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Southern California Swarm

March 21, 2009, M4.8 Earthquake

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On Saturday, March 21, 2009, an earthquake swarm began near Bombay Beach, near the southern end of the San Andreas fault on the eastern side of the Salton Sea in southern California. Within the first week of the swarm, over 350 earthquakes were recorded, the largest of which was a magnitude 4.8 event that occurred on March 24 at 4:55 AM PST. It was felt by over 400 people within the San Diego region. The M4.8 moment tensor solution displays strike-slip motion, with the preferred nodal plane strike of N57E. This solution is consistent with a fault or faults orthogonal to the San Andreas fault, located in the northern Brawley Seismic Zone.

The NNW-trending Brawley Seismic Zone overlies an inferred short spreading center segment between the San Andreas and Imperial (transform) faults in the southern Salton Trough. Precisely relocated seismicity in this zone indicates that it consists of a series of left-stepping, left-lateral faults. From 1932 to 2008, 722 events were recorded within a 5-km radius of this current swarm. Prior to this sequence, the largest event recorded in this region was M4.2 on 11/13/99, which was triggered by the Hector Mine earthquake on 10/16/1999. For more information, visit