Seismic Swarm S20050107.1: Analysis of Activity Near Fontana, California

Seismic swarm S20050107.1 occurred 3 km north of Fontana, California, from 12:11 on 6 January 2005 to 23:22 on 8 January 2005. Over 59 hours and 11 minutes, the sequence produced 30 earthquakes. The events clustered in time and space without a dominant mainshock, characteristic of swarm behavior in tectonically active regions.

The sequence began with two events within 16 seconds: a magnitude 2.5 quake at 2 km depth followed by a magnitude 3.5 event at 4 km depth. Activity intensified later on 6 January, culminating in the largest event of the swarm—a magnitude 4.4 earthquake at 6 km depth at 14:35:27. Subsequent events ranged from magnitude 1.0 to 3.3, with focal depths predominantly between 3 km and 6 km. Smaller events continued through 7 and 8 January, tapering off by the final recorded magnitude 1.0 quake at 4 km depth.

This swarm represents the second such episode recorded in the area since 1 January 2000, following one in 2003. Swarm sequences like this often reflect fluid migration or stress adjustments along fault networks rather than a single rupture.

Fontana lies within the Transverse Ranges of Southern California, a zone of active compression and strike-slip faulting driven by the interaction between the Pacific and North American plates. The region experiences right-lateral shear along the San Andreas Fault system, with nearby structures including the Cucamonga Fault and segments of the Sierra Madre Fault zone. These faults accommodate regional strain accumulation, producing both moderate earthquakes and occasional swarms.

Historical seismicity in the Inland Empire reflects this tectonic setting. The area has recorded numerous small to moderate events, with deeper crustal processes influencing shallow activity. Depths in the 2–6 km range, as observed here, align with brittle failure in the upper crust typical of the Peninsular Ranges and Transverse Ranges transition.

Ongoing monitoring by regional networks continues to track such sequences to refine understanding of fault interactions and improve hazard assessment in this densely populated corridor.

References

• SeismoSight internal swarm classification records
• USGS Earthquake Catalog (historical data for Southern California)
• California Geological Survey fault maps and regional tectonics summaries