Calaveras Fault

Elements of the San Andreas Fault System in the San Francisco Bay Area

The Calaveras Fault is a major branch of the San Andreas Fault System that is located in northern California in the San Francisco Bay Area. Activity on the different segments of the fault includes moderate and large earthquakes as well as aseismic creep. The last large event was the 1984 Morgan Hill event and the last moderate earthquake was the 2007 Alum Rock event.

It is believed to link with the Hayward fault, as well as the West Napa Fault, north of the Carquinez Strait. It passes through or near the cities of Alamo, Danville, San Ramon, Dublin, Pleasanton, Sunol, Milpitas, San Jose, Gilroy, and Hollister.

Location

To the east of the Hayward-Rodgers Creek fault, the Calaveras fault extends 123 km (76 mi), splaying from the San Andreas fault near Hollister and terminating at Danville at its northern end.^[1] It runs east of the San Andreas, diverging from it in the vicinity of Hollister, California, and is responsible for the formation of the Calaveras Valley there. Between the San Andreas Fault and the Calaveras Fault lies the Hayward Fault, which diverges from the Calaveras Fault east of San Jose, California. To the east lies the Clayton-Marsh Creek-Greenville Fault. These four fault structures are some of the major faults in California at the latitude of San Francisco. All are right lateral-moving strike-slip faults.

The Calaveras Fault was named for Calaveras Creek in Santa Clara County east of San Jose where it was first identified. (Calaveras is Spanish for "skulls".) "Calaveras" is also the name of a California county in the foothills of the Sierra Nevada mountains, some 100 miles (160 km) east of Santa Clara County, far from the Calaveras Fault. Some of the cities which the Calaveras Fault passes through or near are: Alamo, Danville, San Ramon, Dublin, Pleasanton, Sunol, Milpitas, San Jose, Gilroy, and Hollister.

Related faults

The West Napa Fault in Napa County is believed to be a continuation of the Calaveras Fault north of the Carquinez Strait. Between the faults lies an area of minor faults aligned en echelon known as the Contra Costa Shear Zone.^[2]

The minor Concord Fault lies to the east of the Calaveras Fault, and small earthquakes occur in the gap between faults, mostly in the vicinity of Alamo, California, relieving stresses generated by the displacement between the two faults. Stresses are also produced by offset and converging slip-strike motions between the Calaveras and Clayton-Marsh Creek-Greenville Fault that continue to elevate Mount Diablo. The compressive pressure is manifest in a significant thrust fault nearby on the western slope of the mountain, the Mount Diablo Thrust Fault,^[3] the most active of its kind in the region and which is also capable of producing significant local earthquakes affecting the Alamo-Danville area.

Tectonic forces

The Pacific Plate is a major section of the Earth's crust, gradually expanding by the eruption of magma along the East Pacific Rise to the southeast. It is also being subducted far to the northwest into the Aleutian Trench under the North American Plate well north of San Francisco. In California, the plate is sliding northwestward along a transform boundary, the San Andreas Fault, toward the subduction zone. At the same time, the North American Plate is moving southwestward, but relatively southeast along the fault. The westward component of the North American Plate's motion results in some compressive force along the San Andreas and its associated faults such as the Calaveras Fault, thus helping lift the Coast Ranges. The Calaveras Fault shares the same relative motions of the San Andreas.

Seismic activity

USGS ShakeMaps showing intensity patterns for the 1984 Morgan Hill (left) and 2007 Alum Rock events

A number of magnitude 6 earthquakes have been recorded on the fault throughout recorded history, the largest of which was a magnitude 6.5 that occurred in 1911 in the Morgan Hill area.^[4] The most recent of these was a magnitude 6.2 earthquake near Morgan Hill in 1984.

A magnitude 5.6 earthquake occurred on the Calaveras Fault on October 30, 2007 at 20:04 PDT (October 31, at 03:04 UTC), near Alum Rock. Prior to the 2014 South Napa earthquake, it was the most powerful quake to hit the Bay Area since the Loma Prieta earthquake of 1989, and the largest on the Calaveras Fault since 1984.

Hayward Fault connection

Although it has been known for some time that the Calaveras and Hayward faults merge in the South Bay region, recent geological studies^[5] suggest that the Calaveras and Hayward faults may be even more closely connected horizontally deep beneath the surface, angling toward each other with depth until they become a single fault. If true, this would have significant implications for the potential maximum strength of earthquakes on the Hayward, since this strength is determined by the maximum length of the fault rupture and this rupture could extend beyond the juncture point to include some portion of the Calaveras.

Recent assessment

Recent (January 2008) assessments^[6] suggest that the northern Calaveras fault (the portion between Sunol and Danville) may be more likely to fail in the next few decades than previously thought.

References

↑ Working Group On California Earthquake Probabilities, "Characterization of the SFBR Earthquake Sources", Earthquake Probabilities in the San Francisco Bay Region: 2002–2031 (PDF), Open-File Report 03-214, United States Geological Survey, p. 16
↑ http://gsa.confex.com/gsa/2005CD/finalprogram/abstract_85559.htm gsa.confex.com
↑ http://www.pbs.org/newshour/indepth_coverage/science/1906quake/map6.html pbs.org
↑ "Historic Earthquakes: Calaveras fault, California". Retrieved 25 November 2008.
↑ "Hayward, Calaveras faults may be connected, geologist says", San Francisco Chronicle (December 12, 2007)
↑ Hayward fault might be more dangerous than scientists thought (San Jose Mercury News)

External links

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 Hollywood 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 Sierra Madre Fault Zone 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 Nacimiento Fault Pleasanton Fault Rinconada 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)

Significant seismically active faults of North America

North America
(crosses national borders)

Alaska-Aleutian megathrust (US-RU)
Basin and Range Province (US-MX)
Cascadia subduction zone (US-CA)
Clarendon-Linden fault system (US-CA)
Denali Fault (US-CA)
Northern Cordilleran Volcanic Province (CA-US)
Puget Sound faults (US-CA)
Rio Grande rift (US-MX)
San Andreas Fault (US-MX)
Southern Great Lakes Seismic Zone (US-CA)
Tintina Fault (US-CA)

Canada

United States

Washington and Oregon	Brothers Fault Zone Olympic-Wallowa Lineament Puget Sound (Seattle Fault, Tacoma Fault) Walker Lane
California	Calaveras Fault Garlock Fault Hayward Fault Zone Mendocino Fracture Zone Newport–Inglewood Fault Puente Hills Fault Rose Canyon Fault San Jacinto Fault Zone (Elsinore Fault Zone)
Great Basin	Independence Valley fault system Moab Fault Teton Fault Wasatch Fault
Great Plains	Central Oklahoma Fault Zone Humboldt Fault Long Point–Eureka Heights fault system Nemaha Fault Zone Wilzetta Fault
Midwest	Marianna Fault New Madrid Seismic Zone Sandwich Fault Zone Wabash Valley Seismic Zone
Appalachian Mountains and Atlantic Coast	Aiken-Augusta Fault Eastern Tennessee Seismic Zone Middleton Place-Summerville Seismic Zone (Woodstock Fault) Northern Appalachians Seismic Zone Ramapo Fault Virginia Seismic Zone

Caribbean & Mexico

Cayman Trough
Chixoy-Polochic Fault
Enriquillo–Plantain Garden fault zone (Cuba)
Gulf of California Rift Zone
Lesser Antilles subduction zone
Middle America Trench (Central America)
Motagua Fault (Central America)
Pedro Miguel Fault
Puerto Rico Trench

San Francisco Bay Area
Bodies of water	Bodega Bay Carquinez Strait Clifton Forebay Golden Gate Grizzly Bay Guadalupe River Half Moon Bay Lake Berryessa Napa River Oakland Estuary Petaluma River Richardson Bay Richmond Inner Harbor Russian River Sacramento River San Francisco Bay San Leandro Bay San Pablo Bay Sonoma Creek Suisun Bay Tomales Bay
Counties	Alameda Contra Costa Marin Napa San Francisco San Mateo Santa Clara Solano Sonoma
Major cities	San Jose San Francisco Oakland
Cities and towns 100k–250k	Antioch Berkeley Concord Daly City Fairfield Fremont Hayward Richmond Santa Clara Santa Rosa Sunnyvale Vallejo
Cities and towns 50k–99k	Alameda Brentwood Castro Valley Cupertino Livermore Milpitas Mountain View Napa Novato Palo Alto Petaluma Pittsburg Pleasanton Redwood City San Leandro San Mateo San Rafael San Ramon South San Francisco Union City Vacaville Walnut Creek
Cities and towns 25k-50k	Belmont Benicia Burlingame Campbell Danville Dublin East Palo Alto Foster City Gilroy Los Altos Los Gatos Martinez Menlo Park Morgan Hill Newark Oakley Pacifica Pleasant Hill Rohnert Park San Bruno San Carlos San Pablo Saratoga Suisun City Windsor
Cities and towns 10k–25k	Alamo Albany Alum Rock American Canyon Ashland Bay Point Cherryland Clayton Discovery Bay Dixon El Cerrito El Sobrante Emeryville Fairview Half Moon Bay Healdsburg Hercules Hillsborough Lafayette Larkspur Millbrae Mill Valley Moraga North Fair Oaks Orinda Piedmont Pinole San Anselmo San Lorenzo Sonoma Stanford Tamalpais-Homestead Valley
Sub-regions	East Bay North Bay San Francisco Peninsula Silicon Valley South Bay
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