TheBrawley Seismic Zone (BSZ), also known as theBrawley fault zone, is a predominantlyextensional tectonic zone that connects the southern terminus of theSan Andreas Fault with theImperial Fault inSouthern California.[1] The BSZ is named for the nearby town ofBrawley inImperial County, California, and the seismicity there is characterized by earthquake swarms.
The Brawley Seismic Zone represents the northernmost extension of the spreading center axis associated with theEast Pacific Rise which runs up the axis of theGulf of California and is in the process ofrifting theBaja California peninsula away from the mainland ofMexico, with significantsubsidence taking place at southern California's Salton Sea and atLaguna Salada in Baja California. Other major locations along the axis include theCerro Prieto spreading center located south of Mexicali, andWagner Basin (a submarine depression in the Gulf of California).[2]
TheSalton Buttes on the south shore of theSalton Sea are on the north margin of the Brawley Seismic Zone and are linked tovolcanic andgeothermal activity within the zone. The Brawley Seismic Zone has been interpreted as an "onshore spreading center" which runs diagonally across theSalton Trough.[1]
On August 26, 2012, nearBrawley, aswarm 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.[3] According to a geologist at theUnited 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."[4]
TheImperial 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 the1940 El Centro earthquake and the1979 Imperial Valley earthquake. There was more than 30 km (19 mi) ofsurface rupture associated with the 1979 event along the northwest trending Imperial fault from just north of theMexico–United States border to an area south of Brawley and the BSZ was found to have ruptured, with surface cracks, for a length of 13 km (8.1 mi).[5] The 1940 and 1979 mainshocks were of similar size and both were followed by damaging aftershocks near Brawley.[6]
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