Seismic Swarm S20030524.1: Analysis of Activity Near Brawley, California

Seismic swarm S20030524.1 occurred 5 km south-southwest of Brawley in Imperial County, California. The sequence began at 15:39 UTC on 23 May 2003 and concluded at 13:39 UTC on 25 May 2003, producing 161 earthquakes over 46 hours. This event represents one of two swarms recorded in the region during 2003 and fits within a broader pattern of five swarms documented since 1 January 2000.

The Brawley area lies within the Imperial Valley, a tectonically active zone at the southern termination of the San Andreas Fault system. Here, the Pacific-North American plate boundary transitions through a series of right-lateral strike-slip faults, including the Imperial Fault and the Brawley Seismic Zone. The valley floor consists of thick sedimentary deposits overlying basement rock, with geothermal fields and the nearby Salton Sea contributing to elevated heat flow that influences local seismicity. Earthquake swarms in this setting commonly arise from fluid migration along fault networks rather than from a single mainshock-aftershock sequence.

Analysis of the first 100 events reveals a typical swarm signature: low-to-moderate magnitudes clustered in time and space. Magnitudes ranged from 1.2 to 4.1, with the largest event (magnitude 4.1) occurring at 02:04 UTC on 24 May at a depth of 16 km. Depths for these events concentrated between 12 km and 16 km, consistent with the brittle-ductile transition zone in the regional crust. Early activity included several events of magnitude 2.5–3.5 within the first eight hours, followed by a steady rate of smaller shocks. No events exceeded magnitude 4.1 in the initial 100 recordings.

Historical context shows recurrent swarm behavior in the Brawley area. Prior episodes since 2000 comprised two swarms in 2000, one in 2002, and a second swarm in 2003. Such clustering reflects the influence of transtensional tectonics and possible magmatic or hydrothermal processes at depth. The 2003 swarm aligns with this pattern, underscoring the Imperial Valley’s elevated seismic hazard relative to surrounding regions.

Overall, swarm S20030524.1 exemplifies the persistent microseismicity that characterizes the Imperial Valley. Continued monitoring remains essential for understanding fault interactions and assessing potential links to larger events along the San Andreas and Imperial fault systems.

References

• United States Geological Survey Earthquake Catalog
• California Geological Survey Regional Fault Maps
• SeismoSight internal swarm classification records