Seismic Swarm S20110409.1: Analysis of Activity Near Progreso, Baja California

Seismic swarm S20110409.1 occurred approximately 16 km west-northwest of Progreso in Baja California, Mexico. The sequence began at 10:09 on 8 April 2011 and concluded at 07:24 on 11 April 2011, spanning 69 hours and 14 minutes. During this period, 41 earthquakes were recorded, with magnitudes ranging from 1.2 to 4.1 and focal depths between 2 km and 13 km.

The largest event reached magnitude 4.1 at 08:58 on 9 April at a depth of 9 km. Other notable shocks included a magnitude 3.3 event at 10:34 on 8 April (9 km depth) and a magnitude 3.1 shock at 10:42 on 9 April (8 km depth). The majority of events clustered at depths of 5–9 km, consistent with shallow crustal faulting typical of the region.

Baja California lies along the tectonically active boundary between the Pacific and North American plates. Right-lateral strike-slip motion along transform faults accommodates plate motion, producing frequent seismic activity. The Progreso area sits within this transform domain, where distributed faulting and occasional swarm sequences occur due to stress transfer and minor fluid involvement in the crust.

Historical records indicate elevated swarm frequency in recent decades. Since 1 January 2000, fifteen swarms have been documented in the vicinity. Activity intensified in 2010 with twelve swarms, followed by three in 2009. These episodes reflect ongoing strain accumulation along regional fault systems without producing a single dominant mainshock.

Swarm characteristics observed in S20110409.1 align with patterns seen elsewhere along the plate boundary. Events occurred in rapid succession with no clear foreshock-mainshock-aftershock progression, and magnitudes remained moderate. Depths stayed within the upper crust, suggesting activation of near-surface fault segments.

The swarm provides insight into localized stress redistribution. Clusters of similar-depth events on 8–9 April indicate progressive failure along adjacent fault patches. Later activity on 10–11 April showed slightly shallower depths, possibly reflecting upward migration of stress or minor pore-pressure changes.

Such sequences contribute to understanding seismic hazard in northern Baja California. While individual swarms rarely generate damaging ground motion, they highlight active fault networks that warrant continued monitoring. Updated regional seismic catalogs confirm persistent low-to-moderate activity consistent with the tectonic regime established over the past several million years.

References

US Geological Survey Earthquake Catalog
Servicio Sismológico Nacional, Mexico
Southern California Earthquake Data Center regional reports