The Calaveras Fault is a major branch of the

San Andreas Fault The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonics, tectonic boundary between the Pacific Plate and the North American Plate, and its motion is Fault (geology)#Strike-slip fau ...

System that is located in northern

California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...

in the

San Francisco Bay Area The San Francisco Bay Area, often referred to as simply the Bay Area, is a populous region surrounding the San Francisco, San Pablo, and Suisun Bay estuaries in Northern California. The Bay Area is defined by the Association of Bay Area Go ...

. Activity on the different segments of the fault includes moderate and large earthquakes as well as

aseismic creep In geology, aseismic creep or fault creep is measurable surface displacement along a fault in the absence of notable earthquakes. Aseismic creep may also occur as "after-slip" days to years after an earthquake. Notable examples of aseismic slip in ...

. The last large event was the magnitude 6.2 1984 Morgan Hill event. The most recent moderate earthquakes were the magnitude 5.1 event on 25 October 2022, and the magnitude 5.6 2007 Alum Rock event. It is believed to link with the Hayward fault, as well as the

West Napa Fault The West Napa Fault is a long geologic fault in Napa County, in the North Bay (San Francisco Bay Area), North Bay region of the San Francisco Bay Area in northern California. It is believed to be the northern extension of the Calaveras Fault in th ...

, north of the Carquinez Strait. It passes through or near the cities of Alamo, Danville, San Ramon,

Dublin Dublin (; , or ) is the capital and largest city of Republic of Ireland, Ireland. On a bay at the mouth of the River Liffey, it is in the Provinces of Ireland, province of Leinster, bordered on the south by the Dublin Mountains, a part of th ...

, Pleasanton, Sunol,

Milpitas Milpitas (Spanish for "little milpas") is a city in Santa Clara County, California, in Silicon Valley. As of the 2020 census, the city population was 80,273. The city's origins lie in Rancho Milpitas, granted to Californio ranchero José Marí ...

, San Jose, Gilroy, and Hollister.

Location

Major California faults and ages of volcanics

To the east of the Hayward-Rodgers Creek fault, the Calaveras fault extends , splaying from the San Andreas fault near Hollister and terminating at Danville at its northern end. It runs east of the San Andreas, diverging from it in the vicinity of

Hollister, California Hollister is a city in and the county seat of San Benito County, located in the Central Coast region of California. With a 2020 United States census population of 41,678, Hollister is one of the largest cities in the Monterey Bay Area and a ...

, and is responsible for the formation of the

Calaveras Valley Calaveras Valley is a valley east of Milpitas, California in the San Francisco Bay Area. It has formed primarily as a result of the actions of the Calaveras Fault. The southern end of the Calaveras Valley is a few miles south of Calaveras Reservoir ...

there. Between the San Andreas Fault and the Calaveras Fault lies the

Hayward Fault The Hayward Fault Zone is a right-lateral strike-slip geologic fault zone capable of generating destructive earthquakes. This fault is about long, situated mainly along the western base of the hills on the east side of San Francisco Bay. It runs ...

, which diverges from the Calaveras Fault east of

San Jose, California San Jose, officially San José (; ; ), is a major city in the U.S. state of California that is the cultural, financial, and political center of Silicon Valley and largest city in Northern California by both population and area. With a 2020 popul ...

. To the east lies the

Clayton-Marsh Creek-Greenville Fault The Clayton-Marsh Creek-Greenville Fault is a fault located in the eastern San Francisco Bay Area of California, in Alameda County and Contra Costa County. It is part of the somewhat parallel system of faults that are secondary to the San Andreas ...

. These four fault structures are some of the major faults in California at the

latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...

San Francisco San Francisco (; Spanish language, Spanish for "Francis of Assisi, Saint Francis"), officially the City and County of San Francisco, is the commercial, financial, and cultural center of Northern California. The city proper is the List of Ca ...

. All are right lateral-moving strike-slip faults. The Calaveras Fault was named for Calaveras Creek in

Santa Clara County Santa Clara County, officially the County of Santa Clara, is the sixth-most populous county in the U.S. state of California, with a population of 1,936,259, as of the 2020 United States Census, 2020 census. Santa Clara County and neighboring Sa ...

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 The Sierra Nevada () is a mountain range in the Western United States, between the Central Valley of California and the Great Basin. The vast majority of the range lies in the state of California, although the Carson Range spur lies primarily ...

mountains, some 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,

, Pleasanton, Sunol,

, San Jose, Gilroy, and Hollister.

Related faults

The

Napa County Napa County () is a county north of San Pablo Bay located in the northern portion of the U.S. state of California. As of the 2020 census, the population was 138,019. The county seat is the City of Napa. Napa County was one of the original c ...

is believed to be a continuation of the Calaveras Fault north of the

Carquinez Strait The Carquinez Strait (; Spanish: ''Estrecho de Carquinez'') is a narrow tidal strait in Northern California. It is part of the tidal estuary of the Sacramento and the San Joaquin rivers as they drain into the San Francisco Bay. The strait is lo ...

. Between the faults lies an area of minor faults aligned ''en echelon'' known as the Contra Costa Shear Zone. The minor

Concord Fault The Concord Fault is a geologic fault in the San Francisco Bay Area. The reason it is called that is because it is located under the city of Concord. It is connected to, and considered to be part of, the same fault zone as the Green Valley 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 Alamo (Spanish: ''Álamo''; meaning "Poplar tree") is an unincorporated community and census-designated place (CDP) in Contra Costa County, California, in the United States. It is a suburb located in the San Francisco Bay Area's East Bay re ...

, 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 Mount Diablo is a mountain of the Diablo Range, in Contra Costa County, California, Contra Costa County of the eastern San Francisco Bay Area in Northern California. It is south of Clayton, California, Clayton and northeast of Danville, Califo ...

. The compressive pressure is manifest in a significant

thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...

nearby on the western slope of the mountain, the Mount Diablo Thrust Fault, 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 The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and Iza ...

is a major section of the Earth's crust, gradually expanding by the eruption of magma along 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 along ...

to the southeast. It is also being subducted far to the northwest into the

Aleutian Trench The Aleutian Trench (or Aleutian Trough) is an oceanic trench along a convergent plate boundary which runs along the southern coastline of Alaska and the Aleutian islands. The trench extends for from a triple junction in the west with the Ulakh ...

under the

North American Plate The North American Plate is a tectonic plate covering most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. With an area of , it is the Earth's second largest tectonic plate, behind the Pacific ...

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

Here are the notable recorded earthquakes on the Calaveras fault: 2022 (5.1), 2007 (5.6), 1984 (6.2) and 1911 (6.5). 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 Morgan Hill is a city in Santa Clara County, California, at the southern tip of Silicon Valley, in the San Francisco Bay Area. Morgan Hill is an affluent residential community, the seat of several high-tech companies, and a dining, entertainmen ...

area. 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. On October 25, 2022 at 11:42 PDT (October 25, at 18:42 UTC), a magnitude 5.1 earthquake occurred on the fault near

Lick Observatory The Lick Observatory is an astronomical observatory owned and operated by the University of California. It is on the summit of Mount Hamilton, in the Diablo Range just east of San Jose, California, United States. The observatory is managed by th ...

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 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

Assessments in January 2008 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. ''Hayward fault might be more dangerous than scientists thought''
(''San Jose Mercury News'')

References

External links

M6.9 Earthquake on the Calaveras Fault
—

Southern California Earthquake Center The Southern California Earthquake Center (SCEC) is a collaboration of more than 1,000 scientists across 100 research institutions with a mission to: conduct research on earthquakes in Southern California and elsewhere by gathering data, conductin ...

ABAG shaking hazard maps
{{SF Bay Area Seismic faults of California Strike-slip faults Geology of Contra Costa County, California Geology of San Benito County, California Geology of Santa Clara County, California Diablo Range Danville, California San Ramon, California Dublin, California Geography of Pleasanton, California Milpitas, California Geography of San Jose, California Gilroy, California Hollister, California Geography of the San Francisco Bay Area