A seismic swarm designated S20220905.1 was recorded 4 km north-northeast of Pinnacles, California. The sequence began at 18:38 UTC on 4 September 2022 and concluded at 07:28 UTC on 6 September 2022, spanning 36 hours and 50 minutes. During this interval, 27 earthquakes were detected, with magnitudes ranging from 0.9 to 3.4 and focal depths predominantly between 5 km and 10 km.
The largest event reached magnitude 3.4 at a depth of 9 km shortly after initiation. Subsequent activity included multiple events of magnitude 2.0–2.5 clustered within the first 12 hours, followed by a gradual decline in both frequency and size. Later shocks on 5 and 6 September remained below magnitude 2.5, with the final recorded event at magnitude 1.8. Depths stayed consistent in the upper crust, indicating shallow brittle failure typical of the local fault network.
The Pinnacles region lies within the central California Coast Ranges, adjacent to the San Andreas Fault system. The area is underlain by volcanic and sedimentary rocks of the Pinnacles Formation, remnants of Miocene volcanism associated with the Neenach volcanic center. Right-lateral strike-slip motion along the San Andreas and subsidiary faults, such as the Chalone Creek Fault, accommodates ongoing Pacific–North American plate boundary deformation. This tectonic setting produces frequent microseismicity and occasional swarms driven by fluid migration or stress transfer along fault segments.
Historical records indicate 21 similar swarms in the vicinity since 2000. These episodes occurred in 2000, 2001 (two events), 2003, 2004, 2005, 2006 (two), 2007 (two), 2008, 2009, 2011 (two), 2012, 2014, 2017, 2018 (two), 2019, and 2021. Swarm recurrence suggests episodic strain release rather than isolated mainshock-aftershock sequences.
The September 2022 swarm exhibited classic characteristics of fault-zone swarm activity: a rapid onset, lack of a dominant mainshock, and spatial clustering at shallow depths. No damage or felt reports beyond minor shaking were associated with the largest events. Continued monitoring remains important given the proximity to the San Andreas Fault, where accumulated strain could influence future regional seismicity.
USGS Earthquake Catalog
California Geological Survey Regional Fault Maps
Pinnacles National Park Geologic Resources Inventory