Seismic Swarm S20210609.1 Near Aguanga, California: June 2021 Analysis

The region surrounding Aguanga in Riverside County, California, lies within the Peninsular Ranges province, a tectonically active zone shaped by the interaction between the Pacific and North American plates. This area experiences frequent seismic activity due to its proximity to major fault systems, including strands of the San Andreas Fault system and associated secondary faults such as the Elsinore and San Jacinto faults. The local geology features Mesozoic granitic and metamorphic rocks overlain by Quaternary sediments, with faulting that accommodates right-lateral strike-slip motion and contributes to the region's elevated seismicity.

Earthquake swarms are common in this setting, reflecting fluid migration or stress transfer along fault networks rather than a single mainshock-aftershock sequence. Since 2000, 57 swarms have occurred in the broader area, with annual counts varying from one in 2001 to a peak of 11 in 2020. This pattern underscores persistent low-level tectonic strain release in southern California.

Swarm S20210609.1 began at 21:59 on 8 June 2021 and concluded at 14:30 on 14 June 2021, spanning 136 hours and 30 minutes. During this period, 126 earthquakes were recorded 8 km east-northeast of Aguanga. Analysis of the first 100 events reveals predominantly low magnitudes, with the majority below 1.0 and a maximum of 2.3. Depths ranged from 1 km to 14 km, clustering most frequently between 2 km and 4 km, consistent with shallow crustal faulting in the Peninsular Ranges.

Event timing showed an initial sparse phase on 8 June, followed by increased activity on 9 June with multiple events near 09:00–09:20. Further clusters emerged on 10 June around 13:00–14:00 and 18:00–19:00, featuring the swarm's largest events. Subsequent days through 11 June exhibited continued but diminishing frequency, with events spaced at intervals of minutes to hours. This temporal distribution indicates episodic energy release typical of swarm behavior, without a dominant triggering shock.

Overall, the swarm remained at microseismic levels, posing minimal hazard but providing valuable data on local stress regimes. Such sequences contribute to understanding fault interactions in a region where historical seismicity has included larger events along nearby faults.

References

USGS Earthquake Catalog California Geological Survey regional reports Southern California Seismic Network bulletins