Seismic Swarm S20201219.1: Analysis of Activity Near Anza, California
Seismic swarm S20201219.1 was recorded 6 km west-southwest of Anza, California, beginning at 09:03 on 18 December 2020 and concluding at 09:57 on 23 December 2020. Over this 120-hour, 54-minute period, 66 earthquakes were detected. The Anza region lies within the San Jacinto Fault Zone, a major component of the broader San Andreas transform boundary system characterized by right-lateral strike-slip motion. This zone accommodates significant strain release through frequent small-to-moderate events and occasional larger ruptures, reflecting the ongoing tectonic interaction between the Pacific and North American plates.
The swarm exhibited typical characteristics of clustered seismicity, with events predominantly of low magnitude. Depths ranged from 2 km to 17 km, indicating activity across shallow crustal levels. The largest event reached magnitude 2.1 at 23:03 on 18 December at 2 km depth, while the majority remained below magnitude 1.0. Temporal distribution showed peak activity on 18–19 December, followed by a gradual decline, consistent with swarm behavior driven by fluid migration or aseismic slip rather than a single mainshock-aftershock sequence.
Historical records since 1 January 2000 document 34 swarms in the same area. Yearly occurrences include one each in 2003 and 2009; two in 2011; three in 2014; one in 2015; two in 2016; six in 2017; seven in 2018; two in 2019; and nine in 2020. This pattern underscores persistent seismic unrest along the fault zone, where swarm episodes recur amid background seismicity rates influenced by the fault's high slip rate and complex geometry.
Such swarms contribute to long-term strain accommodation without producing surface rupture. Monitoring data from regional networks reveal that events in this locale often occur at depths of 2–16 km, aligning with the brittle-ductile transition in the local crust. Continued observation supports improved understanding of fault mechanics and hazard assessment in this tectonically active corridor.
References
SeismoSight internal swarm classification S20201219.1 dataset (2020).
United States Geological Survey Earthquake Catalog (historical swarm statistics, 2000–2020).