Seismic Swarm S20091102.1 Near Progreso, Baja California

Seismic swarm S20091102.1 occurred in the region 11 km NNW of Progreso, Baja California, Mexico. The sequence began at 05:57 on 1 November 2009 and concluded at 23:05 on 5 November 2009, spanning 113 hours and 7 minutes. During this period, 286 earthquakes were recorded.

Analysis of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from 0.5 to 4.1, with the majority between 1.0 and 2.5. Depths were shallow, typically 3 to 14 km, consistent with crustal faulting in the area. The largest event reached magnitude 4.1 at 19:27 on 2 November 2009 at a depth of 7 km. Several events of magnitude 3.0 and above clustered between 18:00 and 21:00 on 2 November, indicating a peak in energy release during that interval. Event frequency was highest in the initial 48 hours, with smaller aftershocks continuing at a declining rate.

The Progreso area lies within the tectonically active Salton Trough, where the Pacific and North American plates interact along a transform boundary that extends southward into the Gulf of California. This setting produces frequent small earthquakes and occasional swarms driven by strike-slip faulting and minor extension. Historical records since 2000 document only three prior swarms in the immediate vicinity, occurring in 2000, 2005, and 2008. These episodes underscore the region's pattern of episodic, clustered seismicity rather than isolated mainshock-aftershock sequences.

Such swarms are commonly associated with fluid migration or minor aseismic slip along local fault segments. The 2009 activity remained below levels that would generate significant surface damage, reflecting the modest energy release characteristic of the zone.

References

SeismoSight internal swarm classification S20091102.1
USGS Earthquake Catalog (regional Baja California events)
Geological Survey of Mexico tectonic framework reports