Seismic Swarm VS20001102.1: Analysis of Activity near Puebla, Baja California

A seismic swarm designated VS20001102.1 occurred south of Puebla in Baja California, Mexico, from 09:51 on 2 November 2000 to 08:38 on 5 November 2000. Over approximately 70 hours and 46 minutes, the sequence produced 63 earthquakes. The events clustered at shallow depths, predominantly between 5 and 6 km, with a single outlier recorded at 16 km. Magnitudes ranged from 1.5 to 3.5, with the largest shocks reaching 3.5 at 10:49 on 2 November and 3.4 and 3.3 on the same day and 4 November, respectively. Activity was most intense during the first 24 hours before declining steadily.

The swarm unfolded in a region situated within the Pacific–North America plate boundary zone. Baja California lies along the western margin of the North American plate, where right-lateral transform motion is accommodated by the San Andreas fault system and its southern extensions into the Gulf of California rift. The town of Puebla sits roughly 13 km north of the swarm centroid, placing the sequence near the western edge of the Mexicali Valley. This area experiences distributed deformation associated with the Cerro Prieto and Imperial faults, both of which have produced significant historical earthquakes.

Geologically, the subsurface consists of Quaternary alluvial and lacustrine sediments overlying Cretaceous granitic basement and Tertiary volcanic rocks. Shallow seismicity is commonly linked to fluid migration or aseismic slip on subsidiary faults within the broader transform system. Depths of 5–6 km recorded during the swarm are consistent with the brittle–ductile transition in this high-heat-flow setting. No surface rupture was reported, typical of low-magnitude swarms in sedimentary basins.

The 2010 Sierra El Mayor–Cucapah earthquake (Mw 7.2) occurred on 4 April 2010 with an epicenter approximately 13 km from the 2000 swarm centroid. That event ruptured a complex set of faults within the same tectonic corridor, highlighting the persistent seismic hazard of the region. Post-2000 monitoring by regional networks has documented recurrent swarm activity along the same structural trend, underscoring the value of dense seismic catalogs for understanding strain release patterns.

Analysis of the 2000 sequence shows a classic swarm signature: rapid onset, lack of a single dominant mainshock, and exponential decay in event rate. Most events remained below magnitude 3.0, limiting potential for damage yet providing insight into background seismicity levels. Depths clustered tightly around 5–6 km suggest a common source volume, possibly influenced by local hydrothermal or magmatic processes within the rift margin.

Continued instrumentation since 2000 has improved location precision and focal-mechanism coverage, revealing that many small events align with northwest-striking right-lateral strike-slip planes. Such data refine models of fault segmentation and stress transfer along the plate boundary. The VS20001102.1 swarm remains a useful case study for characterizing pre-2010 background activity in an area that later hosted a major rupture.

References

USGS Earthquake Hazards Program – El Mayor–Cucapah Earthquake Summary
Servicio Sismológico Nacional, Universidad Nacional Autónoma de México – Regional Seismicity Catalog
SeismoSight internal swarm classification VS20001102.1