Seismic Swarm S20131109.1: Northern California Earthquake Activity on November 9, 2013

Northern California lies within a tectonically active zone shaped by the interaction between the Pacific Plate and the North American Plate along the San Andreas Fault system and associated subsidiary faults. This region experiences frequent low-magnitude seismicity due to strike-slip faulting and crustal deformation. Historical records document major events such as the 1906 San Francisco earthquake and the 1989 Loma Prieta quake, underscoring the area's long-term seismic hazard. Smaller events, including earthquake swarms, contribute to ongoing strain release without typically producing significant damage.

On November 9, 2013, SeismoSight recorded Swarm S20131109.1 in Northern California. The sequence began at 01:11 and concluded at 22:50, encompassing 28 earthquakes over 21 hours and 39 minutes. Magnitudes ranged from 0.0 to 2.9, with focal depths between 6 and 13 km. The largest event reached magnitude 2.9 at 01:57:55. Activity showed clusters in the early morning hours, a brief lull midday, and a final event at 22:50:13. Depths remained consistent in the upper crust, consistent with regional faulting patterns.

This swarm provides insight into localized stress adjustments along minor faults. Sequences like this often occur without a dominant mainshock, instead reflecting fluid migration or aseismic slip influences common in the area. Compared to prior activity, only two swarms had been noted since January 1, 2000: one in 2001 and another in 2011. The 2013 event adds to this sparse record, highlighting episodic rather than continuous swarm behavior in the monitored zone.

The distribution of events indicates a compact source area, with most shocks clustered between 7 and 11 km depth. Low-magnitude events dominated, suggesting limited energy release overall. Such patterns align with Northern California's background seismicity, where swarms serve as indicators of evolving fault conditions rather than precursors to larger quakes.

Further monitoring by regional networks continues to track similar activity, contributing to improved hazard assessments for the San Andreas system and nearby structures.

References

USGS Earthquake Hazards Program
California Geological Survey seismic reports
SeismoSight internal swarm classification data