Seismic Swarm S20180811.1 Near Aguanga, California: Geological Context and Event Analysis
The seismic swarm designated S20180811.1 occurred 8 km northeast of Aguanga in Riverside County, Southern California. This region lies within the Peninsular Ranges province, characterized by a complex network of faults associated with the broader San Andreas Fault system. The Elsinore Fault Zone and nearby San Jacinto Fault contribute to the area's tectonic activity, where right-lateral strike-slip motion accommodates Pacific-North American plate boundary deformation. Shallow crustal depths in this zone, typically under 10 km, facilitate frequent microseismicity and occasional swarms driven by fluid migration or stress transfer along fault segments.
The swarm initiated at 00:28 on 11 August 2018 and concluded at 16:05 on 8 March 2019, spanning 5031 hours and 36 minutes with a total of 6032 registered earthquakes. This prolonged sequence exemplifies the swarm-type behavior common in Southern California, where events cluster without a dominant mainshock. Historical records indicate 45 such swarms in the region since 1 January 2000, with annual frequencies varying: one each in 2001, 2003, 2005, and 2009; two in 2002; three in 2010, 2012, and 2013; four in 2011 and 2014; two in 2015; six in 2016 and 2018; and seven in 2017. These patterns reflect persistent tectonic loading in the area.
Analysis of the first 100 events reveals predominantly low-magnitude activity at shallow depths. The sequence began with a magnitude 0.5 event at 4 km depth, followed by numerous events below magnitude 1.0 clustered at 2–4 km. Notable larger events included a magnitude 2.3 at 2 km depth on 11 August at 21:17, a magnitude 2.8 at 2 km on 11 August at 23:46, and a magnitude 2.2 at 2 km on 12 August at 08:33. Depths ranged from 2 km to 8 km, with most events between 2 km and 4 km, consistent with the brittle upper crust in this faulted terrain. Magnitudes generally remained below 2.0 after the initial peak, indicating a diffuse energy release typical of swarm dynamics.
The geological setting supports such activity due to the intersection of multiple fault strands and potential hydrothermal influences from nearby geothermal features. Updated regional monitoring confirms ongoing microseismicity in the Aguanga area, underscoring the importance of continuous observation for hazard assessment in this populated portion of Southern California.
References
SeismoSight internal classification data for swarm parameters and event lists.
USGS Earthquake Catalog for regional fault and tectonic background.