Seismic Swarm S20240518.1: Analysis of Activity Near Brawley, California
A seismic swarm designated S20240518.1 occurred 6 km south-southwest of Brawley, California, from 05:17 on 18 May 2024 to 00:18 on 21 May 2024. Over 67 hours and one minute, the sequence produced 306 earthquakes. Analysis of the first 100 events reveals a concentration of low-magnitude activity, with events ranging from 0.6 to 3.7. Depths clustered primarily between 5 km and 14 km, though several reached 18 km. The initial event registered magnitude 1.7 at 9 km depth. Subsequent activity included multiple magnitude 2+ events, such as a 2.7 at 11 km early on 18 May and a 3.7 at 11 km later that morning. Magnitudes above 2.5 appeared sporadically, often at depths exceeding 10 km, while shallower events (under 5 km) remained below magnitude 2.0. Temporal distribution showed peak frequency in the first 12 hours, with gradual decline thereafter.
The Brawley area lies within the Imperial Valley of southern California, part of the Salton Trough, a tectonically active pull-apart basin formed by the interaction of the San Andreas Fault system and the Imperial Fault. This region experiences frequent seismicity due to right-lateral strike-slip faulting and associated normal faults. The Brawley Seismic Zone, a north-south trending cluster of faults, accommodates strain transfer between the San Andreas and Imperial faults. Historical records indicate elevated swarm activity linked to fluid migration and aseismic slip along these structures. Depths in the provided sequence align with typical brittle failure in the upper crust of this extensional regime.
Seismic swarms have characterized the region since at least 2000. From 1 January 2000 through 2024, 80 swarms were documented. Annual counts varied, with notable peaks in 2010 (10 swarms), 2013 (12 swarms), and 2021 (7 swarms). Lower activity occurred in years such as 2002, 2015, and 2018, each recording one swarm. The 2024 count reached three swarms by mid-year, consistent with episodic clustering observed in prior decades. These sequences typically involve hundreds of events without a dominant mainshock, distinguishing them from classic aftershock sequences.
Such activity underscores the ongoing tectonic deformation in the Imperial Valley. Monitoring networks provide real-time data essential for assessing potential escalation, though most swarms remain minor. Continued observation supports refined models of fault interactions in this high-strain corridor.
References
SeismoSight internal swarm classification records (S20240518.1 parameters and event list).
USGS Earthquake Hazards Program regional reports on Imperial Valley tectonics.
California Geological Survey summaries of Brawley Seismic Zone history.