Seismic Swarm Analysis: Anza, California, May 2014

An earthquake swarm designated S20140510.1 occurred 8 km west of Anza in Riverside County, California. The sequence began at 15:13 on 9 May 2014 and concluded at 00:21 on 12 May 2014, spanning 57 hours and 8 minutes. During this interval, 44 earthquakes were recorded.

The events exhibited low magnitudes, ranging from 0.0 to 1.6. Depths were predominantly shallow, concentrated between 4 km and 17 km. The majority of activity clustered in the first 24 hours, with peak frequency around 22:57–23:01 on 9 May and additional notable events on 10 and 11 May. Larger events within the swarm included two magnitude 1.6 earthquakes and several magnitude 1.3 shocks, all at depths of 5–16 km.

Earthquake swarms in this region reflect the tectonic setting of the San Jacinto Fault Zone, a major right-lateral strike-slip system within the broader San Andreas transform boundary. The Anza area lies in the Peninsular Ranges, where northwest-trending faults accommodate a significant portion of Pacific–North American plate motion. Historical seismicity shows recurrent swarm activity linked to fluid migration along fault segments and stress interactions within the fault network.

Since 2000, six swarms have been documented near Anza: one each in 2003 and 2009, two in 2011, and two in 2014. These episodes typically involve dozens of microearthquakes without a single dominant mainshock, distinguishing them from typical aftershock sequences. The 2014 swarm aligns with this pattern, occurring within the Anza seismic gap segment known for both swarm behavior and occasional larger events.

No surface rupture or damage was associated with this swarm. Such activity underscores the ongoing seismic hazard in Southern California, where monitoring networks provide detailed records of fault-zone processes.

References

USGS Earthquake Hazards Program – San Jacinto Fault Zone overview
Southern California Seismic Network – Regional tectonics and swarm documentation