Seismic Swarm S20170907.1 Near Anza, California

The Anza region in Riverside County, Southern California, lies within the San Jacinto Fault Zone, a major right-lateral strike-slip system that forms part of the broader Pacific-North American plate boundary. This zone accommodates roughly 20 percent of the relative plate motion and has produced multiple magnitude 6+ earthquakes historically. The local geology features a network of northwest-trending faults cutting through Mesozoic crystalline basement rocks and overlying sedimentary units, creating conditions favorable for both mainshock-aftershock sequences and earthquake swarms.

Seismic swarms in the Anza area arise from fluid migration and aseismic slip along fault segments, often occurring at shallow to mid-crustal depths. Since 2000, 33 swarms have been documented in the immediate vicinity, with annual counts varying from one to five events per year and a notable cluster of activity between 2010 and 2017.

Swarm S20170907.1 began at 05:37 UTC on 6 September 2017, 5 km west-southwest of Anza, and continued until 03:34 UTC on 21 September 2017. Over 357 hours and 57 minutes, 245 earthquakes were recorded. Analysis of the first 100 events shows predominantly low-magnitude activity, with values ranging from 0.0 to 2.6. Depths clustered between 2 km and 6 km, although a few events reached 15 km. The largest event in this subset measured magnitude 2.6 at 10:15 UTC on 7 September at 2 km depth. Most events occurred within the first three days, with a secondary pulse of activity around 7–8 September.

This pattern is consistent with swarm behavior observed along the San Jacinto Fault, where rapid sequences of small events reflect localized stress perturbations rather than a single large rupture. Depths indicate slip within the seismogenic zone above the brittle-ductile transition, typical for this segment of the fault.

References

USGS Earthquake Hazards Program – San Jacinto Fault Zone overview
Southern California Seismic Network catalog summaries
California Geological Survey – Regional fault and seismicity reports