Seismic Swarm S20170609.1 Near Anza, California

The seismic swarm designated S20170609.1 occurred approximately 10 km NNW of Anza in Riverside County, California. It initiated at 13:21 on 8 June 2017 and concluded at 12:13 on 12 June 2017, spanning 94 hours and 51 minutes. During this interval, 61 earthquakes were recorded, with the majority exhibiting low magnitudes and focal depths clustered around 13–17 km.

Event parameters reveal a typical swarm pattern of clustered, low-energy seismicity without a dominant mainshock. The largest event reached magnitude 2.7 at 09:24 on 9 June at 13 km depth. Other notable events included magnitudes of 2.0 on 9 June and 1.8 earlier that day, both near 14 km depth. Depths remained consistent until later phases, when isolated shallower events appeared at 3 km and 12 km. Magnitudes stayed predominantly below 1.5 after the initial peak, indicating rapid energy dissipation through numerous small ruptures along fault segments.

Geologically, the Anza region occupies the central San Jacinto Fault Zone within the Peninsular Ranges province of Southern California. This zone forms part of the active Pacific–North American plate boundary, characterized by right-lateral strike-slip faulting. The local geology comprises Mesozoic granitic and metamorphic rocks of the Peninsular Ranges batholith, overlain by thin Quaternary alluvial deposits. The San Jacinto Fault accommodates a significant portion of the regional slip rate, estimated at 10–15 mm per year, contributing to elevated seismicity.

Historical records document recurrent swarm activity in this area. Since 1 January 2000, 16 swarms have occurred, distributed across specific years: 2002 (1), 2003 (1), 2005 (1), 2009 (1), 2011 (3), 2014 (3), 2015 (1), 2016 (3), and 2017 (2). These episodes reflect episodic fluid migration or aseismic slip triggering on the complex fault network.

Such swarms provide insight into fault mechanics, highlighting distributed microseismicity that releases strain without producing larger events. Depths near 14 km align with the seismogenic zone base in this region, where temperature and pressure conditions favor brittle failure.

References

• USGS Earthquake Hazards Program catalog data
• California Geological Survey regional fault maps
• SeismoSight internal swarm classification records