Seismic Swarm S20100314.1: Analysis of Earthquake Activity Near Pine Valley, California

Swarm S20100314.1 occurred in a region 25 km northeast of Pine Valley, California, within the Peninsular Ranges province. This area lies along the tectonically active boundary between the Pacific and North American plates, where right-lateral strike-slip faulting dominates. The Elsinore Fault Zone and associated structures, including the Agua Caliente Fault, accommodate regional strain and have produced historical seismicity in San Diego County.

The swarm began at 13:29 on 13 March 2010 and concluded at 18:55 on 15 March 2010, spanning 53 hours and 26 minutes. During this interval, 39 earthquakes were recorded. Event magnitudes ranged from 1.0 to 4.2, with the largest shock occurring at 16:32 on 13 March at a depth of 4 km. Focal depths clustered between 4 km and 9 km, consistent with shallow crustal faulting typical of the Peninsular Ranges.

Sequence timing showed an initial energetic phase on 13 March, featuring the magnitude-4.2 mainshock followed by multiple events above magnitude 1.5 within the first eight hours. Activity then declined gradually, with smaller events persisting through 14 and 15 March. Depths remained stable across the swarm, indicating a consistent seismogenic zone without significant migration.

This swarm represents one of only three documented swarms in the locale since 1 January 2000. Earlier episodes occurred in 2007 (one swarm) and 2009 (two swarms). Such clustered activity reflects episodic strain release on local faults rather than mainshock-aftershock sequences driven by a single large rupture.

Regional geology features Mesozoic granitic and metamorphic rocks of the Peninsular Ranges batholith, overlain in places by younger sedimentary units. Ongoing Pacific-North American plate motion of approximately 35–40 mm per year sustains background seismicity, with swarms occasionally triggered by fluid migration or aseismic slip transients at depth.

References

USGS Earthquake Hazards Program, California Geological Survey Peninsular Ranges reports, Southern California Earthquake Data Center catalog summaries.