Seismic Swarm S20230326.1 Near Anza, California: Analysis and Regional Context

A notable earthquake swarm, designated S20230326.1, occurred 8 km north of Anza in Riverside County, California. The sequence began at 14:48 on 25 March 2023 and concluded at 20:53 on 27 March 2023, spanning 54 hours and 5 minutes. During this period, 38 earthquakes were recorded, with magnitudes ranging from 0.1 to 1.8 and focal depths primarily between 3 km and 15 km.

The swarm initiated with a magnitude 0.7 event at 14 km depth. Subsequent activity clustered around shallow depths of 3–4 km during the afternoon of 25 March, including multiple events near magnitude 0.5–0.9. Activity shifted to greater depths of 12–15 km on 26 March, featuring several events of magnitude 1.1–1.6. The sequence tapered off on 27 March with events at 13–15 km depth, the final event registering magnitude 0.5.

This swarm aligns with patterns observed in the Anza region, where seismic swarms are common due to the underlying fault mechanics. Historical records since 2000 indicate 69 swarms in the area, with notable annual counts including 12 in 2020, 7 in 2022, and 7 in 2018. Earlier years such as 2001–2005 recorded fewer events, reflecting variable swarm frequency over two decades.

The Anza area sits within the San Jacinto Fault Zone, a major component of the broader San Andreas transform system in Southern California. This right-lateral strike-slip fault zone accommodates significant tectonic strain between the Pacific and North American plates. The region experiences frequent microseismicity and occasional moderate earthquakes, consistent with its position along active fault strands.

Geological studies confirm that the San Jacinto Fault has produced several historical events exceeding magnitude 6.0, underscoring the zone's potential for larger ruptures. Depths recorded in the March 2023 swarm correspond to typical brittle failure zones in the upper crust of this area, where fluid migration or stress transfer may trigger clustered activity.

References

USGS Earthquake Hazards Program reports on San Jacinto Fault Zone.
California Geological Survey regional fault maps.
Peer-reviewed publications on Southern California seismicity in Bulletin of the Seismological Society of America.