Seismic Swarm S20180203.1: Analysis of Activity Near Aguanga, California

Seismic swarm S20180203.1 occurred 9 km east-northeast of Aguanga in Riverside County, California. The sequence began at 08:25 on 2 February 2018 and concluded at 05:32 on 3 March 2018, spanning 693 hours and 7 minutes. During this period, 426 earthquakes were recorded.

The swarm unfolded in the Peninsular Ranges province of Southern California, where the Pacific and North American plates interact along a network of strike-slip faults. The Elsinore Fault Zone lies nearby and accommodates a portion of the regional strain. This tectonic setting produces both mainshock-aftershock sequences and swarm-like activity characterized by numerous events of similar magnitude without a dominant mainshock.

Examination of the first 100 events reveals predominantly low-magnitude earthquakes. Magnitudes ranged from -0.3 to 2.4, with the majority below 1.0. Depths clustered between 2 km and 12 km, indicating shallow crustal sources typical of the area. Early activity on 2 February featured events near 0.3–0.4 magnitude at depths of 3–6 km. By 4 February, a cluster included the largest event of this subset at magnitude 2.4 at 3 km depth, followed by several events above magnitude 1.0. Subsequent days showed continued low-level seismicity with occasional spikes to magnitude 1.4–1.7, often at depths of 4–6 km. Negative and zero magnitudes appeared regularly, reflecting the high sensitivity of the monitoring network.

Historical records indicate persistent swarm behavior in the region. Since 1 January 2000, 40 swarms have been identified. Annual counts include one swarm each in 2001, 2003, and 2009; two in 2002 and 2005; four each in 2010, 2011, 2014, and 2017; three each in 2012 and 2013; two in 2015; six in 2016; and seven in 2017. This pattern underscores the area's tendency toward episodic, clustered seismicity rather than isolated large events.

Such swarms contribute to ongoing strain release along subsidiary faults and help refine models of fault interaction in the Elsinore system. Continued monitoring supports improved understanding of seismic hazard in this portion of Southern California.

References

• SeismoSight internal swarm classification records
• USGS earthquake catalog for Southern California
• California Geological Survey fault database