Seismic Swarm S20101210.1: Activity Near Progreso, Baja California
Seismic swarm S20101210.1 occurred approximately 17 km west-northwest of Progreso, Baja California, Mexico. The sequence began at 07:09 on 9 December 2010 and concluded at 03:42 on 15 December 2010, spanning roughly 140 hours and 33 minutes. During this interval, 59 earthquakes were recorded.
The events were predominantly shallow, with focal depths ranging from 0 to 10 km. Magnitudes remained modest, peaking at 4.0. The strongest shock occurred on 9 December at 09:02, while additional notable events included magnitudes of 3.8, 3.6, and 2.6. Most activity clustered between 1.0 and 2.0 in magnitude, illustrating the typical character of swarm sequences where numerous smaller events occur without a dominant mainshock.
This swarm unfolded within the tectonically active zone of northern Baja California. The region lies along the diffuse Pacific–North American plate boundary, where right-lateral transform motion is accommodated by a network of faults. Key structures include segments of the Agua Blanca Fault and related systems that extend from the Gulf of California rift toward the international border. These faults produce frequent microseismicity and occasional moderate earthquakes. Historical records document repeated swarms in the vicinity, consistent with fluid migration or aseismic slip triggering clustered seismicity rather than large stress accumulation on a single fault plane.
Since 1 January 2000, sixteen swarms have been identified in the area. Annual counts include three in 2006, one in 2007, three in 2009, and nine in 2010. The elevated number recorded in 2010 reflects improved detection capabilities and possibly heightened tectonic or hydrothermal activity during that period.
The temporal distribution of S20101210.1 showed the highest event rate on the first day, followed by a gradual decline. Depths remained consistently shallow throughout, supporting interpretations of activity within the upper crust. No damage or felt reports of significance were associated with the swarm, underscoring its low-hazard profile despite the volume of events.
Ongoing monitoring by regional seismic networks continues to refine understanding of these sequences and their relationship to the broader fault system in Baja California.
References
USGS Earthquake Catalog
Servicio Sismológico Nacional (Mexico)
SeismoSight internal swarm classification records