Seismic Swarm S20220922.1: Analysis of Activity Near Anza, California
Seismic swarm S20220922.1 occurred 17 km east-southeast of Anza, California, in the San Jacinto Fault Zone. The sequence began at 00:54 on 22 September 2022 and concluded at 19:18 on 23 September 2022, spanning 42 hours and 23 minutes. During this period, 31 earthquakes were recorded, with magnitudes ranging from 0.2 to 2.2 and focal depths between 8 and 15 km.
The swarm exhibited two main phases of elevated activity. The initial cluster on 22 September featured events clustered between 03:46 and 06:19, followed by a more energetic episode starting at 19:04 that included the largest event of magnitude 2.2 at 13 km depth. Subsequent activity on 23 September was sparser, with isolated events through the evening. Depths remained consistent in the mid-crustal range, typical for this portion of the fault zone.
This swarm aligns with patterns observed in the region since 2000, during which 68 swarms have been documented. Annual counts show variability, with peaks in 2020 (11 swarms) and 2018 (8 swarms), and lower activity in years such as 2001 and 2019 (1–2 swarms each). Such sequences reflect the fault zone’s tendency toward clustered, low-magnitude seismicity rather than isolated mainshock-aftershock patterns.
Geologically, the 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 motion. The fault zone features multiple parallel strands and has produced numerous moderate earthquakes historically, with recurrence intervals for larger events estimated in the range of centuries. Depths of 8–15 km correspond to the seismogenic zone where brittle failure occurs in the crystalline basement rocks.
Swarm activity in this setting often results from fluid migration or aseismic slip triggering small failures along fault segments. The 2022 sequence did not exceed magnitude 3.0, consistent with the majority of prior swarms in the catalog.
References
- SeismoSight internal swarm classification S20220922.1
- USGS Quaternary Fault and Fold Database (San Jacinto Fault Zone)
- Southern California Seismic Network event catalog (2000–2022)