Seismic Swarm S20180403.1 Near Aguanga, California

A notable earthquake swarm, designated S20180403.1, occurred 6 km northwest of Aguanga in Riverside County, Southern California. The sequence began at 02:53 on 2 April 2018 and concluded at 17:43 on 12 April 2018, spanning 254 hours and 49 minutes. During this period, 197 earthquakes were recorded. This activity aligns with the region's established pattern of episodic seismic swarms driven by the complex fault network of the Peninsular Ranges.

The Aguanga area lies within a tectonically active zone influenced by the San Andreas Fault system and subsidiary structures, including strands of the Elsinore and San Jacinto fault zones. These faults accommodate right-lateral strike-slip motion associated with the Pacific-North American plate boundary. Depths of events in the swarm ranged primarily between 2 km and 10 km, consistent with shallow crustal seismicity typical of the Peninsular Ranges province. The geology features Mesozoic granitic and metamorphic rocks overlain by Quaternary sediments, with historical seismicity reflecting both mainshock-aftershock sequences and swarm-type behavior.

Analysis of the first 100 events reveals predominantly low-magnitude activity. The initial event registered magnitude 3.3 at 2 km depth. Subsequent shocks included a magnitude 2.3 event and several between 1.0 and 2.0, with the majority falling below magnitude 1.0. Depths clustered around 4–5 km for most events, though occasional deeper occurrences reached 10 km. Temporal distribution showed higher rates in the first 48 hours, followed by a gradual decline, characteristic of swarm evolution without a dominant mainshock.

Since 1 January 2000, 39 swarms have been documented in the same locale. Yearly counts include single swarms in 2001, 2002, 2003, 2005 (two), 2009, and 2010 (three), with increased frequency in later years: four in 2011, three in 2012, three in 2013, four in 2014, two in 2015, five in 2016, seven in 2017, and two in 2018. This recurrence underscores persistent strain accumulation and release along local fault segments.

Such swarms contribute to ongoing seismic hazard assessment in Southern California, where fluid migration or aseismic slip may trigger clustered events. Monitoring by regional networks continues to refine understanding of these patterns within the broader plate-boundary framework.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey regional fault maps
SeismoSight internal swarm classification records