The "Brawley Seismic Zone" ("BSZ"), also known as the "Brawley fault zone", is a predominantly extensional tectonic zone that connects the southern terminus of the San Andreas Fault with the Imperial Fault in Southern California.
The Salton Buttes on the south shore of the Salton Sea are on the north margin of the Brawley Seismic Zone and are linked to volcanic and geothermal activity within the zone. The Brawley Seismic Zone has been interpreted as an "onshore spreading center" which runs diagonally across the Salton Trough.
On August 26, 2012 near Brawley, California, a Earthquake swarm of more than 300 small to moderate earthquakes occurred with the largest two reaching a maximum of 5.3 and 5.5. Some light cosmetic damage was found on some older buildings in downtown Brawley. Earthquake swarms are not unusual for the area. In 2005, also near Brawley, there was a swarm with a peak magnitude of 5.1, while in 1981 a swarm included one quake which was measured at 5.8. The swarm activity is not understood completely, but the new restlessness could generate data that will help scientists to gain a better understanding of the region. According to a geologist at the United States Geological Survey (USGS), such swarms are typical for the Brawley zone: "The area sees lots of events at once, with many close to the largest magnitude, rather than one main shock with several much smaller aftershocks."
The Imperial Valley of southern California experiences high rates of seismicity with a pronounced portion of the activity occurring within the Brawley Seismic Zone that separates the San Andreas fault to the north and the Imperial fault to the south. The Imperial fault was the source of the 1940 El Centro earthquake and the 1979 Imperial Valley earthquake. There was more than of surface rupture associated with the 1979 event along the northwest trending Imperial fault from just north of the Mexico-United States border to an area south of Brawley and the BSZ was found to have ruptured, with surface cracks, for a length of . The 1940 and 1979 mainshocks were of similar size and both were followed by damaging aftershocks near Brawley.