Seismic Swarm S20200301.1 Near Aguanga, California

A seismic swarm designated S20200301.1 was recorded 10 km northeast of Aguanga in Riverside County, California. The sequence began at 12:49 on 29 February 2020 and concluded at 05:23 on 7 March 2020, spanning 160 hours and 34 minutes. During this period, 67 earthquakes were registered.

Event parameters show a predominance of microseismic activity. Magnitudes ranged from -0.3 to 1.6, with the majority below 1.0. Focal depths varied between 1 km and 19 km, indicating activity across shallow to mid-crustal levels. Notable events included a magnitude 1.6 earthquake at 13 km depth on 1 March and another magnitude 1.6 event at 16 km depth on 7 March. The swarm exhibited clustered occurrence, with multiple events on 1 March and 5 March, consistent with episodic stress release along local fault structures.

The Aguanga area lies within the northern Peninsular Ranges geologic province. This region is underlain by Mesozoic granitic plutons and associated metamorphic rocks formed during subduction along the western North American margin. Modern seismicity is driven by the Pacific–North America plate boundary, where right-lateral transform motion is distributed across the San Andreas, San Jacinto, and Elsinore fault systems. The Elsinore Fault Zone, located immediately west of Aguanga, accommodates a portion of this slip and hosts both mainshock–aftershock sequences and swarm-type activity. Historical records indicate that fluid migration and aseismic creep can trigger swarm behavior in this setting by altering pore pressures on favorably oriented faults.

Seismic swarms have been a recurring feature near Aguanga. Since 1 January 2000, 27 swarms have been identified in the immediate vicinity. Yearly counts include single swarms in 2003, 2009, and 2011; three in 2014; one in 2015; two in 2016; six in 2017; seven in 2018; two in 2019; and three in 2020. These episodes typically involve low-magnitude events at depths of 2–15 km and rarely produce felt shaking or surface rupture.

Analysis of S20200301.1 reveals a compact spatiotemporal footprint and a b-value consistent with swarm populations elsewhere in the Elsinore Fault Zone. Depths clustered between 4 km and 14 km during the first three days before shifting deeper toward the sequence’s end. The absence of a dominant mainshock and the gradual decay of activity align with fluid-driven or aseismic-slip models proposed for similar swarms in the Peninsular Ranges.

Continued monitoring by regional networks remains essential for refining fault models and assessing whether future swarms may precede larger events on the Elsinore or nearby structures.

References

USGS Earthquake Catalog (ANSS Comprehensive Catalog)
Southern California Seismic Network (SCSN) annual reports
California Geological Survey, Peninsular Ranges geologic maps
USGS Quaternary Fault and Fold Database of the United States