Seismic Swarm S20170727.1: Analysis of Activity Near Aguanga, California

The seismic swarm designated S20170727.1 occurred 9 km northeast of Aguanga in Riverside County, California. It began at 06:38 on 26 July 2017 and concluded at 15:53 on 9 August 2017, spanning 345 hours and 15 minutes. During this interval, 192 earthquakes were recorded. Analysis of the first 100 events reveals predominantly low-magnitude activity concentrated at shallow depths. Initial events included a magnitude 2.8 quake at 3 km depth, followed rapidly by smaller tremors mostly between 4 km and 5 km. Magnitudes ranged from 0.0 to 2.8, with the majority below 1.0. Depths generally remained under 6 km, though occasional events reached 13–17 km, indicating limited vertical migration. Temporal distribution showed clustering in the first 48 hours, with peak activity on 26–27 July, followed by a gradual decline. This pattern is consistent with fluid-driven swarm behavior rather than a mainshock-aftershock sequence. The Aguanga area lies within the Peninsular Ranges province of Southern California, part of the broader Pacific–North American plate boundary. Regional tectonics are dominated by northwest-striking strike-slip faults, including segments of the Elsinore and San Jacinto fault zones. These structures accommodate a portion of the relative plate motion and have produced historical moderate earthquakes. The local geology features Mesozoic granitic and metamorphic basement rocks overlain by thin Quaternary sediments. Seismicity in this zone frequently manifests as swarms, reflecting complex fault interactions and possible hydrothermal influences. Historical records indicate 31 swarms in the region since 1 January 2000. Annual counts include single swarms in 2001, 2002, 2003, and 2009; two each in 2005, 2012, 2013, and 2015; three in 2010 and 2017; four in 2011 and 2014; and five in 2016. This recurrence underscores the persistent seismic character of the area.

References

SeismoSight internal swarm classification S20170727.1 USGS Earthquake Catalog (regional context)