Seismic Swarm S20230221.1 Near Anza, California

A seismic swarm designated S20230221.1 was recorded 6 km west-southwest of Anza, California, beginning at 19:47 on 20 February 2023 and concluding at 13:12 on 22 February 2023. Over 41 hours and 24 minutes, the sequence produced 25 earthquakes, with magnitudes ranging from 0.0 to 1.2 and focal depths between 3 km and 14 km. The events clustered primarily in two depth bands around 4 km and 12 km, indicating activity along shallow crustal structures typical of the region.

The Anza area sits within the San Jacinto Fault Zone, a major right-lateral strike-slip system that forms part of the boundary between the Pacific and North American plates. This zone accommodates roughly 20 percent of the total plate-boundary slip rate in Southern California. The local geology features a complex network of northwest-trending faults cutting through Mesozoic crystalline basement rocks and overlying sedimentary units. Historical seismicity shows that the Anza gap segment of the San Jacinto Fault has remained relatively quiet for large events since the early twentieth century, yet it experiences frequent small-magnitude swarms driven by fluid migration and stress transfer within the fault network.

The February 2023 swarm fits the established pattern of episodic activity in this zone. Since 2000, 72 swarms have been documented near Anza, with notable increases in frequency during 2018 (7 swarms), 2020 (11 swarms), and 2022 (8 swarms). Earlier decades recorded fewer events, averaging one to four swarms per year between 2001 and 2017. These sequences typically last from several hours to a few days and consist of low-magnitude earthquakes, consistent with the 25 events observed in S20230221.1.

The temporal distribution of the 2023 swarm showed an initial cluster on 20 February, followed by heightened activity on 21 February between 05:00 and 14:00 UTC, and a final event on 22 February. Magnitudes remained below 1.3, and no damage or felt reports were associated with the sequence. Such swarms contribute to ongoing strain release without indicating an immediate large-earthquake trigger, although they underscore the persistent seismic hazard posed by the San Jacinto Fault Zone.

Continued monitoring of swarm characteristics in the Anza region supports improved understanding of fault mechanics and aids in refining probabilistic seismic hazard assessments for Southern California.

References

United States Geological Survey Earthquake Catalog
Southern California Earthquake Data Center
California Geological Survey Fault Activity Map