Seismic Swarm S20140808.1: Analysis of Activity Near Anza, California

The seismic swarm designated S20140808.1 occurred approximately 20 km east-southeast of Anza in Riverside County, California. It began at 03:15 UTC on 7 August 2014 and concluded at 16:52 UTC on 8 August 2014, spanning 37 hours and 37 minutes. During this period, 28 earthquakes were recorded, with magnitudes ranging from -0.1 to 3.2 and focal depths primarily between 7 and 15 km.

The sequence exhibited typical swarm characteristics, featuring a gradual onset followed by clustered events rather than a distinct mainshock-aftershock pattern. The largest event reached magnitude 3.2 at a depth of 13 km on 7 August at 21:45:53 UTC. Several events clustered around this time, including a magnitude 2.5 at 12 km depth and multiple smaller shocks between magnitudes 0.5 and 0.9. Activity remained elevated through the evening of 7 August before tapering on 8 August, with the final recorded event at magnitude 0.3 and 13 km depth.

Event depths showed consistency, with the majority occurring between 12 and 15 km, indicating activity within the seismogenic zone of the local fault network. A single shallower event at 7 km depth occurred on 8 August at 01:26:05 UTC. Magnitudes were predominantly below 1.0, underscoring the swarm's low-energy nature despite the notable peak event.

The Anza region lies within the San Jacinto Fault Zone, a major component of the southern San Andreas Fault system. This right-lateral strike-slip fault accommodates significant strain between the Pacific and North American plates. The area experiences frequent microseismicity due to its complex fault geometry, including multiple strands and step-overs that promote swarm activity. Historical records document 26 swarms in the vicinity since 2000, with notable concentrations in 2010 (5 swarms) and 2011 (4 swarms). Earlier occurrences include single or paired events in 2001, 2002, 2003, 2005, 2009, 2012, and 2013.

Such swarms often reflect fluid migration or aseismic slip along fault segments, contributing to stress redistribution without producing large-magnitude ruptures. The 2014 sequence aligns with this regional pattern, adding to the documented seismic productivity of the zone.

References

USGS Earthquake Hazards Program catalog data
Southern California Seismic Network reports
California Geological Survey fault zone maps