Seismic Swarm S20200404.1: Analysis of Activity Near Anza, California

The seismic swarm designated S20200404.1 occurred 17 km east-southeast of Anza in Riverside County, California. It began at 01:53 UTC on 4 April 2020 and concluded at 05:26 UTC on 14 May 2020, spanning 963 hours and 33 minutes. During this period, 2,928 earthquakes were recorded, marking a significant episode of clustered seismicity in the region.

The swarm initiated with a magnitude 4.8 event at a depth of 10 km. Subsequent activity included a magnitude 3.7 shock one minute later, followed by numerous smaller events. Examination of the first 100 earthquakes reveals a rapid onset of activity dominated by events between magnitude 0.4 and 3.7, with the majority falling below magnitude 2.0. Depths clustered tightly between 8 km and 12 km, consistent with shallow crustal faulting. Notable events in this initial sequence included a magnitude 3.6 at 02:05 and several magnitude 2.4–2.5 shocks distributed over the first hours.

This swarm unfolded within the San Jacinto Fault Zone, part of the broader San Andreas transform system that accommodates right-lateral strike-slip motion between the Pacific and North American plates. The Peninsular Ranges province, underlain by Mesozoic granitic and metamorphic rocks, experiences ongoing deformation along multiple northwest-trending faults. The Anza region sits at a structural transition where the San Jacinto Fault interacts with adjacent strands, producing both mainshock-aftershock sequences and swarm-like behavior driven by fluid migration or aseismic slip.

Historical records document persistent swarm activity in this locale. Since 1 January 2000, 47 swarms have been identified in the immediate area. Yearly counts show variability, with peaks of seven swarms in 2018 and five each in 2010 and 2017. Earlier decades recorded lower frequencies, such as single swarms in 2001 and 2003. These episodes reflect the fault zone’s capacity for distributed, low-magnitude release rather than isolated large ruptures.

The 2020 swarm’s characteristics align with prior activity, featuring an energetic start that decayed into sustained low-level events. Depths remained stable near 10 km, suggesting a consistent seismogenic layer. Such patterns underscore the importance of continuous monitoring in this portion of Southern California, where cumulative strain release helps inform regional seismic hazard assessments.

Overall, Swarm S20200404.1 exemplifies the dynamic seismicity of the Anza segment and contributes to understanding fault interactions within the San Jacinto system.