Seismic Swarm S20080602.1: Analysis of Activity Near Brawley, California

The seismic swarm designated S20080602.1 occurred approximately 6 km north of Brawley in Imperial County, California. It began at 00:49 UTC on 2 June 2008 and concluded at 14:46 UTC on 4 June 2008, spanning 61 hours and 57 minutes. During this interval, 145 earthquakes were recorded. Analysis of the first 100 events reveals a predominance of low-magnitude activity, with magnitudes ranging from 0.9 to 2.9. Depths clustered primarily between 13 km and 18 km, though several events occurred at shallower levels of 6–11 km. The sequence initiated with events of magnitude 1.2–1.7, followed by a gradual increase in frequency and occasional peaks reaching 2.8–2.9. Later portions of the dataset showed a return to smaller events below magnitude 2.0. This swarm aligns with the established pattern of clustered seismicity in the region. Historical records indicate 14 swarms in the area since 1 January 2000, distributed across the following years: two in 2000, one in 2001, one in 2002, four in 2003, one in 2004, three in 2005, and two in 2008. Such episodic activity reflects ongoing tectonic processes rather than isolated mainshock-aftershock sequences. The Brawley area lies within the Salton Trough, a tectonically active pull-apart basin formed by the interaction of the San Andreas Fault system and the Imperial Fault. The region experiences right-lateral strike-slip motion combined with extension, resulting in frequent small earthquakes and occasional swarms. Depths in the 10–20 km range correspond to the brittle-ductile transition zone influenced by elevated geothermal gradients from nearby volcanic and hydrothermal systems associated with the Salton Sea geothermal field. Geological evolution of the Imperial Valley traces to Miocene extension along the Pacific–North America plate boundary, with rapid sedimentation from the Colorado River filling the subsiding basin. The Brawley Seismic Zone, immediately north of the town, accommodates transfer of slip between the San Andreas and Imperial faults through a network of cross-cutting structures. This setting produces diffuse seismicity, often in swarms lasting hours to days, driven by fluid migration and stress perturbations rather than large single ruptures. Recent monitoring by regional networks confirms continued low-level activity consistent with the long-term rate observed in the early 2000s. No events in the 2008 swarm exceeded magnitude 3.0, underscoring the typically modest energy release characteristic of Imperial Valley swarms.

References

United States Geological Survey Earthquake Hazards Program – Imperial Valley seismicity summaries Southern California Earthquake Data Center – Regional fault and swarm documentation California Geological Survey – Salton Trough tectonic framework reports