Seismic Swarm S20100405.2: Analysis of Activity Near Progreso, Baja California

Seismic swarm S20100405.2 was recorded southwest of Progreso in Baja California, Mexico. The sequence began at 22:45 on 4 April 2010 and concluded at 13:57 on 25 April 2010. Over 495 hours and 12 minutes, a total of 840 earthquakes were registered. The swarm occurred within a tectonically active zone where the Pacific and North American plates interact along the Gulf of California rift system.

Baja California experiences frequent seismicity due to right-lateral strike-slip faults and spreading centers associated with the San Andreas transform boundary. The region lies near the Cerro Prieto fault and other segments of the plate boundary that accommodate northwestward motion of the Baja California peninsula. Historical records indicate elevated activity in the area, consistent with ongoing rifting that has shaped the Gulf of California over millions of years.

Analysis of the first 100 events reveals a rapid onset with several events exceeding magnitude 4.0 within the initial hours. The largest magnitude recorded among these events reached 4.7 at a depth of 9 km on 4 April at 23:37. Depths ranged primarily between 0 and 16 km, indicating shallow crustal sources typical of the region’s transform and normal faulting. Magnitudes clustered between 2.0 and 3.6 for the majority of events, with only a small number surpassing 4.0. Temporal distribution showed intense activity in the first 12 hours, followed by a gradual decline in both frequency and peak magnitudes.

This swarm fits within a broader pattern of seismic sequences documented since 2000. Six swarms have occurred in the area through the present, with prior episodes recorded in 2008 (one swarm), 2009 (three swarms), and two additional swarms in 2010. Such clustering reflects episodic strain release along the plate boundary rather than isolated mainshock-aftershock sequences.

The shallow focal depths and moderate magnitudes observed are consistent with the geological setting of Baja California, where brittle failure occurs in the upper crust. No events in the initial 100 exceeded the magnitude threshold that would typically generate significant surface rupture or widespread damage in this sparsely populated region.

Further monitoring by regional networks continues to refine understanding of swarm dynamics and their relation to long-term plate motion rates of approximately 4–5 cm per year.

References

USGS Earthquake Catalog Servicio Sismológico Nacional (Mexico) Geological Survey of Baja California publications on regional tectonics