Brawley Seismic Zone

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.^[1] The BSZ is named for the nearby town of Brawley in Imperial County, California, and the seismicity there is characterized by earthquake swarms.

Geology

Earthquake swarms

On August 26, 2012 near Brawley, a 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.^[3] 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."^[4]

Other activity

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 30 km (19 mi) 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 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]

References

1 2 Fuis, Gary S.; Mooney, Walter D. (1990), "Lithospheric structure and tectonics from seismic-refraction and other data", The San Andreas Fault System, California, Geological Survey Professional Paper 1515, United States Geological Survey, pp. 207–236
↑ David L. Alles (2011). "Geology of the Salton Trough" (PDF). Western Washington University. Retrieved September 30, 2012.
↑ Sewell, Abby (August 26, 2012). ""Earthquake storm" continues to rattle Imperial County, surrounding areas". Los Angeles Times. Retrieved August 26, 2012.
↑ Associated Press (August 26, 2012). "Series of earthquakes rattle Southern California" The Oklahoman
↑ Larsen, Shawn; Reilinger, Robert (1991). "Age constraints for the present fault configuration in the Imperial Valley, California - Evidence for northwestward propagation of the Gulf of California rift system" (PDF). Journal of Geophysical Research (American Geophysical Union) 96 (B6): 10,339–10,340. doi:10.1029/91jb00618.
↑ Bolt, Bruce (August 2005), Earthquakes: 2006 Centennial Update – The 1906 Big One (Fifth ed.), W. H. Freeman and Company, p. 61, ISBN 978-0716775485

Sources

Hauksson, E.; Stock, J.; Bilham, R.; Boese, M.; Chen, X.; Fielding, E. J.; Galetzka, J.; Hudnut, K. W.; Hutton, K.; Jones, L. M.; Kanamori, H.; Shearer, P. M.; Steidl, J.; Treiman, J.; Weia, S.; Yang, W. (March–April 2013), "Report on the August 2012 Brawley Earthquake Swarm in Imperial Valley, Southern California" (PDF), Seismological Research Letters (Seismological Society of America) 84 (2): 177–189, doi:10.1785/0220120169
Meltzner, A J.; Rockwell, T. K.; Owen, L. A. (2006), "Recent and long-term behavior of the Brawley Fault Zone, Imperial Valley, California: An escalation of slip rate?" (PDF), Bulletin of the Seismological Society of America (Seismological Society of America) 96 (6): 2304–2328, doi:10.1785/0120050233

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
List of all events for Brawley Swarm – Southern California Seismic Network
August 2012 Brawley, California Earthquake Swarm – United States Geological Survey

Seismically active fault zones and tectonic blocks of California

Statewide Faults	San Andreas Fault

Faults, zones, and blocks of Southern California	Brawley Seismic Zone Chino Fault Elsinore Fault Zone Elysian Park Fault Garlock Fault Hosgri Fault Imperial Fault Zone Laguna Salada Fault Newport–Inglewood Fault Peninsular Ranges Puente Hills Fault Raymond Fault Rose Canyon Fault Salton Trough Salinian Block San Cayetano Fault San Felipe Fault Zone San Gabriel Fault San Jacinto Fault Zone Santa Maria River Fault Santa Ynez Fault Shoreline Fault Ventura Fault White Wolf Fault Whittier Fault Yorba Linda Fault

Faults and zones of Northern California	Bartlett Springs Fault Calaveras Fault Clayton-Marsh Creek-Greenville Fault Concord Fault Hayward Fault Zone Healdsburg Fault Maacama Fault Mendocino Fracture Zone Mendocino Triple Junction Mount Diablo Thrust Fault Monta Vista Fault Pleasanton Fault San Gregorio Fault San Pablo Fault Seal Cove Fault Serra Fault Silver Creek Fault Tesla Fault West Napa Fault

Sierra Nevada faults, zones, and blocks	Death Valley Fault Zone Furnace Creek Fault Zone Honey Lake Fault Zone Kern Canyon Fault Maria fold and thrust belt Owens Valley Fault Rescue Lineament-Bear Mountains fault zone Sierra Nevada Batholith Sierra Nevada Fault Sierra Nevada-Great Valley Block Smartville Block Walker Lane

Category California Geological Survey Seismic Hazards Mapping Act Mexico (list)

Earthquakes in the United States

Historical	1638 New Hampshire 1663 Charlevoix 1700 Cascadia 1755 Cape Ann 1811–12 New Madrid 1812 Wrightwood 1857 Fort Tejon 1865 Memphis 1867 Manhattan, Kansas 1868 Hawaii 1868 Hayward 1872 Lone Pine 1872 North Cascades 1886 Charleston 1887 Sonora 1892 Laguna Salada

20th-century	1906 San Francisco 1906 Aleutian Islands 1909 Wabash River 1915 Pleasant Valley 1916 Irondale 1925 Santa Barbara 1931 Valentine 1933 Long Beach 1935 Helena 1940 El Centro 1940 New Hampshire 1944 Cornwall–Massena 1946 Aleutian Islands 1947 Wisconsin 1948 Desert Hot Springs 1949 Olympia 1952 Kern County 1957 Andreanof Islands 1957 San Francisco 1959 Hebgen Lake 1964 Alaska 1965 Rat Islands 1965 Olympia 1968 Illinois 1969 Santa Rosa 1971 San Fernando 1975 Near Islands 1975 Morris 1975 Hawaii 1979 Coyote Lake 1979 Imperial Valley 1980 Eureka 1983 Coalinga 1983 Borah Peak 1984 Morgan Hill 1986 North Palm Springs 1986 Chalfant Valley 1987 Whittier Narrows 1989 Loma Prieta 1991 Sierra Madre 1992 Cape Mendocino 1992 Landers 1992 Big Bear 1993 Scotts Mills 1993 Klamath Falls 1994 Northridge 1995 Marathon 1996 Duvall 1998 Pymatuning 1999 Hector Mine 2000 Yountville

21st-century	2001 Nisqually 2002 Denali 2003 Alabama 2003 San Simeon 2006 Gulf of Mexico 2006 Hawaii 2007 Alum Rock 2007 Andreanof Islands 2008 Illinois 2008 Chino Hills 2010 Eureka 2010 Illinois 2010 Baja California 2010 Indiana 2011 Colorado 2011 Virginia 2011 Oklahoma 2014 Aleutian Islands 2014 Southeast Alaska 2014 South Napa

Swarms	2001 Enola 2008 Reno 2010–11 Guy-Greenbrier 2012 Brawley 2009–16 Oklahoma

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