Seismic Swarm Near Anza, California: Analysis of the June 2024 Event

A seismic swarm designated S20240621.1 occurred 17 km east-southeast of Anza, California, between 21:25 on 20 June 2024 and 21:30 on 23 June 2024. In approximately 72 hours, the sequence produced 41 earthquakes. The events were characterized by low magnitudes, predominantly between 0.2 and 1.9, with focal depths ranging from 5 to 14 km. The largest event reached magnitude 1.9 at a depth of 7 km on 21 June at 18:34.

The Anza region lies within the Peninsular Ranges of Southern California, where the San Jacinto Fault Zone forms a major component of the Pacific–North American plate boundary. This right-lateral strike-slip system accommodates a significant portion of the relative plate motion and is known for producing both large earthquakes and episodic swarm activity. The fault zone’s complex geometry, including multiple strands and step-overs, promotes distributed microseismicity and swarm sequences rather than isolated mainshock–aftershock patterns.

Seismic swarms in this area typically arise from fluid migration along fault planes or from aseismic slip that triggers small brittle failures at depths of 5–15 km. The June 2024 swarm fits this pattern, with the majority of events clustered between 6 and 10 km depth. Activity began with a magnitude-0.6 event, built gradually over the first 24 hours, and reached peak rates on 21 June before declining steadily through 23 June.

Historical records indicate that swarm sequences have been recurrent in the Anza area since at least 2000. A total of 46 swarms have been documented in that period, with notable concentrations in 2010 (six swarms), 2022 (seven swarms), and 2013 and 2016 (four swarms each). These episodes demonstrate that swarm behavior is a persistent feature of the local fault system rather than an anomalous occurrence.

The June 2024 swarm did not produce any events of sufficient magnitude to cause damage or trigger a formal aftershock sequence. Its limited energy release and short duration are consistent with the background microseismicity observed along the central San Jacinto Fault. Continued monitoring remains important because the same fault segments have hosted larger earthquakes in the past and retain the potential for significant seismic release.

References
United States Geological Survey Earthquake Catalog (ANSS Comprehensive Catalog)
Southern California Seismic Network (SCSN) regional bulletins
California Geological Survey, Fault Activity Map of California (2023 update)