Calaveras Fault

[[File:Calaveras Fault Creep.png|thumb|Calaveras Fault creep in downtown Hollister in April 2009]] 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 magnitude 6.2 [[1984 Morgan Hill earthquake|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 earthquake|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, California|Alamo]], [[Danville, California|Danville]], [[San Ramon, California|San Ramon]], [[Dublin, California|Dublin]], [[Pleasanton, California|Pleasanton]], [[Sunol, California|Sunol]], [[Milpitas, California|Milpitas]], [[San Jose, California|San Jose]], [[Gilroy, California|Gilroy]], and [[Hollister, California|Hollister]].

==Location== [[File:Major California faults and ages of volcanics.png|thumb|260px|[[Geological map]] of the Calaveras Fault]] To the east of the Hayward-Rodgers Creek fault, the Calaveras fault extends {{convert|123|km|abbr=on|order=flip}}, splaying from the San Andreas fault near Hollister and terminating at Danville at its northern end.<ref>{{citation|title=Earthquake Probabilities in the San Francisco Bay Region: 2002–2031|chapter-url=https://pubs.usgs.gov/of/2003/of03-214/OFR-03-214_FullText.pdf|author=Working Group on California Earthquake Probabilities|publisher=United States Geological Survey|chapter=Characterization of the SFBR Earthquake Sources|series=Open-File Report 03-214|page=16}}</ref> (As of March 1978 it was believed that they did not quite connect.)<ref>{{Cite web |date=March 1978 |title=A STUDY OF FAULT PLANE MECHANISMS PRECEDING THE THANKSGIVING DAY, 1974 EARTHQUAKE AT HOLLISTER, CALIFORNIA |url=https://pubs.usgs.gov/of/1978/0255/report.pdf |access-date=11 Oct 2025 |website=pubs.usgs.gov}}</ref> 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 Zone|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 [[Fault (geology)|faults]] in California at the [[latitude]] of [[San Francisco]]. All are right lateral-moving strike-slip faults.

[[File:122-38HaywardFault.jpg|thumb|300px|Elements of the San Andreas Fault System in the San Francisco Bay Area]] The Calaveras Fault was named for [[Calaveras Creek (California)|Calaveras Creek]] (in [[Santa Clara County, California|Santa Clara County]], east of San Jose), where it was first identified. ("Calaveras" is also the name of an unrelated [[Calaveras County, California|California county]] in the western foothills of the [[Sierra Nevada]] mountains, some {{convert|100|mi|abbr=on}} east of Santa Clara County, far from the Calaveras Fault.) Some of the cities which the Calaveras Fault passes through or near are: [[Alamo, California|Alamo]], [[Danville, California|Danville]], [[San Ramon, California|San Ramon]], [[Dublin, California|Dublin]], [[Pleasanton, California|Pleasanton]], [[Sunol, California|Sunol]], [[Milpitas, California|Milpitas]], [[San Jose, California|San Jose]], [[Gilroy, California|Gilroy]], and [[Hollister, California|Hollister]].

==Related faults== The [[West Napa Fault]] in [[Napa County, California|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 [[Echelon formation#Geology|''en echelon'']] known as the Contra Costa Shear Zone.<ref>[http://gsa.confex.com/gsa/2005CD/finalprogram/abstract_85559.htm http://gsa.confex.com/gsa/2005CD/finalprogram/abstract_85559.htm] ''gsa.confex.com''</ref>

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]],<ref>[https://www.pbs.org/newshour/indepth_coverage/science/1906quake/map6.html https://www.pbs.org/newshour/indepth_coverage/science/1906quake/map6.html] {{Webarchive|url=https://web.archive.org/web/20121102221326/http://www.pbs.org/newshour/indepth_coverage/science/1906quake/map6.html |date=2012-11-02 }} PBS</ref> 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== {{multiple image | align = right | image1 = USGS Shakemap - 1984 Morgan Hill earthquake.jpg | image2 = CISN Shakemap - 2007 Alum Rock earthquake.jpg | footer = USGS ShakeMaps showing intensity patterns for the 1984 Morgan Hill (left) and 2007 Alum Rock events }}

Here are the notable recorded earthquakes on the Calaveras fault: 2022 (5.1), [[2007 Alum Rock earthquake|2007 (5.6)]], [[1984 Morgan Hill earthquake|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, California|Morgan Hill]] area.<ref>{{cite web |url=https://earthquake.usgs.gov/earthquakes/states/events/1911_07_01.php |title=Historic Earthquakes: Calaveras fault, California |access-date=25 November 2008 |archive-url=https://web.archive.org/web/20091110071300/http://earthquake.usgs.gov/earthquakes/states/events/1911_07_01.php |archive-date=2009-11-10 |url-status=dead }}</ref> The most recent of these was a magnitude 6.2 earthquake near [[1984 Morgan Hill earthquake|Morgan Hill in 1984]].

A [[2007 Alum Rock earthquake|magnitude 5.6 earthquake]] occurred on the Calaveras Fault on October 30, 2007, at 20:04 [[Pacific Time Zone#Daylight saving time|PDT]] (October 31, at 03:04 [[Coordinated Universal Time|UTC]]), near [[Alum Rock, San Jose|Alum Rock]]. Prior to the [[2014 South Napa earthquake]], it was the most powerful quake to hit the Bay Area since the [[1989 Loma Prieta earthquake]], 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]].<ref>{{cite web |title=M 5.1 – 15 km ESE of Alum Rock, CA |url=https://earthquake.usgs.gov/earthquakes/eventpage/nc73799091/executive |archive-url=https://web.archive.org/web/20221025184758/https://earthquake.usgs.gov/earthquakes/eventpage/nc73799091/executive |archive-date=2022-10-25 |publisher=United States Geological Survey |access-date=26 October 2022}}</ref>

==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<ref>{{Cite web|url=https://www.sfgate.com/bayarea/article/Hayward-Calaveras-faults-may-be-connected-3234475.php|title=Hayward, Calaveras faults may be connected, geologist says|first=David|last=Perlman|date=December 12, 2007|website=San Francisco Chronicle}}</ref> 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, California|Sunol]] and [[Danville, California|Danville]]) may be more likely to fail in the next few decades than previously thought.<ref>[http://www.mercurynews.com/news/ci_8028422 ''Hayward fault might be more dangerous than scientists thought''] (''San Jose Mercury News'')</ref>

==See also== *[[Bernal Subbasin]] *[[Tesla Fault]]

==References== <references/>

==Further reading== * {{cite journal | last1=Chaussard | first1=E. | last2=Bürgmann | first2=R. | last3=Fattahi | first3=H. | last4=Johnson | first4=C. W. | last5=Nadeau | first5=R. | last6=Taira | first6=T. | last7=Johanson | first7=I. | title=Interseismic coupling and refined earthquake potential on the Hayward-Calaveras fault zone | journal=Journal of Geophysical Research: Solid Earth | volume=120 | issue=12 | year=2015 | issn=2169-9313 | doi=10.1002/2015jb012230 | pages=8570–8590| doi-access=free | bibcode=2015JGRB..120.8570C | url=https://authors.library.caltech.edu/records/64pcy-n1945/files/Chaussard_et_al-2015-Journal_of_Geophysical_Research__Solid_Earth.pdf?download=1 }} * {{cite journal | last1=Harris | first1=R. A. | last2=Barall | first2=M. | last3=Lockner | first3=D. A. | last4=Moore | first4=D. E. | last5=Ponce | first5=D. A. | last6=Graymer | first6=R. W. | last7=Funning | first7=G. | last8=Morrow | first8=C. A. | last9=Kyriakopoulos | first9=C. | last10=Eberhart-Phillips | first10=D. | title=A Geology and Geodesy Based Model of Dynamic Earthquake Rupture on the Rodgers Creek-Hayward-Calaveras Fault System, California | journal=Journal of Geophysical Research: Solid Earth | volume=126 | issue=3 | year=2021 | article-number=e2020JB020577 | issn=2169-9313 | doi=10.1029/2020jb020577| doi-access=free | bibcode=2021JGRB..12620577H | url=https://escholarship.org/content/qt953625x2/qt953625x2.pdf?t=qoh57h }} * {{cite journal | last1=McClellan | first1=P. H. | last2=Hay | first2=E. A. | title=Triggered slip on the Calaveras Fault during the magnitude 7.1 Loma Prieta, California, Earthquake | journal=Geophysical Research Letters | volume=17 | issue=8 | year=1990 | issn=0094-8276 | doi=10.1029/gl017i008p01227 | pages=1227–1230| bibcode=1990GeoRL..17.1227M }}

==External links== *[https://www.youtube.com/watch?v=-mY9c-xC2fM M6.9 Earthquake on the Calaveras Fault] – [[Southern California Earthquake Center]] *[https://web.archive.org/web/20141011070221/http://resilience.abag.ca.gov/earthquakes/ ABAG shaking hazard maps] *[https://web.mst.edu/~rogersda/forensic_geology/calaveras_fault/calaveras_fault.htm Tracking the Elusive Calavaras Fault from Sunol to San Ramon]

[[Category:Seismic faults of California]] [[Category:Strike-slip faults]] [[Category:Geology of Contra Costa County, California]] [[Category:Geology of San Benito County, California]] [[Category:Geology of Santa Clara County, California]] [[Category:Diablo Range]]  [[Category:Danville, California]] [[Category:San Ramon, California]] [[Category:Dublin, California]] [[Category:Geography of Pleasanton, California]] [[Category:Milpitas, California]] [[Category:Geography of San Jose, California]] [[Category:Gilroy, California]] [[Category:Hollister, California]] [[Category:Geography of the San Francisco Bay Area]]