Seismic Swarm S20200526.3 Near Anza, California

The seismic swarm designated S20200526.3 occurred approximately 10 km north-northwest of Anza in Riverside County, California. This sequence began at 12:18 UTC on 25 May 2020 and concluded at 07:17 UTC on 2 June 2020, spanning 186 hours and 59 minutes. During this period, 100 earthquakes were recorded, with magnitudes ranging from -0.4 to 1.7 and focal depths primarily between 3 km and 17 km.

The Anza region lies within the San Jacinto Fault Zone, a major right-lateral strike-slip system that forms part of the broader San Andreas Fault transform boundary. This zone accommodates a significant portion of the relative motion between the Pacific and North American plates. The local geology consists of Mesozoic granitic rocks of the Peninsular Ranges Batholith overlain by Quaternary alluvial deposits. The fault zone here exhibits a complex geometry with multiple strands that facilitate swarm activity through fluid migration and aseismic slip processes.

The swarm initiated with low-magnitude events clustered in the afternoon of 25 May, including several below magnitude 0.5 at depths of 4–6 km. Activity persisted through the following days with intermittent peaks, such as a magnitude 1.3 event on 29 May at 5 km depth and two magnitude 1.7 events on 31 May. Depths remained shallow to moderate throughout, consistent with brittle failure in the upper crust. No single dominant mainshock occurred; instead, the sequence displayed the characteristic diffuse energy release of a swarm.

Historical data indicate that swarm activity is recurrent in this area. Since 2000, 52 swarms have been documented near Anza, with notable increases in frequency during 2016–2018. Earlier swarms occurred in years including 2001, 2005, 2010, 2012, 2014, and 2017. These events reflect ongoing tectonic strain accumulation along the San Jacinto Fault, where small-magnitude sequences often precede or accompany larger regional earthquakes.

Analysis of the 2020 swarm reveals a typical pattern of microseismicity, with the majority of events below magnitude 1.0. The temporal distribution shows clustering in the first 48 hours followed by a gradual decline, suggesting a triggered cascade driven by pore-pressure changes rather than static stress transfer from a primary rupture. Depths concentrated around 5–7 km align with the seismogenic zone in this segment of the fault.

Such swarms contribute to the broader understanding of fault mechanics in Southern California. Continuous monitoring by regional networks helps track these sequences, providing insights into potential precursors or stress evolution along active structures.

References
SeismoSight internal classification data for swarm S20200526.3
United States Geological Survey regional fault maps and historical seismicity records for the San Jacinto Fault Zone