Seismic Swarm S20241205.1 Near Petrolia, California: Geological Context and Event Analysis

A seismic swarm designated S20241205.1 was recorded 53 km west-northwest of Petrolia in Humboldt County, California. The sequence began at 18:46 UTC on 5 December 2024 and concluded at 09:48 UTC on 14 December 2024, spanning 207 hours and 1 minute. During this interval, 194 earthquakes were registered.

Analysis of the first 100 events reveals a rapid onset with several events exceeding magnitude 3.0 in the initial hours. The largest event reached magnitude 4.6 at a depth of 12 km shortly after initiation. Subsequent activity included multiple magnitude 3.5–4.4 shocks at depths ranging from 0 to 31 km, with the majority clustered between 4 and 20 km. Shallow events (less than 5 km) occurred alongside deeper ones, indicating distributed fault slip across a vertically extensive volume. Magnitudes generally declined after the first day, though isolated events above magnitude 3.0 continued through early 6 December.

The Petrolia region lies near the Mendocino Triple Junction, where the Pacific, North American, and Juan de Fuca plates converge. This tectonic setting produces complex deformation involving the Cascadia subduction zone to the north and the San Andreas fault system to the south. The area experiences frequent moderate seismicity due to right-lateral strike-slip motion and subduction-related stresses. Historical records document significant events, including the 1992 magnitude 7.2 Petrolia earthquake, which caused surface rupture and coastal uplift.

Since 2000, only three prior swarms have been identified in the immediate vicinity: one in 2002, one in 2015, and one in 2021. These episodes underscore the episodic nature of clustered seismicity in this high-strain zone, distinct from typical mainshock-aftershock sequences.

The December 2024 swarm aligns with patterns observed in prior activity, featuring a broad magnitude range and variable focal depths consistent with reactivation of subsidiary faults within the broader plate-boundary deformation zone.

References

USGS Earthquake Catalog
California Geological Survey Regional Fault Maps
NOAA National Centers for Environmental Information Earthquake Database