Seismic swarm S20180124.7 occurred 14 km east-southeast of Pinnacles, California, between 18:41 UTC on 23 January 2018 and 17:03 UTC on 25 January 2018. Over this 46-hour period, 29 earthquakes were recorded, with magnitudes ranging from 0.5 to 3.4 and focal depths between 5 and 8 km. The largest event reached magnitude 3.4 at 21:00 UTC on 23 January.
This swarm exemplifies typical low-magnitude seismic sequences common in central California. Events clustered tightly in time and space, beginning with a magnitude-1.4 quake, followed by a rapid increase in activity that included the peak magnitude event within three hours. Subsequent events showed a gradual decline, with the final recorded shock at magnitude 2.3 on 25 January. Depths remained consistent in the mid-crust, suggesting activation along a shallow fault segment.
The Pinnacles region lies within the California Coast Ranges, where the transform boundary between the Pacific and North American plates drives ongoing deformation. The nearby San Andreas Fault system accommodates most of the relative plate motion, while subsidiary faults such as the Calaveras and San Andreas itself contribute to distributed seismicity. Pinnacles National Park preserves remnants of a Miocene volcanic complex that was displaced northwestward along the San Andreas Fault over millions of years, providing a geological marker of long-term strike-slip displacement.
Historical records indicate that earthquake swarms have occurred repeatedly in this area. Since 2000, sixteen swarms have been documented, with notable years including 2006 and 2007 (two each), 2011 and 2012 (two each), and single swarms in 2000, 2001, 2003, 2004, 2005, 2008, 2014, and 2017. These sequences typically lack a dominant mainshock-aftershock pattern and are often linked to aseismic slip or fluid migration along fault zones in the region.
The 2018 swarm fits within this established pattern of episodic, clustered activity. Such events contribute to the cumulative strain release along the broader fault network without producing damaging ground motion, given their modest magnitudes and moderate depths. Continued monitoring by regional seismic networks remains essential for characterizing fault behavior in this tectonically active setting.
USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey regional fault maps
Pinnacles National Park geologic resources inventory (NPS)