Seismic Swarm S20221026.1 Near Idyllwild, California

A seismic swarm designated S20221026.1 occurred 10 km south of Idyllwild in Riverside County, California, from 10:17 on 25 October 2022 to 21:08 on 26 October 2022. Over 34 hours and 50 minutes, the sequence produced 35 earthquakes, with magnitudes ranging from 0.2 to 2.7 and focal depths between 8 km and 17 km. The largest events reached magnitude 2.7 at 08:15 on 26 October and magnitude 2.3 at 08:14 the same day, both at depths of 13 km.

The swarm clustered tightly in both space and time, with the majority of events occurring between 08:12 and 08:27 on 26 October. Magnitudes remained modest overall, consistent with swarm behavior in which no single mainshock dominates the sequence. Depths concentrated around 13–14 km, typical for the brittle-ductile transition zone in this part of the Peninsular Ranges.

Idyllwild 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 trends northwest–southeast through the San Jacinto Mountains and has produced multiple historical earthquakes exceeding magnitude 6. Regional geology comprises Mesozoic granitic and metamorphic rocks of the Peninsular Ranges batholith, cut by active faults that extend to depths of 15–20 km. Seismicity in the area is driven by the broader transform boundary between the Pacific and North American plates.

Since 2000, 64 swarms have been recorded in the region. Annual counts show a marked increase after 2016, with 2020 recording the highest number (10 swarms). This long-term pattern reflects persistent tectonic loading along the San Jacinto Fault Zone and adjacent structures.

The S20221026.1 swarm adds to the documented seismic activity south of Idyllwild. Its compact spatial footprint and lack of a dominant mainshock align with fluid-migration or aseismic-slip triggering mechanisms commonly invoked for swarms in southern California. Continued monitoring remains essential given the proximity of the sequence to populated areas and critical infrastructure.

References

United States Geological Survey Earthquake Catalog (earthquake.usgs.gov)
Southern California Seismic Network (scsn.org)
California Geological Survey Fault Activity Map