Seismic Swarm S20160610.1 Near Anza, California: Geological Context and Initial Event Analysis

The region 23 km east-southeast of Anza, California, lies within the tectonically active San Jacinto Fault Zone of southern California. This area experiences frequent seismicity due to right-lateral strike-slip motion along the San Jacinto fault system, a major branch of the broader San Andreas transform boundary. The local geology features a network of northwest-trending faults cutting through Mesozoic granitic basement rocks and overlying sedimentary units, with historical slip rates on the order of several millimeters per year contributing to elevated earthquake hazard.

Seismic swarms have long characterized this portion of the fault zone. Records since 2000 document 30 such swarms, with annual counts varying from one to five events per year in the listed intervals. These swarms typically involve clustered microseismicity without a single dominant mainshock, reflecting fluid migration or aseismic slip processes along fault segments.

Swarm S20160610.1 began at 15:31 UTC on 9 June 2016 and concluded at 19:53 UTC on 12 July 2016. Over 796 hours and 22 minutes, 1773 earthquakes were recorded. The sequence initiated with low-magnitude events near 14 km depth. Within the first day, activity intensified, culminating in a magnitude 5.1 earthquake at approximately 08:04 UTC on 10 June at 12 km depth. This event was followed within minutes by a magnitude 3.7 shock and additional aftershocks ranging from 2.0 to 3.2, all concentrated between 10 and 13 km depth.

Analysis of the first 100 events reveals a clear temporal and spatial pattern. Early events remained small (magnitudes 0.0–1.2) and deep (11–14 km). Following the magnitude 5.1 mainshock, magnitudes increased briefly before declining, with most subsequent events falling between 0.6 and 2.3. Depths stayed consistently shallow to mid-crustal (9–14 km), indicating rupture within the brittle seismogenic zone of the local fault network. The sequence exhibited classic swarm behavior, with no single event dominating energy release and activity decaying gradually over subsequent hours.

This swarm aligns with the region’s established pattern of episodic clustered seismicity. Continued monitoring of such sequences supports improved understanding of fault mechanics and hazard assessment along the San Jacinto system.

References

• USGS Earthquake Catalog (earthquake.usgs.gov)
• Southern California Seismic Network reports
• California Geological Survey fault zone maps