An earthquake swarm designated S20110827.1 occurred 21 km northeast of Soledad in Monterey County, California. The sequence began at 10:24 on 26 August 2011 and concluded at 22:13 on 4 September 2011, spanning 227 hours and 49 minutes. During this period, 162 earthquakes were recorded.
The swarm's location places it within the California Coast Ranges, specifically in the Gabilan Range foothills adjacent to the Salinas Valley. This region forms part of the Salinian Block, a geologic terrane bounded by the San Andreas Fault to the east and the Sur-Nacimiento Fault to the west. The area's seismicity stems from right-lateral strike-slip motion along the San Andreas system and associated subsidiary faults, including the Calaveras and San Gregorio faults. Crustal deformation here reflects ongoing Pacific-North American plate interaction at a rate of approximately 35–40 mm per year.
Analysis of the first 100 events reveals a typical swarm pattern dominated by low-magnitude earthquakes. Magnitudes ranged from 0.2 to 4.6, with the majority below 2.0. Depths clustered between 4 and 8 km, consistent with shallow crustal faulting in the region. The largest event, magnitude 4.6, occurred at 07:18 on 27 August at 7 km depth, followed closely by a 3.6 event at 07:21 and a 3.2 at 07:20. Subsequent activity included several events above magnitude 3.0, such as 3.3 on 26 August and 3.1 on 27 August, with aftershocks decaying rapidly in size.
Temporal distribution shows an initial burst of activity on 26–27 August, with events concentrated within hours of the main shocks. Depths remained stable around 5–6 km for most events after the initial deeper 9 km shock. No significant migration of hypocenters was evident in the early sequence, suggesting localized slip on a small fault segment rather than widespread fluid-driven migration.
Historical records indicate this was one of 12 swarms in the area since 2000, with prior episodes in 2000 (1), 2001 (2), 2003 (1), 2004 (1), 2005 (1), 2006 (2), 2007 (2), 2008 (1), and 2009 (1). Such recurrent swarms highlight the region's persistent low-level seismic productivity without progression to a major mainshock.
The 2011 swarm produced no reported damage or injuries, consistent with its modest magnitudes and rural setting. Monitoring by regional seismic networks continues to track background seismicity in this tectonically active corridor.
USGS Earthquake Catalog
California Geological Survey Fault Database
USGS National Seismic Hazard Model (2023 update)