Seismic Swarm S20210328.1 Near Anza, California: Analysis and Regional Context

A seismic swarm designated S20210328.1 occurred approximately 13 km west-northwest of Anza in Riverside County, California. The sequence began at 22:09 on 27 March 2021 and concluded at 19:41 on 30 March 2021, spanning 69 hours and 32 minutes. During this interval, 33 earthquakes were recorded. Magnitudes ranged from -0.1 to 1.9, with focal depths distributed between 6 km and 16 km. The largest event reached magnitude 1.9 at a depth of 14 km shortly after initiation, while subsequent activity consisted predominantly of microearthquakes below magnitude 1.0. Activity was concentrated in the initial 24 hours, with diminishing frequency over the following two days.

The Anza region lies within the San Jacinto Fault Zone, a major component of the Pacific-North American plate boundary in Southern California. This right-lateral strike-slip fault system accommodates a significant portion of the relative plate motion and has produced multiple historical earthquakes exceeding magnitude 6. The fault zone exhibits complex segmentation, with the Anza section characterized by a combination of locked and creeping segments. Seismicity in this area frequently manifests as swarms rather than isolated mainshock-aftershock sequences, often linked to fluid migration along fault planes or transient aseismic slip.

Since 2000, the Anza vicinity has experienced 49 documented swarms. Annual counts show variability, with notable increases in later years: single events in 2002 and 2003, rising to 10 swarms in 2020. This pattern suggests ongoing tectonic strain release modulated by local crustal conditions. Depths of swarm events typically fall within the upper 20 km of the crust, consistent with the brittle-ductile transition zone along the San Jacinto Fault.

The March 2021 swarm displayed typical characteristics of Anza-area sequences, including low maximum magnitudes and a wide depth range. Such swarms contribute to the cumulative seismic energy release without producing damaging ground motion. Monitoring by regional networks indicates that similar activity may recur, reflecting the fault zone’s persistent microseismicity.

References

USGS Earthquake Hazards Program – San Jacinto Fault Zone overview
Southern California Seismic Network – Regional fault and swarm documentation
SeismoSight internal swarm classification records