Seismic Swarm S20240512.1: Analysis of Activity Near Delta, Baja California

A seismic swarm designated S20240512.1 occurred 5 km northwest of Delta in Baja California, Mexico. The sequence began at 14:39 UTC on 12 May 2024 and concluded at 01:16 UTC on 15 May 2024, spanning 58 hours and 36 minutes. During this period, 93 earthquakes were recorded, with magnitudes ranging from 1.1 to 4.8 and focal depths predominantly between 3 km and 27 km.

The swarm exhibited peak intensity on 12 May, particularly between 18:00 and 19:00 UTC, when multiple events exceeded magnitude 3.0. The largest shock reached magnitude 4.8 at 18:22:42 UTC at a depth of 8 km. Subsequent notable events included magnitudes 4.5 at 19:04:00 UTC, 4.1 at 04:02:18 UTC on 13 May, and 4.0 at 18:33:44 UTC on 12 May. Activity declined steadily thereafter, with only isolated events above magnitude 2.0 recorded on 14 and 15 May. Depths clustered mostly between 5 km and 11 km, consistent with shallow crustal faulting in the region.

This swarm aligns with the tectonic framework of the Mexicali Valley, situated within the Pacific–North American plate boundary. The area forms part of a transtensional zone influenced by the Cerro Prieto and Imperial faults, which accommodate right-lateral shear and extension. Geothermal fields such as Cerro Prieto, located nearby, indicate elevated heat flow and fluid circulation that can trigger swarm-like sequences through pore-pressure changes.

Historical records document 21 prior swarms in the same locale since 2000. These episodes cluster in specific years, with the highest frequency in 2010 (nine swarms), followed by 2002 and 2008 (three each). Earlier activity occurred in 2000, 2005, 2006, 2009, and 2012. Such recurrent swarms reflect ongoing strain accumulation and release along the regional fault network.

Swarm events in this setting typically arise from aseismic slip, fluid migration, or stress triggering rather than a single mainshock-aftershock cascade. The 2024 sequence displayed classic swarm characteristics: a rapid onset, lack of a dominant foreshock, and gradual decay without a clear aftershock sequence. Magnitudes remained moderate, limiting potential for significant damage, though the proximity to populated areas and infrastructure warrants continued monitoring.

Regional seismic hazard assessments identify the Mexicali Valley as moderately active, with historical earthquakes exceeding magnitude 7.0 along nearby segments of the plate boundary. Ongoing geothermal operations and agricultural development in the delta region increase exposure to ground shaking and liquefaction.

References

• Servicio Sismológico Nacional (SSN) earthquake catalog, Mexico.
• USGS Earthquake Hazards Program, regional tectonics summaries.
• Geological Society of America, studies on Cerro Prieto fault system.