Seismic Swarm S20190426.1: Activity Near Anza, California

Seismic swarm S20190426.1 occurred 9 km west-southwest of Anza, California, from 08:34 on 25 April 2019 to 07:12 on 27 April 2019. Over 46 hours and 37 minutes, the swarm produced 37 earthquakes. Magnitudes ranged from -0.3 to 2.3, with the largest event recorded at magnitude 2.3 and depth of 2 km on 25 April at 20:56. Depths were predominantly shallow, between 2 km and 12 km, consistent with activity in the upper crust.

The sequence began with several micro-earthquakes below magnitude 0.5 on 25 April. Activity intensified midday with events up to magnitude 0.5, followed by the peak magnitude 2.3 shock in the evening. Smaller events continued through 26 April, including a magnitude 1.7 quake at 4 km depth. The swarm concluded with a magnitude 0.9 event on 27 April.

This swarm aligns with patterns observed in the region since 2000, when 47 swarms have been documented. Annual counts show variability, with peaks of seven swarms each in 2017 and 2018. Earlier years recorded fewer episodes, such as one in 2001 and three in 2010.

The Anza area sits within the San Jacinto Fault Zone, a major right-lateral strike-slip system in Southern California. This zone forms part of the boundary between the Pacific and North American plates and accommodates roughly 20 percent of the relative plate motion. The fault zone trends northwest-southeast and exhibits complex segmentation with multiple strands that have produced historical earthquakes exceeding magnitude 6.

Geological studies indicate that the San Jacinto Fault Zone has been active throughout the Quaternary period, with slip rates estimated at 10–15 mm per year on principal strands near Anza. The region’s seismicity reflects both tectonic loading and fluid-related processes that can trigger swarm sequences. Depths recorded in S20190426.1 fall within the typical 2–15 km range for crustal events along this fault.

Swarm activity in this setting often lacks a clear mainshock-aftershock decay and instead shows clustered occurrence over hours to days. The 2019 swarm displayed this behavior, with events distributed across a compact area and no single dominant rupture.

References

USGS Earthquake Catalog
Southern California Earthquake Data Center
California Geological Survey Fault Activity Map