Earthquake Swarm S20200607.1 Near Anza, California

An earthquake swarm designated S20200607.1 was recorded 13 km west-northwest of Anza in Riverside County, California. The sequence began at 17:01 on 6 June 2020 and concluded at 12:54 on 16 June 2020, spanning 235 hours and 52 minutes. A total of 109 earthquakes were registered during this period.

The Anza region lies within the San Jacinto Fault Zone, one of Southern California’s most active strike-slip fault systems. This zone forms part of the broader Pacific–North American plate boundary and accommodates a significant portion of the relative motion between the two plates. The fault zone comprises multiple strands that have produced both large historical earthquakes and frequent seismic swarms. Depths of events in the swarm ranged from 2 km to 18 km, consistent with the seismogenic zone along the San Jacinto Fault, where brittle failure occurs in the upper crust.

Analysis of the first 100 events reveals predominantly microseismic activity. Magnitudes clustered between –0.2 and 1.7, with the majority falling below 0.8. The largest event reached magnitude 1.7 on 11 June 2020 at a depth of 5 km. Most events occurred at depths of 5–7 km, although a subset extended to 14–18 km, indicating activity across a vertical range typical of the fault zone. Temporal distribution showed peak rates on 7 June and 12 June, with events occurring in tight spatial clusters that migrated slightly over the ten-day span.

Seismic swarms are a recurring feature in this area. Since 1 January 2000, 49 swarms have been identified in the Anza vicinity. Annual counts increased notably after 2010, reaching six swarms in 2020 alone. These episodes typically involve hundreds of small events without a distinct mainshock–aftershock pattern, reflecting fluid migration or aseismic slip along fault segments.

The geological setting features Quaternary alluvium overlying Mesozoic crystalline basement rocks of the Peninsular Ranges. The San Jacinto Fault Zone cuts through this terrain, creating a complex network of faults that facilitate swarm activity. Updated seismic monitoring by regional networks confirms that swarm recurrence remains elevated compared with earlier decades, consistent with long-term strain accumulation along the fault.

This swarm produced no reported damage or felt shaking beyond instrumental detection. Continued monitoring is warranted given the fault zone’s history of generating moderate to large earthquakes.

References

USGS Earthquake Catalog
California Geological Survey Fault Database
Southern California Seismic Network reports