Seismic Swarm S20131109.2 Near Redlands, California

A seismic swarm designated S20131109.2 occurred 7 km SSW of Redlands, California, from 18:13 on 8 November 2013 to 10:20 on 10 November 2013. Over 40 hours and 6 minutes, 33 earthquakes were recorded, with magnitudes ranging from 0.6 to 2.8 and depths primarily between 9 km and 18 km.

The sequence began with a magnitude 1.0 event at 5 km depth. Activity intensified on 9 November, including a magnitude 2.8 quake at 12 km depth at 00:58:46, followed by multiple events clustered around 10–12 km depth. Notable later shocks reached magnitudes of 2.1 and 2.2. Depths showed minor variation, with the final recorded event at 18 km on 10 November.

This swarm aligns with patterns observed in the region since 2000, marking the fourth such episode. Earlier swarms occurred in 2003 (one event), 2005 (two events), and 2010 (one event). Swarms in this area typically involve low-to-moderate magnitudes and reflect localized stress adjustments rather than mainshock-aftershock sequences.

The Redlands area lies within the tectonically active Transverse Ranges of Southern California, where the Pacific and North American plates interact along the San Andreas Fault system. This setting produces frequent small earthquakes and occasional swarms due to complex fault networks and possible fluid migration at depth. Depths of 9–18 km correspond to the brittle upper crust in this zone, where most regional seismicity occurs.

Seismic swarms like S20131109.2 provide data for monitoring fault behavior and assessing background seismicity levels. No damage or felt reports beyond minor shaking were associated with these events.

References
SeismoSight internal swarm classification records
USGS Earthquake Catalog (regional tectonics and historical context)