Seismic Swarm S20100701.1: Analysis of Activity near Progreso, Baja California
The seismic swarm designated S20100701.1 occurred approximately 16 km south of Progreso in Baja California, Mexico. It began at 16:29 on 30 June 2010 and concluded at 18:36 on 2 July 2010, spanning 50 hours and 7 minutes. During this interval, 35 earthquakes were recorded, with magnitudes ranging from 1.4 to 3.1 and focal depths between 0 and 14 km.
This sequence exemplifies a typical earthquake swarm, characterized by a concentrated cluster of events without a dominant mainshock or clear foreshock-aftershock pattern. The events initiated with a magnitude 1.8 quake at 6 km depth on 30 June at 16:29:14. Subsequent activity included multiple events of magnitude 2.8, such as those at 19:50:04 (13 km depth) and 19:51:00 (6 km depth). Peak magnitudes reached 3.1 at 01:26:41 on 1 July at 5 km depth. Later events on 2 July remained below magnitude 2.5, marking the swarm's gradual decline.
Geologically, the Progreso region lies within the Pacific-North American plate boundary zone. Baja California experiences ongoing transform faulting associated with the San Andreas system, particularly the Cerro Prieto and Imperial faults. These structures accommodate right-lateral strike-slip motion at rates of several centimeters per year. Shallow crustal depths recorded in the swarm align with the brittle upper crust in this tectonically active corridor, where fluid migration and stress transfer often trigger clustered seismicity.
The area has a documented history of swarm activity. Since 1 January 2000, eleven such swarms have occurred, with three in 2009 and eight in 2010. These episodes reflect episodic strain release along secondary fault segments rather than large single ruptures. No damage or felt reports above intensity III were associated with S20100701.1, consistent with its modest magnitudes.
Seismic monitoring in northern Baja California benefits from regional networks that provide precise hypocentral locations. Continued observation supports hazard assessment in this portion of the plate boundary, where swarm recurrence underscores the need for sustained vigilance.
References
USGS Earthquake Catalog (historical data 2000–2010)
Servicio Sismológico Nacional, Mexico (regional tectonics reports)
SeismoSight internal swarm classification records