Seismic Swarm S20220930.2 Near Anza, California: Analysis and Regional Context

A seismic swarm designated S20220930.2 occurred approximately 18 km east-southeast of Anza in Riverside County, California. The sequence began at 16:59 on 29 September 2022 and concluded at 22:52 on 3 October 2022, spanning 101 hours and 53 minutes. During this period, 69 earthquakes were recorded, with magnitudes ranging from 0.0 to 3.4 and focal depths primarily between 4 km and 14 km.

The swarm initiated with smaller events of magnitude 0.9 and 0.4 at 13 km depth. Activity intensified on 30 September, featuring a peak event of magnitude 3.4 at 9 km depth around 11:50, accompanied by numerous aftershocks clustered at depths of 9–11 km. Subsequent days showed continued low-magnitude seismicity, including events of magnitude 1.6 and 1.4 on 2 October, before tapering off with final tremors of magnitude 0.8 and 1.1 on 3 October at depths around 12–14 km. Depths remained relatively shallow throughout, consistent with activity in the upper crust.

The Anza region lies within the San Jacinto Fault Zone, a major right-lateral strike-slip system in southern California that accommodates a significant portion of the Pacific-North American plate boundary motion. This fault zone branches from the San Andreas Fault and exhibits high strain rates, frequent moderate earthquakes, and episodic swarm activity. The local geology comprises Mesozoic crystalline basement rocks overlain by Quaternary sediments, with faulting influenced by the broader transform boundary tectonics.

Seismic swarms are common in this area due to fluid migration and stress interactions along fault segments. Historical data indicate 71 swarms in the region since 2000, with notable increases in frequency during certain years, such as 11 events in 2020 and 8 in 2018. These sequences typically involve hundreds of small-magnitude events without a dominant mainshock, reflecting the fault zone's segmented and creeping behavior.

The S20220930.2 swarm aligns with this pattern, contributing to ongoing monitoring of strain accumulation. No significant surface rupture or damage was associated with the events, underscoring the low hazard of individual swarm episodes despite the region's overall seismic potential.

References

USGS Earthquake Catalog
California Geological Survey Fault Database
SeismoSight internal swarm classification records