Map of notable earthquakes in California since 1900– (map data)

The earliest known earthquake in theU.S. state ofCalifornia was documented in 1769 by the Spanish explorers and Catholic missionaries of thePortolá expedition as they traveled northward fromSan Diego along theSanta Ana River near the present site ofLos Angeles. Ship captains and other explorers also documented earthquakes. AsSpanish missions were constructed beginning in the late 18th century, earthquake records were kept. After the missions weresecularized in 1834, records were sparse until theCalifornia gold rush in the 1840s. From 1850 to 2004, there was about one potentially damaging event per year on average, though many of these did not cause serious consequences or loss of life.^[1][2]

Since the three damaging earthquakes that occurred in theAmerican Midwest and theUnited States East Coast (1755 Cape Ann,1811–1812 New Madrid,1886 Charleston) were well known, it became apparent tosettlers that the earthquake hazard was different in California. While the1812 San Juan Capistrano,1857 Fort Tejon, and1872 Owens Valley shocks were in mostly unpopulated areas and only moderately destructive, the1868 Hayward event affected the thrivingfinancial hub of theSan Francisco Bay Area, with damage fromSanta Rosa in the north toSanta Cruz in the south. By this time, scientists were well aware of the threat, butseismology was still in its infancy. Following destructive earthquakes in the late 19th and early 20th centuries,real estate developers,press, andboosters minimized and downplayed the risk of earthquakes out of fear that the ongoingeconomic boom would be negatively affected.^[3][4]

According to seismologistCharles Richter, the1906 San Francisco earthquake moved theUnited States Government into acknowledging the problem. Prior to that, no agency was specifically focused on researching earthquake activity. TheUnited States Weather Bureau did record when they happened and severalUnited States Geological Survey scientists had briefly disengaged from their regular duties of mappingmineral resources to write reports on the New Madrid and Charleston events, but no trained geologists were working on the problem until after 1906 when theUnited States Coast and Geodetic Survey was made responsible. The outlook improved when ProfessorAndrew Lawson brought the state'sfirst monitoring program online at theUniversity of California, Berkeley in 1910 with seismologistHarry Wood, who was later instrumental in getting theCaltech Seismological Laboratory inPasadena operational in the 1920s.^[3][5]

Early developments at theCaltech lab included an earthquake observation network using their own custom-built short-periodseismometers, theRichter scale, and theModified Mercalli intensity scale (an updated version of the Mercalli intensity scale). In 1933, theLong Beach earthquake occurred in a populated area and damaged or destroyed many public school buildings in Long Beach and Los Angeles. Some decades later, theSan Fernando earthquake affected theSan Fernando Valley north of Los Angeles with heavy damage to several hospitals. In both cases, the perception of California policy makers changed, and state laws andbuilding codes were modified (with much debate) to require commercial and residential properties to be built towithstand earthquakes. Higher standards were established for fire stations, hospitals, and schools, and construction of dwellings was also restricted nearactive faults.^[4][5]

During thelast 66 million years, nearly the entire west coast of North America has been dominated by asubduction zone, with theFarallon plate subducting beneath theNorth American plate. Presently, theJuan de Fuca plate (with itsExplorer andGorda satellite plates) and theRivera andCocos plates are the only remnants of the once much larger Farallon plate. The plate margin that remains in California is that of thestrike-slipSan Andreas Fault (SAF), thediffusePacific–North American plate boundary that extends east into theBasin and Range Province ofeastern California andwestern Nevada (a seismically active area calledWalker Lane) and southwest into the California Continental Borderland region off the central and southern coasts. This system offaults terminates in the north at theMendocino triple junction, one of the most seismically active regions in the state, where earthquakes are occasionally the result ofintraplate deformation within the Gorda plate. It terminates in the south at theSalton Sea where displacement transitions to a series ofspreading centers andtransform faults, beginning with theBrawley seismic zone in theImperial Valley.^[6]

In theSan Francisco Bay Area, the San Andreas system of faults spans offshore and into the East Bay area, with the bulk of the faults lying to the east of the main San Andreas fault (SAF). There is a 70% probability that one of these faults will generate a 6.7 M_w or greater earthquake before 2030, including theHayward Fault Zone, which has gone beyond its averagereturn period of 130 years (157 years ago as of February 2026). While the SAF is quiet north ofSan Francisco, the central SAF segment nearSan Juan Bautista is whereaseismic creep was first studied, and to the south is where the recurringParkfield earthquakes occur. The secondary faults lay to the west of the main SAF at the extreme southern portion, including the active and youngSan Jacinto Fault Zone, which may be taking over as the primary boundary south ofCajon Pass. Apaleoseismic investigation usingLidar revealed that more than 16 feet (5 m) of slip has accumulated since the1857 event on the southern SAF, which borders theMojave Desert to the north and east of the Greater Los Angeles Area. Near theTransverse Ranges,reverse andthrust faults have produced damaging earthquakes inSanta Barbara and theSan Fernando Valley.^[6]

• Alquist Priolo Special Studies Zone Act
• California earthquake forecast
• Field Act
• Geography of California
• Geology and geological history of California
• Southern California Earthquake Center
• Timeline of the Portolá expedition

Sources

• Rodda, P. U.; Leviton, A. E. (1983), "Nineteenth Century Earthquake Investigations in California",Earth Sciences History,2 (1):48–56,Bibcode:1983ESHis...2...48R,doi:10.17704/eshi.2.1.576065779g862057,JSTOR 24135726
• Wood, H. O. (1916), "The earthquake problem in the western United States",Bulletin of the Seismological Society of America,6 (4):197–217,Bibcode:1916BuSSA...6..197W,doi:10.1785/BSSA0060040197,S2CID 130712233

Wikimedia Commons has media related toEarthquakes in California.

Look upearthquake in Wiktionary, the free dictionary.

• Information by Region-California – USGS
• Southern California Earthquake Data Center
• Putting Down Roots in Earthquake Country – Earthquake Country Alliance
• Broadband Seismic Data Collection Center –University of California, San Diego
• What Will Happen After The 'Big One' Hits SoCal? –KCET
• Little-known quake, tsunami hazards lurk offshore of Southern California –American Geophysical Union
• California’s other drought: A major earthquake is overdue –The Conversation
• New California bill aims to create a public inventory of collapse-risk buildings –Temblor, Incorporated

• Earthquakes in California
• California history-related lists
• California nature-related lists
• Natural disasters in California
• Lists of earthquakes in the United States
• Tsunamis in the United States

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