Seismic Swarm S20100620.1: Analysis of Activity Near Delta, Baja California
Seismic swarm S20100620.1 occurred in a tectonically active zone 23 km west of Delta, Baja California, Mexico. The sequence began at 18:47 on 19 June 2010 and concluded at 12:08 on 3 July 2010, spanning 329 hours and 21 minutes. During this period, 161 earthquakes were recorded, with the first 100 events providing key insight into the swarm’s characteristics.
Event magnitudes ranged primarily between 1.3 and 3.3, with the majority falling between 1.9 and 2.7. Depths were predominantly shallow, concentrated between 5 and 14 km, though several events reached 16 km. The sequence exhibited temporal clustering, with elevated rates on 19–20 June and again around 23–25 June. Notable events included a magnitude 3.2 quake at 21:51 on 19 June (depth 14 km) and a magnitude 3.3 event at 08:53 on 20 June (depth 16 km). Later peaks featured two magnitude 3.2 and 2.8 events within two minutes on 23 June.
The swarm took place within the Salton Trough, a pull-apart basin marking the transition between the San Andreas fault system and the Gulf of California rift. This region experiences distributed strike-slip and normal faulting driven by Pacific–North American plate motion. The nearby Cerro Prieto geothermal field and associated fault zone contribute to elevated background seismicity through fluid migration and heat flow.
Since 1 January 2000, fifteen swarms have been documented in the area, with yearly counts as follows: 2000 (1), 2002 (2), 2005 (1), 2006 (1), 2008 (3), 2009 (2), and 2010 (5). This pattern indicates recurrent swarm activity linked to the region’s transtensional regime. On 4 April 2010, the magnitude 7.2 Sierra El Mayor (El Mayor–Cucapah) earthquake occurred only 13 km from the swarm center, likely influencing local stress conditions that preceded the June–July sequence.
The 2010 swarm displayed typical features of fluid-driven or triggered activity in a geothermal-tectonic setting, with rapid onset, modest maximum magnitudes, and shallow focal depths. Such episodes underscore the ongoing seismic hazard in the Mexicali Valley and the value of dense monitoring for understanding swarm dynamics along the plate boundary.
References
USGS Earthquake Catalog (historical events and magnitudes)
Servicio Sismológico Nacional, Mexico (regional seismicity records)
SeismoSight internal swarm classification S20100620.1