Seismic Swarm S20011209.1 Near Estación Coahuila, Baja California

Seismic swarm S20011209.1 was recorded between 23:36 on 8 December 2001 and 15:16 on 12 December 2001, approximately 10 km southeast of Estación Coahuila in Baja California, Mexico. Over 87 hours and 40 minutes, the sequence produced 57 earthquakes. The event began with a magnitude 5.7 shock at 6 km depth, followed by a rapid succession of smaller events that declined in frequency over the subsequent days.

The region lies within the Pacific–North American plate boundary zone. Baja California forms part of the Baja California microplate, which moves northwestward relative to the North American plate along a system of right-lateral strike-slip faults. Estación Coahuila is situated near the southern extension of the Imperial fault and the northern margin of the Cerro Prieto fault, both of which accommodate transform motion associated with the opening of the Gulf of California. The shallow crustal depths recorded during the swarm (primarily 6–7 km) are consistent with the brittle behavior of the upper crust in this tectonically active corridor.

The initial magnitude 5.7 event was followed within minutes by a magnitude 3.4 shock at the same depth. Subsequent activity included multiple events in the magnitude 2.0–3.1 range, with the largest aftershock reaching magnitude 4.8 roughly two hours after the mainshock. Activity remained elevated through 9 December, with events occurring at intervals of minutes to hours, before tapering to isolated shocks by 11–12 December. Depths remained tightly clustered between 6 and 7 km throughout the sequence, indicating a compact source volume.

This swarm occurred in a setting known for both tectonic and geothermal seismicity. The nearby Cerro Prieto geothermal field, one of Mexico’s largest, has produced persistent microseismicity linked to fluid movement and fault slip. Historical records show that similar swarm-like sequences have occurred in the broader Mexicali Valley–Imperial Valley region, often without a single dominant mainshock. The 2001 swarm followed an earlier episode in 2000, underscoring the episodic nature of clustered seismicity along these faults.

The 57 events documented in the swarm illustrate a classic swarm pattern: an abrupt onset, high initial rate, and gradual decay without a clear aftershock sequence obeying Omori’s law. Most events remained below magnitude 3.0 after the first day, suggesting that the stress perturbation was rapidly accommodated by distributed slip on small fault patches.

Geodetic and seismic monitoring in the area continues to track strain accumulation along the plate boundary. Updated regional catalogs confirm that the Imperial–Cerro Prieto fault system remains capable of producing both swarms and larger mainshock–aftershock sequences, as demonstrated by subsequent events in the region.

References

• Servicio Sismológico Nacional (SSN), Universidad Nacional Autónoma de México, event catalog 2001.
• United States Geological Survey Earthquake Hazards Program, regional seismicity reports.
• Consejo Nacional de Ciencia y Tecnología, geothermal studies of the Cerro Prieto field.