Seismic Swarm PS20100405.1: Activity Near Progreso, Baja California
Seismic swarm PS20100405.1 was recorded between 22:43 on 4 April 2010 and 00:07 on 5 April 2010, approximately 20 km west-northwest of Progreso, Baja California, Mexico. The sequence lasted 1 hour and 24 minutes and included five earthquakes. Event parameters were as follows: a magnitude 5.2 event at 9 km depth at 22:43; a magnitude 5.7 event at 10 km depth at 22:50; a magnitude 5.4 event at 40 km depth at 23:25; a magnitude 3.0 event at 4 km depth at 00:01; and a magnitude 5.2 event at 9 km depth at 00:07.
This swarm occurred within the tectonically active boundary zone between the Pacific and North American plates. Baja California lies along a system of transform faults and pull-apart basins that accommodate right-lateral shear and oblique extension. The regional geology features Quaternary alluvial deposits overlying Cretaceous granitic basement and Cenozoic volcanic and sedimentary rocks, with active faulting concentrated along the Laguna Salada and related structures.
The swarm took place one day after the magnitude 7.2 Sierra El Mayor earthquake of 4 April 2010, whose epicenter lay only 7 km from the swarm centroid. That mainshock ruptured a northwest-striking normal-oblique fault within the same fault network, producing widespread aftershock activity across northern Baja California. The timing and proximity of swarm PS20100405.1 indicate it formed part of the post-mainshock stress readjustment.
Historical records show elevated swarm activity in the region since 2000. A total of 53 swarms have been documented, with yearly counts of 4 in 2000, 1 in 2001, 4 in 2002, 4 in 2003, 1 in 2004, 5 in 2005, 4 in 2006, 1 in 2007, 9 in 2008, 13 in 2009, and 7 in 2010. These episodes cluster near major fault intersections and often precede or follow larger events, reflecting episodic strain release along the plate-boundary system.
Seismic swarms in this setting typically arise from fluid migration or aseismic slip that triggers successive failures on closely spaced faults. Depths ranging from 4 km to 40 km in the present sequence are consistent with the brittle-ductile transition zone mapped in the area. Continued monitoring of such sequences provides critical data for refining fault models and assessing short-term hazard in this rapidly deforming region.