Seismic Swarm S20091126.1 Near Progreso, Baja California

An earthquake swarm designated S20091126.1 occurred approximately 13 km northwest of Progreso, Baja California, Mexico. The sequence began at 04:52 on 26 November 2009 and concluded at 05:25 on 27 November 2009, encompassing a total duration of 24 hours and 33 minutes. During this interval, 35 earthquakes were recorded.

The events exhibited predominantly shallow focal depths between 0 and 5 km. Magnitudes ranged from 0.7 to 3.6, with the largest shock reaching 3.6 at a depth of 4 km on 26 November at 07:10:46 UTC. Other notable events included a magnitude 3.3 quake at 07:33:17 and a magnitude 2.8 event at 10:10:54. The majority of activity clustered between 04:52 and 10:49 on 26 November, followed by sparse occurrences through the night and into the early morning of 27 November.

This swarm aligns with the broader tectonic setting of northern Baja California, situated along the boundary between the Pacific and North American plates. The region forms part of the San Andreas fault system extension, characterized by right-lateral strike-slip faults and associated pull-apart basins linked to the opening of the Gulf of California. Active structures such as the Cerro Prieto and Imperial faults lie nearby, contributing to frequent microseismicity and occasional swarms driven by fluid migration or stress transfer along these transform boundaries.

Historical records indicate that five swarms have occurred in the area since 2000. These took place in 2000 (one swarm), 2005 (one swarm), 2008 (one swarm), and two in 2009. Such episodic clustering is typical in this transform-margin environment, where low-magnitude events often occur in rapid succession without a distinct mainshock-aftershock pattern.

The shallow depths observed in S20091126.1 are consistent with the brittle upper crust in this part of the Salton Trough–Imperial Valley corridor, where sedimentary basins overlie crystalline basement at relatively shallow levels. No damage or felt reports of significance were associated with the sequence, reflecting the modest magnitudes involved.

References

USGS Earthquake Hazards Program – Regional Tectonic Summary for Baja California
Servicio Sismológico Nacional (Mexico) – Historical Seismicity Catalog
Geological Society of America – Papers on Gulf of California Rift Tectonics