Seismic Swarm S20050201.1 Near Holtville, California
SeismoSight recorded swarm S20050201.1 beginning at 07:49 on 31 January 2005 and concluding at 13:25 on 1 February 2005. The sequence occurred 9 km SSW of Holtville in Imperial County, California, and comprised 32 earthquakes over 29 hours and 36 minutes.
All events shared shallow focal depths, predominantly 10 km, with one event at 6 km. Magnitudes ranged from 1.1 to 2.7. The largest shock reached magnitude 2.7 at 14:05 on 31 January. Additional notable events included magnitude 2.6 shocks at 15:14 on 31 January and 02:04 on 1 February. Smaller events filled the sequence, with many falling between magnitude 1.3 and 2.2.
This swarm reflects typical clustered seismicity in the Imperial Valley. The region lies within the southernmost San Andreas Fault system, where the Pacific and North American plates interact along a network of strike-slip faults. The Imperial Fault and the Brawley Seismic Zone accommodate right-lateral shear and produce frequent earthquake swarms without a single dominant mainshock.
Imperial Valley seismicity has been documented since the early twentieth century. The area’s sedimentary basin and active faulting create conditions favorable for swarm activity driven by fluid migration and stress transfer along fault segments. Historical records show that swarms often occur near the southern end of the San Andreas system and the northern Gulf of California extensional zone.
Since 2000, three swarms have been identified in the immediate area. One occurred in 2000 and two in 2003. Swarm S20050201.1 fits this pattern of episodic, low-magnitude clustering that releases strain without producing damaging ground motion.
No damage or injuries were reported from the 2005 sequence. The events remained below the threshold typically felt by residents, though nearby instrumentation captured the full temporal evolution of the swarm.
References
SeismoSight internal classification records for swarm S20050201.1.
United States Geological Survey Earthquake Hazards Program, Imperial Valley regional tectonics summaries.
California Geological Survey, Imperial County fault and seismicity maps.