Seismic Swarm S20200122.1 Near Anza, California
An earthquake swarm designated S20200122.1 occurred 13 km west-northwest of Anza, California. The sequence began at 15:09 on 21 January 2020 and concluded at 03:45 on 25 January 2020, spanning 84 hours and 36 minutes. During this period, 57 earthquakes were recorded.
The events clustered at shallow depths, predominantly between 3 km and 16 km. Magnitudes ranged from -0.2 to 2.4, with the largest shock registering 2.4 at 12:15 on 22 January 2020 at a depth of 5 km. Other notable events included multiple shocks of magnitude 1.4–1.5 on 22 and 24 January. Activity showed a typical swarm pattern, with frequent small events interspersed with occasional moderate ones and no single dominant mainshock.
This swarm fits within a broader pattern of seismic activity in the region. Since 1 January 2000, 45 swarms have been documented near Anza. Yearly occurrences include single swarms in 2001, 2002, 2003, 2009, and 2020; two each in 2005, 2012, 2013, and 2015; three each in 2010 and 2019; four each in 2011 and 2014; five in 2016; six in 2017; and seven in 2018.
The Anza area lies in the San Jacinto Fault Zone of southern California, a right-lateral strike-slip system that forms part of the boundary between the Pacific and North American tectonic plates. The zone accommodates approximately 1.5 cm of annual slip and has produced multiple magnitude 6–7 earthquakes historically. Local geology consists of Mesozoic granitic and metamorphic basement rocks overlain by Quaternary alluvial sediments. Fluid migration along fault segments is believed to contribute to swarm generation through pore-pressure changes that trigger slip on small fractures.
Seismic monitoring in the region benefits from dense instrumentation by the Southern California Seismic Network, enabling precise location of events at depths typically less than 20 km. The 2020 swarm remained within expected bounds for background activity along this fault zone.
References
USGS Earthquake Hazards Program
Southern California Seismic Network annual reports
California Geological Survey fault-zone maps