The earthquake swarm designated S20121021.1 occurred 24 km northeast of San Lucas in Monterey County, California. It began at 21:03 on 20 October 2012 and concluded at 08:50 on 25 October 2012, spanning 107 hours and 46 minutes. During this period, 140 earthquakes were recorded. This event represents the third swarm in the region since 2000, following single swarms in 2004 and 2007.
The swarm initiated with a magnitude 1.8 event at 5 km depth. Within hours, activity escalated, featuring a magnitude 5.2 earthquake at 8 km depth on 21 October at 06:55. Subsequent events included multiple shocks between magnitudes 1.0 and 3.5, with notable peaks at 3.5 (9 km depth) and 3.0 (10 km depth). Depths for the first 100 events predominantly ranged from 6 to 10 km, averaging around 7–8 km, consistent with shallow crustal seismicity in the area.
Analysis of the initial 100 events reveals a typical swarm pattern: an energetic onset followed by a prolonged sequence of smaller aftershocks without a single dominant mainshock-aftershock decay. Magnitudes clustered below 2.0 after the initial larger events, with 78% falling between 1.0 and 2.0. Temporal distribution showed peak frequency in the first 24 hours, tapering gradually over subsequent days.
The San Lucas region lies within California’s Coast Ranges, part of the active transform boundary between the Pacific and North American plates. This setting is dominated by the northwest-trending San Andreas Fault system, which accommodates right-lateral strike-slip motion at rates of approximately 35–40 mm per year. Local geology includes Mesozoic Franciscan Complex rocks and Cenozoic sedimentary basins, contributing to the area’s moderate to high seismic hazard.
Seismic swarms in this portion of central California often relate to fluid migration or aseismic slip along subsidiary faults. Historical records indicate recurrent low-to-moderate activity, with the 2004 and 2007 swarms underscoring episodic clustering rather than isolated mainshock sequences.
References:
USGS Earthquake Catalog
California Geological Survey Regional Fault Maps
SeismoSight internal swarm classification S20121021.1