Seismic Swarm S20250516.1 Near Anza, California: Characteristics and Regional Context
A seismic swarm designated S20250516.1 occurred 5 km west-northwest of Anza, California, from 03:14 on 16 May 2025 to 10:38 on 20 May 2025. Over 103 hours and 24 minutes, the event sequence produced 45 earthquakes. Magnitudes ranged from 0.0 to 2.2, with the largest event recorded at magnitude 2.2 on 16 May at 16:44:32 UTC and a focal depth of 15 km. Depths throughout the swarm varied between 2 km and 17 km, indicating activity across shallow crustal levels.
The sequence began with a magnitude 0.2 event at 2 km depth and included multiple low-magnitude events clustered in the first 24 hours. Subsequent days showed continued low-level activity, with occasional events reaching magnitude 1.0–1.5. Depths generally remained between 3 km and 16 km after the initial hours, consistent with typical swarm patterns in the area where events do not follow a clear mainshock-aftershock decay.
The Anza region sits within the San Jacinto Fault Zone, a major right-lateral strike-slip system that accommodates a significant portion of the Pacific–North American plate boundary motion. This fault zone has long been recognized for producing both large earthquakes and frequent earthquake swarms. The swarm location aligns with known segments of the fault that exhibit episodic clustered seismicity driven by fluid migration or aseismic slip transients.
Historical records document 80 swarms in the same region since 1 January 2000. Annual counts show variability, with peaks in 2020 (13 swarms) and sustained activity in 2017, 2018, and 2022. Earlier years recorded fewer events, reflecting both natural fluctuation and improvements in seismic network detection. These swarms typically involve dozens of events over days to weeks and rarely produce magnitudes above 3.0, distinguishing them from mainshock sequences.
The May 2025 swarm fits established patterns of low-magnitude, short-duration clustering observed along the San Jacinto Fault. Depths between 3 km and 17 km correspond to the seismogenic zone where brittle failure occurs on fault surfaces. No surface rupture or significant damage was associated with this sequence, consistent with the modest energy release.
Ongoing monitoring by regional seismic networks continues to track activity in this portion of the fault zone. Such swarms provide data on fault behavior and stress conditions without indicating imminent large earthquakes.
References
SeismoSight internal classification and event catalog for swarm S20250516.1.
USGS Earthquake Hazards Program regional fault and seismicity reports for the San Jacinto Fault Zone.