Seismic Swarm S20051018.1 Near Cabazon, California: Event Analysis and Regional Context

The seismic swarm designated S20051018.1 occurred 11 km north of Cabazon in Riverside County, California. Registered between 04:08 on 18 October 2005 and 09:04 on 20 October 2005, the sequence lasted 52 hours and 55 minutes and comprised 46 earthquakes. This event represents the sole swarm recorded in the region since 1 January 2000.

The swarm initiated with a magnitude 4.1 earthquake at 15 km depth. A larger magnitude 4.4 event followed at 17 km depth approximately three hours later. Subsequent activity included a magnitude 2.9 event at 17 km depth on 18 October and a magnitude 2.2 event at 16 km depth later that day. Magnitudes ranged from 0.8 to 4.4, with the majority falling between 1.0 and 1.9. Depths clustered predominantly between 15 km and 19 km, though several shallower events occurred at 4 km.

Temporal distribution showed peak activity on 18 October, with 28 events recorded that day. Activity declined on 19 October (11 events) and tapered to seven events on 20 October. The sequence exhibited typical swarm characteristics: no single dominant mainshock followed by clear aftershocks, but rather a cluster of events of comparable size distributed over two days.

The Cabazon area lies within the San Gorgonio Pass, a structurally complex zone where the San Andreas Fault interacts with the San Jacinto and Banning faults. This region experiences distributed deformation due to the transition from the Coachella Valley segment of the San Andreas Fault to the more segmented Big Bend area. Historical seismicity reflects ongoing right-lateral strike-slip motion between the Pacific and North American plates, with background earthquake rates elevated compared to surrounding stable blocks.

Swarm events in this setting often relate to fluid migration or aseismic slip along fault segments, though precise triggering mechanisms for S20051018.1 remain internal to SeismoSight classification. Depths around 15–19 km align with the seismogenic zone in the Peninsular Ranges and Transverse Ranges transition, where brittle failure occurs in the mid-crust.

No additional swarms have been classified in the area since this 2005 sequence, underscoring its rarity within the local catalog.

References SeismoSight internal swarm classification database USGS regional fault and seismicity reports for Southern California