Seismic Swarm S20210427.1: Analysis of Activity Near Aguanga, California

The seismic swarm designated S20210427.1 occurred approximately 10 km northeast of Aguanga in Riverside County, California. It began at 13:44 on 26 April 2021 and concluded at 18:50 on 1 May 2021, spanning 125 hours and 6 minutes. During this period, 80 earthquakes were recorded, with magnitudes ranging from -0.4 to 2.2. Depths varied from 2 km to 19 km, showing a progression from predominantly shallow events early in the sequence to deeper occurrences later.

Initial activity on 26 April featured events clustered at depths of 2–5 km, including a magnitude 2.0 earthquake at 17:28:13. Subsequent days saw continued shallow seismicity interspersed with slightly deeper shocks. By 28 April, depths increased notably, with multiple events between 11 km and 19 km, such as a magnitude 0.5 event at 20:37:01 recorded at 19 km. This pattern suggests possible downward migration along fault structures or activation of distinct segments within the local fault network. The largest event, magnitude 2.2 at 10 km depth, occurred on 30 April at 17:48:28.

The swarm 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-trending strike-slip faults associated with the San Andreas system, including segments of the Elsinore and San Jacinto fault zones. The crust here experiences distributed deformation through numerous smaller faults and fractures, facilitating frequent low-magnitude earthquake sequences. Historical records indicate that such swarms are common in this setting due to fluid migration and stress transfer along these structures.

Since 2000, the region has experienced 48 documented swarms. Annual counts show variability, with notable increases in later years: 2002 (1), 2003 (1), 2005 (2), 2009 (1), 2010 (2), 2011 (3), 2012 (1), 2013 (1), 2014 (3), 2015 (2), 2016 (4), 2017 (6), 2018 (7), 2019 (3), 2020 (10), and 2021 (1). This trend aligns with ongoing monitoring that highlights episodic swarm activity in the Aguanga vicinity.

The 2021 swarm remained at low magnitudes overall, consistent with background seismicity levels in the area. No significant surface rupture or damage was associated with these events, reflecting the limited energy release typical of swarm-type sequences rather than mainshock-aftershock patterns.

References

• SeismoSight internal swarm classification database
• USGS Earthquake Catalog (earthquake.usgs.gov)
• Southern California Seismic Network reports on Peninsular Ranges tectonics