Seismic Swarm VS20081120.1 Near Puebla, Baja California

The earthquake swarm VS20081120.1 was recorded approximately 14 km south of Puebla in Baja California, Mexico. Activity began at 00:02 on 20 November 2008 and ended at 00:28 on 22 November 2008, producing 35 events over 48 hours and 26 minutes. Magnitudes ranged from 1.4 to 4.9, with the majority of hypocenters located between 3 km and 8 km depth and one outlier at 16 km.

The sequence opened with a magnitude 1.7 event, followed within minutes by events of 2.8, 3.7 and 3.6. A magnitude 4.9 earthquake occurred at 19:23 on 20 November at 6 km depth—the largest of the swarm. Subsequent activity included multiple events above magnitude 3.0 on both 20 and 21 November, tapering to smaller shocks before termination. Depths remained shallow throughout, consistent with brittle failure in the upper crust.

Earthquake swarms represent clusters of seismicity without a single dominant mainshock. In this case the gradual rise and decay of event rates, combined with the absence of a clear foreshock–aftershock pattern, align with typical swarm behavior driven by localized stress perturbations or fluid involvement along active faults.

Since 1 January 2000 the same region has hosted eight documented swarms. These occurred in 2000 (one swarm), 2002 (three swarms), 2005 (one swarm), 2006 (one swarm) and 2008 (two swarms, including VS20081120.1). The repeated occurrence indicates persistent mechanical conditions favoring swarm-type release rather than isolated large events.

On 4 April 2010 a magnitude 7.2 earthquake struck the Sierra El Mayor area, with its epicenter only 14 km from the 2008 swarm centroid. This event ruptured a complex network of faults within the same tectonic corridor, underscoring the area’s capacity for both swarm activity and occasional larger ruptures.

Geologically, the Puebla region lies within the transtensional Pacific–North America plate boundary. Northern Baja California accommodates right-lateral shear and crustal extension linked to the southern continuation of the San Andreas system and the opening of the Gulf of California. Active faults here include northwest-striking strike-slip structures and subsidiary normal faults that produce shallow seismicity. The shallow focal depths observed in VS20081120.1 are typical of this setting, where brittle failure occurs above approximately 10–15 km.

References
SeismoSight internal classification – swarm VS20081120.1 parameters and event list.
USGS Earthquake Catalog – 2010 Sierra El Mayor M7.2 event location and regional tectonics.