Seismic Swarm S20080626.1: Analysis of Activity Near Alberto Oviedo Mota, Baja California

Seismic swarm S20080626.1 was recorded in the Mexicali Valley region of Baja California, Mexico. The sequence began at 19:29 on 25 June 2008 and concluded at 16:42 on 26 June 2008, spanning 21 hours and 13 minutes. A total of 27 earthquakes were detected at a location 15 km west of Alberto Oviedo Mota. Magnitudes ranged from 1.3 to 3.6, with the majority occurring at shallow depths of 5–6 km. Two events reached magnitude 3.6, one at 01:42 on 26 June at 15 km depth and another at 04:03 on the same day at 5 km depth. Additional notable shocks included a magnitude 3.4 event at 20:24 on 25 June and a magnitude 2.9 at 22:15 the same evening at 17 km depth.

The swarm exhibited typical characteristics of clustered microseismicity, with events distributed across a brief time window and concentrated at shallow crustal levels. Depths remained predominantly between 5 km and 6 km, though a few reached 12–17 km, suggesting minor involvement of deeper structures. No single mainshock dominated; instead, the activity reflected distributed strain release along local fault segments.

The region lies within the Salton Trough, a tectonically active pull-apart basin formed at the boundary between the Pacific and North American plates. Right-lateral strike-slip motion along the San Andreas fault system transitions southward into the Imperial and Cerro Prieto faults, which traverse the Mexicali Valley. This setting produces frequent earthquake swarms due to fluid migration, aseismic slip, and stress interactions among en echelon fault strands. The 2008 swarm occurred approximately 5 km from the epicenter of the magnitude 7.2 El Mayor–Cucapah earthquake of 4 April 2010, highlighting the persistent seismic hazard along this portion of the plate boundary.

Historical records since 2000 indicate seven prior swarms in the immediate area, occurring in 2000 (one swarm), 2002 (three swarms), 2005 (one swarm), 2006 (one swarm), and 2008 (this event). Such recurrent swarms underscore the valley’s elevated background seismicity and its role in accommodating plate-boundary deformation. The 2010 magnitude 7.2 mainshock, one of the strongest regional events in recent decades, ruptured multiple fault segments near the same locale, releasing accumulated strain that may have been influenced by preceding swarm activity.

Overall, swarm S20080626.1 represents a short-lived episode of low-to-moderate energy release consistent with the tectonic regime of the northern Baja California fault network. Continued monitoring remains essential for understanding how these sequences relate to larger events along the Imperial–Cerro Prieto fault system.

References

SeismoSight internal swarm classification data.
USGS Earthquake Catalog, El Mayor–Cucapah event records.