Seismic Swarm S20091108.1: Analysis of Activity near Progreso, Baja California

A seismic swarm designated S20091108.1 occurred 10 km northwest of Progreso, Baja California, Mexico. The sequence began at 02:28 on 8 November 2009 and ended at 16:37 on 9 November 2009, lasting 38 hours and 9 minutes. During this period, 54 earthquakes were recorded. Magnitudes ranged from 0.8 to 2.7. Depths were predominantly shallow, concentrated between 4 km and 6 km, with a few events reaching 12–14 km. Activity peaked during the afternoon of 8 November, featuring multiple events above magnitude 2.0, including a 2.7 event at 13:59. The sequence exhibited classic swarm characteristics: rapid onset, high event rate over a short interval, and absence of a dominant mainshock. The Progreso area lies in northern Baja California within the southern Salton Trough, a transtensional zone marking the Pacific–North America plate boundary. The region experiences distributed deformation along northwest-trending strike-slip faults and northeast-striking normal faults linked to the Cerro Prieto and Imperial fault systems. Geothermal activity and shallow seismicity are common due to crustal thinning and magmatic influence from the nearby spreading center. Historical records show that swarms in this tectonic setting often reflect fluid migration or aseismic slip transients rather than large stress accumulation. Since 1 January 2000, four swarms have been documented in the area, occurring in 2000, 2005, 2008, and 2009. The 2009 swarm fits this recurring pattern of episodic, low-magnitude clusters that release strain without producing damaging ground motion. Insights from the event list reveal temporal clustering and depth consistency. Most events occurred at 4–5 km depth, suggesting activity within the brittle upper crust. Magnitudes remained below 3.0 throughout, with 70 % of events between 1.0 and 2.0. The swarm terminated abruptly after the final recorded event at 16:37 on 9 November, indicating rapid decay of the triggering process. Such sequences provide valuable data for understanding short-term seismic hazard in the Salton Trough and for refining models of fault interaction along the plate boundary.

References

SeismoSight internal swarm classification S20091108.1
USGS Earthquake Hazards Program regional tectonic summaries
INEGI and Servicio Sismológico Nacional geological mapping of Baja California faults