Seismic Swarm Near Brawley, California: The June 2021 Event and Regional Context

On 11 June 2021, a seismic swarm designated S20210611.1 was recorded 4 km southeast of Brawley, California. The sequence began at 00:01 UTC and concluded at 04:53 UTC, encompassing 26 earthquakes over 4 hours and 52 minutes. Magnitudes ranged from 0.8 to 2.1, with focal depths primarily between 2 km and 22 km. The largest events reached magnitude 2.1 at depths of 10 km and 7 km, while most activity clustered around 5–8 km depth.

This swarm exemplifies the frequent, low-magnitude earthquake clusters typical of the Imperial Valley. Events occurred in rapid succession, with notable pairs such as the 00:40:09 and 00:40:19 shocks, and a cluster around 03:35–03:49 that included multiple magnitude 2.0+ events. Depths remained shallow to moderate, consistent with the region’s active fault network.

Brawley lies within the Salton Trough, a tectonically active pull-apart basin at the southern terminus of the San Andreas Fault system. The area experiences right-lateral shear between the Pacific and North American plates, accommodated by the Imperial, Brawley, and San Jacinto faults. Geothermal gradients are elevated due to crustal thinning and magmatic intrusions beneath the Salton Sea, contributing to swarm behavior through fluid migration and aseismic slip.

Historically, the Brawley Seismic Zone has produced numerous swarms. Since 2000, 87 such sequences have been documented, with annual counts varying from 1 to 13. Peak activity occurred in 2013 (13 swarms) and 2012 (10 swarms), while quieter years included 2002, 2004, 2014, 2015, and 2018 (1 swarm each). The 2021 total reached 6 swarms, aligning with the long-term average.

These swarms typically involve dozens of events below magnitude 3.0 and rarely produce significant damage. They often correlate with geothermal fluid movement or minor fault adjustments rather than mainshock-aftershock sequences. The June 2021 swarm followed this pattern, remaining well below thresholds for felt shaking or infrastructure impact in nearby communities.

Ongoing monitoring by regional seismic networks continues to track activity in this high-strain zone. Understanding swarm statistics aids in distinguishing background seismicity from potential precursors to larger events along the Imperial Fault.

References

USGS Earthquake Hazards Program – Imperial Valley seismicity reports
California Geological Survey – Salton Trough tectonic summaries
Southern California Earthquake Data Center – regional catalog statistics