Seismic Swarm S20060528.1 Near Delta, Baja California, Mexico
The seismic swarm S20060528.1 occurred in the tectonically active Mexicali Valley of Baja California, Mexico, approximately 7 km west of Delta. This region lies within the Salton Trough, a pull-apart basin formed by the interaction between the Pacific and North American plates along the San Andreas fault system. Right-lateral strike-slip motion dominates, with subsidiary normal faulting contributing to the complex deformation. The swarm began at 16:30 on 27 May 2006 and concluded at 06:36 on 29 May 2006, spanning 38 hours and 5 minutes during which 27 earthquakes were recorded.
Event parameters reveal a shallow sequence concentrated at depths of 5–8 km. The largest events reached magnitudes 4.2 and 4.4 on 28 May, both at 6 km depth. These mainshocks were preceded by smaller foreshocks on 27 May, including magnitudes 3.2 and 2.8, and followed by a rapid decay in activity. Most events clustered tightly in both time and space, consistent with swarm behavior rather than a classic mainshock-aftershock sequence.
Geologically, the swarm epicenters align with the broader Imperial-Cerro Prieto fault zone, where geothermal activity and fluid migration are known to modulate seismicity. Historical records indicate five prior swarms in the immediate area since 2000, occurring in 2000 (one swarm), 2002 (three swarms), and 2005 (one swarm). These recurrent episodes underscore the region’s persistent strain accumulation and release along transform boundaries.
A notable nearby event was the 4 April 2010 Mw 7.2 Sierra El Mayor earthquake, located only 11 km from the 2006 swarm center. That rupture produced widespread surface deformation and triggered aftershocks across the same fault network, highlighting the interconnected nature of seismic sources in the area.
Overall, swarm S20060528.1 exemplifies the frequent, moderate-magnitude activity that characterizes the southernmost San Andreas transform margin. Continued monitoring remains essential for understanding fluid-driven triggering mechanisms and assessing future seismic potential in this densely populated border region.
References
- SeismoSight internal swarm classification records, 2006.
- USGS Earthquake Catalog, Sierra El Mayor-Cucapah event, 2010.