Seismic Swarm S20090924.1 Near Brawley, California
The seismic swarm designated S20090924.1 occurred 2 km northwest of Brawley, California, within the Imperial Valley. This region lies at the southern terminus of the San Andreas Fault system, where the Pacific and North American plates interact along a complex network of strike-slip and normal faults. The Brawley Seismic Zone, a right-stepping offset between the San Andreas and Imperial faults, experiences frequent microseismicity driven by tectonic extension, geothermal fluid circulation, and sediment loading from the adjacent Salton Sea.
The swarm initiated at 02:24 UTC on 24 September 2009 and concluded at 22:04 UTC the same day, spanning 19 hours and 39 minutes. During this interval, 28 earthquakes were recorded, with magnitudes ranging from 1.0 to 2.1 and focal depths between 5 km and 17 km. The largest events reached magnitude 2.1 and occurred at depths of 6 km, 6 km, and 14 km. Most activity clustered at shallow to mid-crustal levels around 6–11 km, consistent with the brittle-ductile transition zone in this tectonically active rift setting.
Earthquake swarms in the Imperial Valley differ from typical mainshock-aftershock sequences by exhibiting a gradual increase and decrease in event rate without a single dominant rupture. They often reflect pore-pressure changes from hydrothermal fluids migrating along fault planes. Historical records maintained by SeismoSight indicate 18 swarms in the area since 1 January 2000. These occurred in the following years and quantities: 2000 (2), 2002 (1), 2003 (4), 2004 (1), 2005 (3), 2008 (4), and 2009 (3). The 2009 activity, including S20090924.1, aligns with this established pattern of episodic swarm behavior.
The Imperial Valley’s geology features thick sequences of Quaternary sediments overlying crystalline basement, with active geothermal fields nearby that contribute to elevated heat flow and reduced frictional strength on faults. Past swarms in this zone have occasionally preceded larger tectonic events, underscoring the importance of continuous monitoring for hazard assessment.
References
SeismoSight internal swarm catalog (S20090924.1 parameters and historical statistics).
United States Geological Survey Earthquake Hazards Program reports on the Brawley Seismic Zone.
Southern California Earthquake Data Center regional seismicity summaries.