Seismic Swarm Offshore Northern California: December 2024 Analysis

A seismic swarm designated S20241205.2 occurred offshore Northern California from 18:57 on 5 December 2024 to 16:01 on 7 December 2024. Over 45 hours and 4 minutes, the sequence produced 108 earthquakes. Analysis of the first 100 events reveals magnitudes ranging from 1.4 to 4.0, with the majority between 2.0 and 3.0. Depths clustered primarily between 5 km and 20 km, though several events reached 0 km or extended to 33 km.

The sequence began with a 3.5 magnitude event at 21 km depth. Subsequent activity included additional 3.5 and 3.2 magnitude shocks within the first few hours, followed by a peak event of magnitude 4.0 at 7 km depth on 6 December at 20:01. Later notable events comprised magnitudes of 3.7, 3.6, and 3.2, all at shallow depths under 10 km. Depths showed no consistent migration pattern, with shallow and mid-crustal events interspersed throughout the swarm duration.

This swarm represents the fifth such sequence in the region since 2000. Prior events occurred in 2002, 2015, 2021, 2022, and an earlier 2024 swarm. The recurrence indicates episodic clustering rather than isolated incidents, consistent with the tectonic setting of the area.

Offshore Northern California lies at the Mendocino Triple Junction, where the Pacific, North American, and Juan de Fuca plates interact. The region features the northern termination of the San Andreas Fault system transitioning into the Cascadia Subduction Zone and the Mendocino Fracture Zone. Transform faulting dominates, producing frequent moderate earthquakes through strike-slip motion. Historical records document recurrent seismicity driven by plate boundary deformation, with swarms often linked to fluid migration or stress transfer along fault segments.

The December 2024 swarm aligns with this framework, exhibiting typical characteristics of swarm behavior: rapid onset, lack of a single dominant mainshock, and gradual decay. Depths mostly within the upper crust reflect brittle failure along active fault zones in this tectonically active margin.

References

USGS Earthquake Catalog
SeismoSight internal swarm classification data