Seismic Swarm S20030301.1 Near Brawley, California

On March 1, 2003, a notable earthquake swarm was recorded 8 km south-southeast of Brawley, California. The sequence began at 00:08 and concluded at 23:37, encompassing 52 events over 23 hours and 29 minutes. Magnitudes ranged from 1.0 to 2.7, with focal depths primarily between 8 km and 18 km. The largest event, magnitude 2.7, occurred at 04:17:25 at a depth of 15 km. Other notable shocks included multiple magnitude 2.4 events distributed throughout the day, alongside numerous smaller events clustered between 1.3 and 2.1.

This swarm exemplifies typical seismic behavior in the region, where clusters of low-to-moderate magnitude earthquakes occur without a single dominant mainshock. Events were concentrated at mid-crustal depths, consistent with activity along local fault structures. The temporal pattern showed an initial phase of activity in the early morning hours, followed by intermittent bursts continuing into the evening.

The Brawley area lies within the Imperial Valley of southern California, a tectonically active zone at the boundary between the Pacific and North American plates. The region features the Brawley Seismic Zone, characterized by right-lateral strike-slip faulting and associated normal faults that accommodate extensional strain. This setting contributes to frequent swarm activity driven by fluid migration and stress transfer along the Imperial and San Andreas fault systems. Historical records indicate recurrent swarms in the Imperial Valley, reflecting ongoing plate-boundary deformation.

Since January 1, 2000, two prior swarms have been documented in the vicinity: one in 2000 and another in 2002. These sequences highlight the episodic nature of seismicity in the Brawley Seismic Zone, where swarms serve as indicators of localized stress accumulation rather than precursors to larger events.

References

SeismoSight internal swarm classification database
USGS Earthquake Catalog (historical context for Imperial Valley)
California Geological Survey regional tectonic summaries