Earthquake Swarm S20100517.2: Seismic Activity Near Ocotillo, California
An earthquake swarm designated S20100517.2 was recorded in the region 8 km east-southeast of Ocotillo, California. The sequence began at 15:49 on 16 May 2010 and concluded at 10:05 on 20 May 2010, spanning 90 hours and 15 minutes. During this period, 47 earthquakes were registered, with magnitudes ranging from 0.6 to 3.5 and focal depths primarily between 2 km and 13 km.
The swarm initiated with a series of small events on 16 May, including a magnitude 2.1 quake at 16:02. Activity intensified on 17 May, featuring the largest event of magnitude 3.5 at 13:13, accompanied by multiple shocks above magnitude 2.0. Subsequent days showed a gradual decline, with the final recorded event being a magnitude 1.2 quake on 20 May. Depths remained shallow throughout, consistent with crustal faulting in the upper 15 km.
This activity aligns with the broader tectonic setting of southern California, where the Pacific and North American plates interact along a network of strike-slip faults. The Ocotillo area lies within the Salton Trough, an extensional basin influenced by the San Andreas Fault system and subsidiary structures such as the Elsinore and San Jacinto fault zones. These faults accommodate right-lateral shear and produce frequent small-magnitude seismicity.
Historical records indicate that the region has experienced multiple swarms since 2000, with nine documented episodes. Prior swarms occurred in 2006 (three events), 2007 (one), 2008 (one), and 2010 (four), highlighting episodic clustered seismicity rather than isolated mainshock-aftershock sequences.
Such swarms typically arise from fluid migration or aseismic slip along fault segments, releasing strain without a dominant triggering event. Depths in this swarm cluster around 5–9 km, suggesting activity within the seismogenic zone of local faults. No damage or felt reports beyond minor shaking were associated with the largest events.
References
- SeismoSight internal swarm classification data (S20100517.2 parameters and event list)
- USGS Earthquake Catalog for regional fault mapping and historical context