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 The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizonta ...

with the

Imperial Fault The Imperial Fault Zone is a system of geological faults located in Imperial County in the Southern California region, and adjacent Baja California state in Mexico. It cuts across the border between the United States and Mexico. Geology The I ...

Southern California Southern California (commonly shortened to SoCal) is a geographic and cultural region that generally comprises the southern portion of the U.S. state of California. It includes the Los Angeles metropolitan area, the second most populous urban a ...

. The BSZ is named for the nearby town of Brawley in

Imperial County, California Imperial County is a county on the southeast border of the U.S. state of California. As of the 2020 census, the population was 179,702, making it the least populous county in Southern California. The county seat is El Centro. Imperial is the m ...

, and the seismicity there is characterized by earthquake swarms.

Geology

The Brawley Seismic Zone represents the northernmost extension of the spreading center axis associated with the

East Pacific Rise The East Pacific Rise is a mid-ocean rise (termed an oceanic rise and not a mid-ocean ridge due to its higher rate of spreading that results in less elevation increase and more regular terrain), a divergent tectonic plate boundary located alon ...

which runs up the axis of the

Gulf of California The Gulf of California ( es, Golfo de California), also known as the Sea of Cortés (''Mar de Cortés'') or Sea of Cortez, or less commonly as the Vermilion Sea (''Mar Bermejo''), is a marginal sea of the Pacific Ocean that separates the Baja C ...

and is in the process of

rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-grabe ...

ing the Baja California Peninsula away from the mainland of

Mexico Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...

, with significant subsidence taking place at southern California's Salton Sea and at Laguna Salada in Baja California. Other major locations along the axis include the Cerro Prieto spreading center located south of Mexicali, and Wagner Basin (a submarine depression in the Gulf of California). The

Salton Buttes The Salton Buttes are a group of volcanoes in California, on the Salton Sea. They consist of a -long row of five lava domes, named Mullet Island, North Red Hill, Obsidian Butte, Rock Hill and South Red Hill. They are closely associated with a ...

on the south shore of the

Salton Sea The Salton Sea is a shallow, landlocked, highly saline body of water in Riverside and Imperial counties at the southern end of the U.S. state of California. It lies on the San Andreas Fault within the Salton Trough that stretches to the Gulf ...

are on the north margin of the Brawley Seismic Zone and are linked to

volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates a ...

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 The Salton Trough is an active tectonic pull-apart basin, or graben. It lies within the Imperial, Riverside, and San Diego counties of southeastern California, United States and extends south of the Mexico–United States border into the stat ...

Earthquake swarms

On August 26, 2012 near Brawley, a

swarm Swarm behaviour, or swarming, is a collective behaviour exhibited by entities, particularly animals, of similar size which aggregate together, perhaps milling about the same spot or perhaps moving ''en masse'' or migrating in some direction. ...

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 The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, ...

(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."

Other activity

The

Imperial Valley , photo = Salton Sea from Space.jpg , photo_caption = The Imperial Valley below the Salton Sea. The US-Mexican border runs diagonally across the lower left of the image. , map_image = Newriverwatershed-1-.jpg , map_caption = Map of Imperial ...

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 In seismology, surface rupture (or ground rupture, or ground displacement) is the visible offset of the ground surface when an earthquake rupture along a fault affects the Earth's surface. Surface rupture is opposed by buried rupture, where th ...

associated with the 1979 event along the northwest trending Imperial fault from just north of the

Mexico–United States border The Mexico–United States border ( es, frontera Estados Unidos–México) is an international border separating Mexico and the United States, extending from the Pacific Ocean in the west to the Gulf of Mexico in the east. The border trave ...

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.

References

Sources * *

External links

Fault History and Architecture of the Southernmost San Andreas Fault and Brawley Seismic Zone: New Constraints from CHIRP Data Acquired in the Salton Sea
– American Geophysical Union, Fall Meeting 2006

–

Southern California Seismic Network The Southern California Seismic Network (SCSN) is a cooperative project of Caltech and the United States Geological Survey. The SCSN has benefited from numerous upgrade projects. TERRAscope, funded by the L. K. Whittier and ARCO Foundations, and NS ...

August 2012 Brawley, California Earthquake Swarm
–

{{Earthquakes in the United States Seismic faults of California Seismic faults of Mexico Geology of Imperial County, California Natural history of Baja California Brawley, California Gulf of California Salton Trough