Seismic Swarm S20180305.1: Analysis of Activity Near Aguanga, California

The seismic swarm designated S20180305.1 occurred approximately 10 km northeast of Aguanga in Riverside County, California. It commenced at 18:27 on 4 March 2018 and concluded at 12:22 on 24 March 2018, spanning 473 hours and 55 minutes. During this period, 203 earthquakes were recorded, reflecting a concentrated episode of low-magnitude seismicity typical of swarm sequences in the region. Examination of the initial 100 events reveals predominantly small-magnitude earthquakes, with values ranging from 0.1 to 2.1. The largest event reached magnitude 2.1 on 5 March 2018 at a depth of 3 km. Depths clustered between 3 km and 8 km, averaging around 5 km, consistent with shallow crustal activity. Early events on 4 March showed magnitudes between 0.2 and 1.3 at depths of 3–13 km, followed by a gradual increase in frequency on 5 March, including multiple events near magnitude 1.0–2.1. Subsequent days through 13 March maintained this pattern, with occasional spikes to magnitude 1.9–2.0 amid numerous sub-magnitude-1.0 tremors. This distribution underscores the swarm's diffuse nature, lacking a dominant mainshock. The Aguanga area lies within the Peninsular Ranges province of Southern California, influenced by the broader San Andreas Fault system and subsidiary structures such as the Elsinore and San Jacinto fault zones. These right-lateral strike-slip faults accommodate Pacific-North American plate motion, generating frequent small earthquakes and occasional swarms driven by fluid movement or localized stress perturbations in the brittle upper crust. Historical records indicate elevated swarm activity here, with 41 documented swarms since 1 January 2000. Yearly occurrences include one in 2001, two in 2002, one in 2003, two in 2005, one in 2009, four in 2010, four in 2011, three in 2012, three in 2013, four in 2014, two in 2015, six in 2016, seven in 2017, and one in 2018. This frequency highlights the region's persistent microseismicity. Such swarms contribute to understanding fault interactions and may signal evolving stress conditions without escalating to larger events. Monitoring continues to refine models of this tectonically dynamic zone.

References

SeismoSight Internal Classification Database USGS Earthquake Catalog (regional context)