Seismic Swarm S20170503.1: Analysis of Activity Near Aguanga, California
The seismic swarm designated S20170503.1 occurred 9 km northeast of Aguanga in Riverside County, California. It initiated at 23:12 on 2 May 2017 and concluded at 04:48 on 8 May 2017, spanning 125 hours and 36 minutes. During this interval, 91 earthquakes were recorded, with magnitudes ranging from 0.0 to 3.3 and focal depths predominantly between 2 km and 6 km.
The largest event reached magnitude 3.3 at 15:44:55 on 3 May 2017 at a depth of 3 km. Subsequent notable shocks included a magnitude 2.6 at 15:47:35 and a magnitude 2.0 at 02:55:00 on 3 May. Depths remained shallow throughout, consistent with activity in the upper crust of the Peninsular Ranges province. The sequence exhibited typical swarm characteristics, featuring a gradual onset, multiple peaks in activity on 3 May, and a steady decline through 8 May without a dominant mainshock-aftershock pattern.
Southern California lies along the transform boundary between the Pacific and North American plates, where right-lateral strike-slip faulting accommodates relative motion. The Aguanga region sits within a structurally complex zone influenced by the Elsinore Fault system and subsidiary structures of the San Jacinto Fault Zone. These faults cut through Mesozoic granitic and metamorphic basement rocks of the Peninsular Ranges batholith, which formed during subduction along the western North American margin. Ongoing transpression and transtension produce frequent small-magnitude seismicity and occasional swarms driven by fluid migration along fault networks or aseismic slip.
Historical records indicate persistent swarm activity in the area. Since 1 January 2000, 30 swarms have been documented, distributed across multiple years: one each in 2001, 2002, 2003, and 2009; two in 2005 and 2015; three in 2010, 2012, and 2013; four in 2011 and 2014; and five in 2016. This pattern underscores the region's recurrent microseismicity, often linked to the same fault segments activated during S20170503.1.
Such swarms provide valuable data for understanding fault mechanics in southern California. Continued monitoring supports improved hazard assessment for nearby communities, where even moderate events can produce felt shaking due to shallow source depths.
References
SeismoSight internal swarm classification database
USGS Earthquake Catalog (queried for regional context)