudes, followed by sustained lower-magnitude events through 21 May before tapering on 22 May.

Magnitude-frequency patterns show a high proportion of events between 1.5 and 2.5, with fewer larger shocks. Depth clustering below 5 km for most events suggests fluid-assisted triggering or localized stress perturbations along fault segments. The absence of a clear magnitude decay sequence aligns with swarm dynamics often observed in this tectonic setting, where pore-pressure changes or aseismic slip may sustain activity over days.

Geological mapping indicates that the Potter Valley vicinity hosts Franciscan Complex bedrock cut by Quaternary-active faults. Updated regional models from the U.S. Geological Survey emphasize the Maacama Fault's slip rate of approximately 5–10 mm per year, contributing to the area's elevated seismic hazard. Similar swarms have occurred historically in adjacent Mendocino County zones, underscoring the role of distributed fault networks in accommodating plate motion.

This 2000 sequence provides a clear record of short-term seismic clustering in a transform-margin environment. Analysis of event timing and depths supports interpretations of shallow crustal processes without requiring magmatic involvement.

References

USGS Earthquake Catalog (ANSS Comprehensive Catalog)
California Geological Survey Fault Activity Map
USGS Quaternary Fault and Fold Database of the United States