Seismic Swarm Near Delta, Baja California: June 2024 Event Analysis

On June 5, 2024, a seismic swarm was recorded 6 km north of Delta, Baja California, Mexico. The sequence began at 07:11 and concluded at 20:31, spanning 13 hours and 20 minutes. During this period, 25 earthquakes were registered, with magnitudes ranging from 1.5 to 4.5 and focal depths primarily between 2 km and 16 km.

The swarm exhibited a clustered temporal pattern, with the majority of events occurring in the morning hours. A magnitude 4.3 earthquake at 08:42:42 was followed closely by several events between 3.8 and 2.1, all at depths of approximately 14–15 km. Later activity included a magnitude 4.5 event at 11:58:40, also at 15 km depth. Shallower events, such as the magnitude 1.8 at 2 km depth recorded at 11:34:56, were less frequent but contributed to the overall sequence.

This activity aligns with the region's established seismic character. Delta lies within the Mexicali Valley, part of the Pacific–North American plate boundary. The area experiences transform faulting associated with the Cerro Prieto and Imperial fault systems, which form an extension of the San Andreas fault zone into northern Mexico. These structures accommodate lateral plate motion and are linked to geothermal fields that influence local seismicity through fluid migration and heat flow.

Historical records indicate recurrent swarm activity in the vicinity. Since January 1, 2000, 19 swarms have been documented. The distribution by year includes one swarm in 2000, three in 2002, one each in 2005 and 2006, three in 2008, two in 2009, six in 2010, one in 2012, and the current event in 2024. Such episodic clusters are typical of the Brawley Seismic Zone and adjacent Mexican segments, where swarms often occur without a single dominant mainshock.

The June 2024 swarm's characteristics—moderate magnitudes, shallow to mid-crustal depths, and rapid succession—reflect stress release along pre-existing faults in a tectonically complex setting. No surface rupture or significant damage has been associated with events of this scale in the historical record for the immediate area.

References
USGS Earthquake Catalog
Servicio Sismológico Nacional (Mexico)
Geological Survey of Baja California publications on the Cerro Prieto fault zone