Seismic Swarm S20220702.1 Near Anza, California: July 2022 Analysis

A seismic swarm designated S20220702.1 occurred 13 km west-northwest of Anza, California, between 20:34 on 1 July 2022 and 20:30 on 5 July 2022. In 95 hours and 56 minutes, the sequence produced 49 earthquakes, with magnitudes ranging from -0.1 to 2.2 and focal depths predominantly between 10 and 16 km.

The swarm began with a magnitude 0.1 event at 14 km depth. Subsequent activity showed a gradual increase in event frequency and a modest rise in maximum magnitudes, peaking at 2.2 on 4 July at 8 km depth. Depths remained consistent within the mid-crustal range typical of the region, suggesting rupture along a localized fault segment without significant migration to shallower or deeper levels.

The Anza area lies 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 zone exhibits complex segmentation and has produced multiple moderate to large earthquakes historically. Seismicity here is influenced by the nearby San Andreas Fault to the north and the Elsinore Fault to the southwest, creating a tectonically active corridor prone to both mainshock-aftershock sequences and swarm-like activity.

Earthquake swarms are recurrent in this portion of the San Jacinto Fault Zone. Since 2000, 53 swarms have been documented in the broader area, with notable increases in frequency during 2018 (7 events), 2020 (10 events), and 2021 (4 events). The 2022 sequence represents the second swarm recorded that year. Such swarms typically involve numerous small-magnitude events without a dominant mainshock, consistent with fluid migration or aseismic slip triggering mechanisms observed in similar tectonic settings.

Event timing within S20220702.1 showed clustering on 2 July, with 22 earthquakes recorded that day alone. Magnitudes remained below 1.0 for most of the sequence, except for isolated events reaching 1.5–2.2 on 2–4 July. Depths showed minor variation, with the shallowest event at 7 km and several deeper occurrences at 16 km, indicating activity distributed across a narrow vertical extent of the fault.

The geological context of the Anza region features Quaternary alluvial deposits overlying Mesozoic basement rocks, with active fault traces expressed as linear scarps and offset drainages. Paleoseismic studies indicate recurrence intervals for large events on nearby San Jacinto segments on the order of centuries, underscoring the importance of monitoring microseismicity for hazard assessment.

This swarm adds to the long-term record of episodic seismic activity in the San Jacinto Fault Zone, providing data for refined models of fault interaction and strain accumulation along the plate boundary.

References

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