Analysis of the October-November 2008 Earthquake Swarm near Ocotillo Wells, California
An earthquake swarm designated S20081030.1 occurred in southern California, beginning at 01:20 on 30 October 2008 and concluding at 15:44 on 1 November 2008. The events were centered 22 km east-southeast of Ocotillo Wells in San Diego County. In total, 105 earthquakes were recorded over 62 hours and 23 minutes. This swarm took place within the tectonically active Imperial Valley region, part of the broader transform boundary between the Pacific and North American plates.
The area lies near the Brawley Seismic Zone, a region characterized by frequent swarm activity due to its position along the San Andreas Fault system and associated secondary faults such as the Coyote Creek and Superstition Hills faults. Shallow crustal deformation and fluid migration along these structures commonly trigger clustered seismicity rather than isolated mainshock-aftershock sequences. Depths of recorded events ranged primarily from 0 to 17 km, consistent with brittle failure in the upper crust of this extensional and strike-slip regime.
Examination of the first 100 events reveals a predominance of low-magnitude earthquakes. Magnitudes spanned 0.6 to 3.6, with the largest event reaching 3.6 at 03:49:45 on 30 October. Multiple events of magnitude 2.9 and above occurred within the initial hours, including a 3.2-magnitude quake at 03:27:38. Depths clustered between 1 and 3 km for most shocks, though occasional deeper events extended to 17 km. Temporal distribution showed peak activity in the first six hours, followed by a gradual decline, with events continuing into early November at reduced rates. This pattern aligns with typical swarm behavior driven by aseismic slip or pore-pressure changes rather than a single large rupture.
Seismic swarms have been recurrent in this locale. Since 2000, ten such episodes have been documented, occurring in 2000 (one swarm), 2002 (one), 2003 (two), 2005 (one), 2006 (two), and 2008 (three). These clusters underscore the ongoing strain accommodation along the plate boundary, where the region absorbs a portion of the relative plate motion through distributed faulting.
The 2008 swarm contributed to ongoing monitoring efforts that help refine hazard assessments for nearby populated areas and infrastructure. No damage was reported from these events, which remained below thresholds typically associated with structural impacts. Continued surveillance of swarm patterns aids in distinguishing background seismicity from potential precursors to larger earthquakes in this high-hazard corridor.
References
USGS Earthquake Catalog California Geological Survey regional fault maps