Seismic Swarm S20130429.1 Near Calipatria, California
An earthquake swarm designated S20130429.1 occurred in the Imperial Valley of southern California, centered 11 km west-northwest of Calipatria. The sequence began at 21:35 UTC on 28 April 2013 and concluded at 00:29 UTC on 1 May 2013, spanning 50 hours and 53 minutes. During this interval, 41 earthquakes were recorded, with magnitudes ranging from 0.2 to 3.0 and focal depths primarily between 0 and 11 km.
The swarm exhibited typical characteristics of clustered seismic activity in the region, featuring a rapid onset of events followed by a gradual decline. The largest event reached magnitude 3.0 at 17:32 UTC on 29 April at a depth of 1 km. Several other notable shocks included magnitudes of 2.4, 2.1, and multiple events between 1.5 and 1.8, concentrated within the first 24 hours. Shallower events dominated the early phase, while a few deeper quakes up to 11 km occurred later in the sequence.
This activity aligns with the tectonic setting of the Imperial Valley, situated within the Brawley Seismic Zone. The area lies at the transition between the San Andreas Fault system and the Imperial Fault, where right-lateral strike-slip motion accommodates plate boundary deformation. Geothermal fields and volcanic features associated with the Salton Trough further contribute to swarm-type seismicity through fluid migration and crustal heating.
Historical records indicate persistent swarm behavior in the vicinity. Since 1 January 2000, 57 swarms have been documented in the region. Annual counts show variability, with elevated activity in 2009 (11 swarms), 2010 (13 swarms), and 2012 (11 swarms). Earlier years recorded fewer events, such as single swarms in 2000, 2001, 2002, 2004, and 2008. The 2013 total reached two swarms by the time of S20130429.1.
Such sequences underscore the Imperial Valley's elevated seismic hazard. Ongoing monitoring supports improved understanding of fault interactions and potential precursors to larger events along major regional structures.
References
SeismoSight internal swarm classification database.
USGS Earthquake Catalog for regional tectonic context.