Seismic Swarm S20200603.1 Near Anza, California

Seismic swarm S20200603.1 occurred 19 km east-southeast of Anza, California, within the tectonically active San Jacinto Fault Zone. This region lies at the boundary between the Pacific and North American plates, where right-lateral strike-slip motion drives frequent small-magnitude earthquakes. The swarm began at 13:33 UTC on 2 June 2020 and concluded at 01:39 UTC on 5 June 2020, lasting 60 hours and 5 minutes. During this interval, 63 earthquakes were recorded.

Event magnitudes ranged from 0.0 to 2.0, with the majority clustered between 0.3 and 1.0. Depths predominantly fell between 8 km and 10 km, though values extended from 2 km to 14 km. Notable events included a magnitude 1.8 quake at 09:21 UTC on 3 June, a magnitude 2.0 event at 17:29 UTC on 3 June, and several magnitude 1.6 shocks later that day. Activity showed a gradual decline after the initial 24 hours, with fewer events recorded on 4 and 5 June.

Swarm sequences such as this lack a dominant mainshock followed by aftershocks, instead featuring distributed energy release over hours to days. In the Anza area, such patterns reflect fluid migration or aseismic slip along fault segments. The 63 events align with typical swarm characteristics observed in this portion of the fault zone.

Historical records indicate 37 swarms in the same locale since 1 January 2000. Yearly counts include one swarm each in 2001, 2003, 2009, 2014, 2017, and 2019; two each in 2002, 2005, and 2015; three each in 2011 and 2012; four each in 2013 and 2016; and five each in 2010 and 2018. The 2020 swarm represents the first recorded that year, continuing a pattern of episodic clustered seismicity.

This activity underscores the persistent strain accumulation along the Anza Gap segment of the San Jacinto Fault, a portion historically associated with moderate events. Ongoing monitoring contributes to refined understanding of fault behavior and regional hazard assessment.

References

SeismoSight internal swarm classification S20200603.1 dataset.
USGS Earthquake Catalog for Southern California fault zone descriptions.