Seismic Swarm S20100724.3: Analysis of Activity in Baja California, Mexico

Seismic swarm S20100724.3 occurred in northern Baja California, Mexico, approximately 18 km west of Alberto Oviedo Mota. The sequence began at 00:58 on 24 July 2010 and concluded at 18:35 on 2 August 2010, spanning 233 hours and 36 minutes. During this period, 117 earthquakes were recorded. The swarm was situated in a tectonically active zone near the boundary between the Pacific and North American plates, where right-lateral strike-slip faulting dominates due to the ongoing rifting associated with the Gulf of California.

The first 100 events of the swarm displayed magnitudes ranging from 1.0 to 3.9, with the majority occurring at shallow depths between 0 and 18 km. Notable events included a magnitude 3.9 earthquake at 18:05 on 24 July at 13 km depth, another magnitude 3.9 at 00:11 on 26 July at 8 km depth, and a magnitude 3.8 at 07:27 on 25 July at 6 km depth. Depths clustered predominantly around 5–9 km, consistent with activity along upper-crustal faults in the region. The temporal distribution showed the highest frequency in the initial 48 hours, followed by a gradual decline, characteristic of swarm behavior rather than a classic mainshock-aftershock sequence.

This swarm unfolded in the context of the broader 2010 El Mayor-Cucapah earthquake sequence. A magnitude 7.2 event on 4 April 2010, centered roughly 13 km from the swarm location, ruptured multiple fault segments in the Sierra El Mayor area. That earthquake highlighted the presence of previously unrecognized faults accommodating oblique extension and strike-slip motion. Post-event aftershocks and subsequent swarms have continued to delineate the complex fault network in the region.

Geologically, northern Baja California lies within the southernmost segment of the San Andreas fault system. The area features a series of northwest-trending faults, including strands of the Laguna Salada and Cerro Prieto systems, which accommodate Pacific-North America plate motion at rates of approximately 4–5 cm per year. The crust here is relatively thin and young, influenced by Miocene extension and ongoing transtensional tectonics. Historical records indicate recurrent seismic swarms, with 18 documented episodes since 2000, concentrated in years such as 2002 and 2010. These swarms often occur in clusters, reflecting fluid migration or stress triggering along interconnected fault zones.

The 2010 swarm contributed to ongoing monitoring of post-seismic relaxation following the April mainshock. Its shallow focal depths and moderate magnitudes suggest activation of secondary faults rather than the primary rupture plane. Such activity underscores the persistent seismic hazard in the Imperial Valley–Mexicali region, where population centers lie close to active structures.

References

SeismoSight internal swarm classification S20100724.3
USGS Earthquake Catalog for the 2010 El Mayor-Cucapah event