Seismic Swarm S20200926.2 Near Anza, California

The region 7 km north of Anza, California, lies within the San Jacinto Fault Zone in Riverside County, part of the broader Peninsular Ranges province. This area experiences frequent seismic activity due to right-lateral strike-slip faulting along the San Jacinto Fault, which forms a key segment of the Pacific-North American plate boundary. The fault zone accommodates approximately 1–2 cm of annual slip and has produced multiple historical earthquakes exceeding magnitude 6.0, including the 1918 San Jacinto event. The Anza seismic gap, characterized by a relative lack of large ruptures in recent centuries, remains a focus of monitoring because of its potential for significant future events.

Earthquake swarms are common in this tectonically complex setting, where fluid migration and stress interactions along fault segments trigger clusters of events without a clear mainshock-aftershock sequence. Since 2000, 53 swarms have been documented in the immediate vicinity, with notable increases in frequency during 2016–2018 and 2020. Prior annual counts include single swarms in 2001–2003 and 2005, 2009; three in 2010; four in 2011; and rising numbers through the late 2010s, reflecting ongoing strain accumulation.

Swarm S20200926.2 began at 22:17 UTC on 25 September 2020 and concluded at 09:15 UTC on 29 September 2020, spanning 82 hours and 58 minutes. During this period, 42 earthquakes were recorded, with magnitudes ranging from negative values to 1.9 and focal depths primarily between 5 km and 18 km. The sequence initiated with a magnitude 0.8 event at 18 km depth, followed by numerous smaller events clustered around 10–15 km depth on subsequent days. Peak activity included a magnitude 1.9 shock on 27 September at 7 km depth and several magnitude 1.5–1.7 events on 28–29 September. Depths generally shallowed slightly during the swarm, consistent with fluid-driven triggering mechanisms observed in similar Southern California sequences.

This swarm aligns with the established pattern of low-magnitude, high-frequency activity in the Anza gap, providing data for refining fault models and stress-transfer analyses. No damage or felt reports were associated with these events, underscoring their minor character relative to the region’s seismic potential.

References

USGS Earthquake Catalog
Southern California Seismic Network reports
California Geological Survey fault-zone maps