Earthquake Swarm S20220519.2 Near Anza, California

An earthquake swarm designated S20220519.2 occurred 3 km southeast of Anza, California. The sequence began at 21:37 on 18 May 2022 and concluded at 06:22 on 20 May 2022, spanning 32 hours and 45 minutes. During this interval, 29 earthquakes were recorded.

The events clustered at depths primarily between 11 km and 15 km, with one shallower event at 3 km and another at 7 km. Magnitudes remained low throughout, ranging from 0.1 to a peak of 2.7. The largest event, magnitude 2.7, took place on 19 May at 12:52:00 at 13 km depth. Most activity concentrated in a brief window on 19 May between 15:10 and 15:22, when 15 events occurred within 12 minutes.

The Anza region sits within the Peninsular Ranges of Southern California, where the San Jacinto Fault Zone forms a major strand of the broader San Andreas system. This right-lateral strike-slip fault accommodates a significant portion of Pacific-North American plate motion. The local geology features crystalline basement rocks of Mesozoic age overlain by Quaternary sediments in adjacent valleys. The fault zone here exhibits a history of both large mainshock-aftershock sequences and swarm-type activity driven by fluid migration and stress transfer along segmented fault strands.

Seismic swarms have been a recurring feature in the Anza area. Since 1 January 2000, 66 swarms have been documented. Yearly counts include single events in 2001, 2022, and multiple episodes in 2018 (8), 2020 (11), and 2017 (6). These clusters typically involve low-magnitude events at mid-crustal depths, consistent with the characteristics observed in S20220519.2.

The swarm data reflect the ongoing tectonic loading and release along the Anza Gap segment of the San Jacinto Fault, an area monitored for its potential to host larger earthquakes. Depths near 12–14 km align with the brittle-ductile transition zone where small ruptures often occur without significant surface rupture.

References

SeismoSight internal classification records for swarm S20220519.2.
USGS Earthquake Hazards Program regional fault database.
California Geological Survey Peninsular Ranges geologic mapping.