Seismic Swarm S20241206.1: Analysis of Activity Near Petrolia, California
Seismic swarm S20241206.1 occurred 46 km west-northwest of Petrolia in Humboldt County, California. The sequence began at 23:04 on 5 December 2024 and concluded at 22:50 on 11 December 2024, spanning 143 hours and 45 minutes. During this period, 65 earthquakes were recorded.
The swarm exhibited a range of magnitudes from 1.7 to 4.1, with events distributed across depths of 0–34 km. Notable activity included two magnitude 4.1 earthquakes on 6 and 10 December, both at 5 km depth, alongside multiple events exceeding magnitude 3.0. Shallower events clustered near the surface, while deeper shocks reached up to 34 km, indicating varied faulting within the crust.
This region lies at the Mendocino Triple Junction, where the Pacific, North American, and Juan de Fuca plates interact. The area features the Mendocino Fracture Zone and proximity to the Cascadia Subduction Zone, producing complex strike-slip and thrust faulting. High seismicity results from ongoing plate convergence and transform motion.
Since 2000, only two prior swarms have been documented in the immediate vicinity: one in 2010 and another in 2014. These earlier episodes were limited in event count compared with the 65 recorded in December 2024. The swarm followed closely after the magnitude 7.0 Offshore Cape Mendocino earthquake of 5 December 2024, located just 7 km from the swarm centroid, suggesting possible stress triggering.
Depth distribution shows most events between 0 and 20 km, consistent with brittle failure in the upper crust of this tectonically active margin. The temporal clustering—peaking on 6 December with multiple magnitude 3+ events—illustrates typical swarm behavior driven by fluid migration or aseismic slip rather than a single mainshock-aftershock sequence.
Monitoring such swarms aids in understanding regional strain accumulation near the Cascadia margin, where large megathrust events remain a long-term hazard. Continued observation supports refined seismic hazard models for northern California coastal communities.
References
USGS Earthquake Hazards Program
California Geological Survey seismic reports
SeismoSight internal swarm classification S20241206.1