Seismic Swarm S20190328.1 Near Anza, California: March-April 2019 Analysis

The Anza region in Riverside County, Southern California, lies within the San Jacinto Fault Zone, a major component of the broader San Andreas Fault system. This right-lateral strike-slip fault accommodates significant tectonic strain between the Pacific and North American plates. The area's geology features crystalline basement rocks of the Peninsular Ranges, intersected by active fault strands that facilitate frequent microseismicity and swarm activity. Depths in the provided swarm data, predominantly 2–6 km with occasional events to 16 km, align with typical shallow crustal seismicity along this fault segment.

Seismic swarms in this region reflect fluid migration and stress triggering rather than a single mainshock-aftershock sequence. Swarm S20190328.1 began at 10:04 on 27 March 2019 and concluded at 06:34 on 2 April 2019, spanning 140 hours and 29 minutes. During this interval, 79 earthquakes were recorded 7 km northwest of Anza. Magnitudes ranged from –0.3 to 2.5, with the largest event occurring at 14:30 on 27 March at 2 km depth. Most events clustered between 3–5 km depth, consistent with the fault zone's shallow seismogenic layer.

Temporal distribution showed peak activity on 27–28 March, followed by a gradual decline. Early events included magnitudes of 0.6–1.3 within the first hours, while later stages featured smaller events around 0.3–0.9. The sequence exhibited typical swarm characteristics: no dominant mainshock, clustered occurrence, and migration along the fault plane.

Historical records indicate 40 swarms in the Anza area since 1 January 2000. Annual counts increased notably after 2013, with 4 swarms in 2014, 5 in 2016, 6 in 2017, and 7 in 2018. This trend suggests evolving stress conditions or improved detection capabilities along the San Jacinto Fault.

Such swarms contribute to long-term strain release without producing damaging ground motion. Continued monitoring remains essential given the fault's proximity to populated areas and its history of moderate earthquakes.

References

United States Geological Survey Earthquake Catalog
Southern California Seismic Network records
California Geological Survey fault zone maps