Analysis of Seismic Swarm S20200202.1 Near Anza, California

The seismic swarm designated S20200202.1 occurred approximately 13 km west-northwest of Anza in Riverside County, California. It initiated at 02:15 on 1 February 2020 and concluded at 19:55 on 9 February 2020, spanning 209 hours and 40 minutes. During this period, 147 earthquakes were recorded. This event aligns with the region's established pattern of swarm activity, where clusters of low-to-moderate magnitude events occur without a dominant mainshock.

Anza lies within the San Jacinto Fault Zone, a major component of the southern San Andreas Fault system. The area experiences frequent seismic activity due to right-lateral strike-slip faulting along northwest-trending faults. Historical records indicate that the region has hosted multiple swarms since 2000, totaling 47 such episodes through early 2020. Annual counts include one swarm each in 2001–2003 and 2005, with increased frequency in later years: six in 2016, six in 2017, seven in 2018, and three in 2019. The 2020 tally reached two swarms, underscoring ongoing tectonic strain accumulation.

Examination of the first 100 events reveals predominantly shallow focal depths, with most occurring between 4 km and 6 km. Magnitudes ranged from negative values to a peak of 3.6, recorded at 00:59 on 2 February 2020 at 5 km depth. Early activity on 1 February featured events below magnitude 1.0, followed by a rapid escalation on 2 February that included multiple events near magnitude 2.0. Subsequent days showed a gradual decline in both frequency and intensity, with isolated events reaching magnitude 2.3 on 3 February. Depths remained consistent, though a few outliers extended to 15–16 km, suggesting limited vertical migration of activity.

This swarm's characteristics—high event count, absence of a clear foreshock-mainshock-aftershock sequence, and clustering in time and space—are typical of fluid-driven processes in the Anza region. Such swarms often reflect pore-pressure changes along fault segments rather than large-scale stress release. The location near the San Jacinto Fault places it in a zone of elevated seismic hazard, where cumulative swarm activity contributes to long-term fault loading.

Geological monitoring in this area benefits from dense seismic networks that capture microseismicity, aiding in the refinement of fault models and hazard assessments. Continued observation of swarm patterns supports improved forecasting of potential larger events in the southern California fault network.

References

SeismoSight internal swarm classification records. USGS Earthquake Catalog for regional fault data.