1968 Borrego Mountain earthquake
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The 1968 Borrego Mountain earthquake occurred on April 8, at 18:28 PST, near the unincorporated community of
Ocotillo Wells Ocotillo Wells is an unincorporated area, unincorporated community in San Diego County, California, United States. It is west of the Imperial County, California, Imperial County line on California State Route 78 at an elevation of . The name be ...
in
San Diego County San Diego County (), officially the County of San Diego, is a county in the southwestern corner of the U.S. state of California. As of the 2020 census, the population was 3,298,634, making it California's second-most populous county and the fi ...
. The
moment magnitude The moment magnitude scale (MMS; denoted explicitly with or Mw, and generally implied with use of a single M for magnitude) is a measure of an earthquake's magnitude ("size" or strength) based on its seismic moment. It was defined in a 1979 pape ...
() 6.6 strike-slip earthquake struck with a
focal depth In seismology, the depth of focus or focal depth refers to the depth at which an earthquake occurs. Earthquakes occurring at a depth of less than are classified as shallow-focus earthquakes, while those with a focal depth between and are commonl ...
of . Damage was relatively moderate, and the mainshock was assigned a maximum
Modified Mercalli intensity The Modified Mercalli intensity scale (MM, MMI, or MCS), developed from Giuseppe Mercalli's Mercalli intensity scale of 1902, is a seismic intensity scale used for measuring the intensity of shaking produced by an earthquake. It measures the eff ...
(MMI) of VII (''Very strong''). Shaking was felt in
Nevada Nevada ( ; ) is a U.S. state, state in the Western United States, Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. N ...
, and
Arizona Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a state in the Southwestern United States. It is the 6th largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix. Arizona is part of the Fou ...
. It was the largest earthquake to strike California since 1952, and its display of afterslip became the subject of scientific interest.


Tectonic setting

A simplified tectonic bounday map of the Salton Trough area. Red lines show major faults and arrows indicate fault motion.
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 ...
(SAF) is the main
plate boundary Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
that defines the margin between the
Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...
and
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 ...
s in California. It is believed to have formed during the
Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...
. The fault has a length of , of which, it is visible for from the
Salton Sea The Salton Sea is a shallow, landlocked, highly saline body of water in Riverside and Imperial counties at the southern end of the U.S. state of California. It lies on the San Andreas Fault within the Salton Trough that stretches to the Gulf o ...
to
Point Arena Point Arena, formerly known as Punta Arena (Spanish language, Spanish for "Sandy Point") is a small coastal city in Mendocino County, California, Mendocino County, California, United States. Point Arena is located west of Hopland, California, H ...
. Divided into four distinct segments, it displays right-lateral strike-slip movement. It accommodates 20–75 percent of plate motion between the Pacific and North American plates its segments. The tectonic boundary in Southern California is complex—plate motion is accommodated by the SAF and a network of subparallel faults. The SAF terminates at the
Salton Trough The Salton Trough is an active tectonic pull-apart basin, or graben. It lies within the Imperial, Riverside, and San Diego counties of southeastern California, United States and extends south of the Mexico–United States border into the state of ...
, a
transtension Transtension is the state in which a rock mass or area of the Earth's crust experiences both ''extensive'' and ''transtensive'' shear. As such, transtensional regions are characterised by both extensional structures ( normal faults, grabens) and wr ...
al zone (
pull-apart basin In geology, a basin is a region where subsidence generates accommodation space for the deposition of sediments. A pull-apart basin is a structural basin where two overlapping (en echelon) strike-slip faults or a fault bend creates an area of cru ...
) that separates it from the
Imperial Fault The Imperial Fault Zone is a system of geological faults located in Imperial County in the Southern California region, and adjacent Baja California state in Mexico. It cuts across the border between the United States and Mexico. Geology The Im ...
in the south. The exact measurement of slip across faults in this zone is poorly understood.


California Borderland

West of the Salton Trough is a largely offshore fault system that accommodates approximately 20 percent of the plate motion. Faults such as the
Rose Canyon Fault The Rose Canyon Fault is a right-lateral strike-slip fault running in a north-south direction through San Diego County, California.Hart, E.W., Bryant, W.A., Wills, C.J., Treiman, J.A., and Kahle, J.E."Summary Report: Fault Evaluation Program, 1987 ...
and the Newport-Inglewood Fault pose large risks to unprepared coastal communities such as San Diego and Los Angeles.


San Jacinto Fault Zone

East of the California Borderland, the plate boundary is a complex zone of faults that run parallel to the SAF. The two main faults are the
San Jacinto Fault Zone The San Jacinto Fault Zone (SJFZ) is a major strike-slip fault zone that runs through San Bernardino, Riverside, San Diego, and Imperial Counties in Southern California. The SJFZ is a component of the larger San Andreas transform system and is ...
(SJFZ) and
Elsinore Fault Zone The Elsinore Fault Zone is a large right-lateral strike-slip geological fault structure in Southern California. The fault is part of the trilateral split of the San Andreas fault system and is one of the largest, though quietest faults in Souther ...
. The SJFZ is a complex, highly segmented, and overlapped fault zone that runs parallel to the San Andreas Fault, but separated by the
San Jacinto Mountains The San Jacinto Mountains (''Avii Hanupach''Munro, P., et al. ''A Mojave Dictionary''. Los Angeles: UCLA. 1992. in Mojave) are a mountain range in Riverside County, located east of Los Angeles in southern California in the United States. The mou ...
. It is located on the eastern Salton Trough, and runs directly beneath the cities of
San Bernardino San Bernardino (; Spanish language, Spanish for Bernardino of Siena, "Saint Bernardino") is a city and county seat of San Bernardino County, California, United States. Located in the Inland Empire region of Southern California, the city had a ...
, Colton, San Jacinto and
Hemet Hemet is a city in the San Jacinto Valley in Riverside County, California. It covers a total area of , about half of the valley, which it shares with the neighboring city of San Jacinto. The population was 89,833 at the 2020 census. The foundin ...
. Segments of the SJFZ are given names despite being one fault system. These segments include the Coyote Creek (CCF), Superstition Hills (SHF), and Superstition Mountain (SMF) faults. The CCF is estimated to be -long and displays right-lateral strike-slip displacement. Considered the most active fault in Southern California, 36 notable earthquakes have been associated with it since 1857. Between
1915 Events Below, the events of World War I have the "WWI" prefix. January * January – British physicist Sir Joseph Larmor publishes his observations on "The Influence of Local Atmospheric Cooling on Astronomical Refraction". *January 1 ...
and 1954, five earthquakes of magnitude 6.0 or greater were damaging. Seismic activity on the SJFZ is greater than on the SAF.


Eastern California Shear Zone

Further east, near the border with Nevada and Arizona, the
Eastern California Shear Zone Eastern may refer to: Transportation *China Eastern Airlines, a current Chinese airline based in Shanghai *Eastern Air, former name of Zambia Skyways *Eastern Air Lines, a defunct American airline that operated from 1926 to 1991 *Eastern Air Li ...
(ECSZ) takes up to 25 percent of the plate motion. The ECSZ consists of north–west trending right-lateral faults in the
Mojave Desert The Mojave Desert ( ; mov, Hayikwiir Mat'aar; es, Desierto de Mojave) is a desert in the rain shadow of the Sierra Nevada mountains in the Southwestern United States. It is named for the indigenous Mojave people. It is located primarily in ...
, and
Walker Lane The Walker Lane is a geologic trough roughly aligned with the California/Nevada border southward to where Death Valley intersects the Garlock Fault, a major left lateral, or sinistral, strike-slip fault. The north-northwest end of the Walker La ...
, which lies at the western margin of the
Basin and Range Province The Basin and Range Province is a vast physiographic region covering much of the inland Western United States and northwestern Mexico. It is defined by unique basin and range topography, characterized by abrupt changes in elevation, alternating ...
. Large earthquakes associated with the ECSZ occurred in
1872 Events January–March * January 12 – Yohannes IV is crowned Emperor of Ethiopia in Axum, the first ruler crowned in that city in over 500 years. * February 2 – The government of the United Kingdom buys a number of forts on ...
,
1992 File:1992 Events Collage V1.png, From left, clockwise: 1992 Los Angeles riots, Riots break out across Los Angeles, California after the Police brutality, police beating of Rodney King; El Al Flight 1862 crashes into a residential apartment buildi ...
,
1999 File:1999 Events Collage.png, From left, clockwise: The funeral procession of King Hussein of Jordan in Amman; the 1999 İzmit earthquake kills over 17,000 people in Turkey; the Columbine High School massacre, one of the first major school shootin ...
and
2019 File:2019 collage v1.png, From top left, clockwise: Hong Kong protests turn to widespread riots and civil disobedience; House of Representatives votes to adopt articles of impeachment against Donald Trump; CRISPR gene editing first used to experim ...
.


Earthquake

The only recorded
foreshock A foreshock is an earthquake that occurs before a larger seismic event (the mainshock) and is related to it in both time and space. The designation of an earthquake as ''foreshock'', ''mainshock'' or aftershock is only possible after the full sequ ...
had a magnitude of 3.7, occurring one minute before the mainshock. There was no foreshock activity recorded in the hours to weeks before the mainshock; seismic activity in the area was lower than usual in the four months before April 1968. The mainshock which measured 6.6 , was the result of shallow
strike-slip In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
faulting which initiated from the
hypocenter In seismology, a hypocenter or hypocentre () is the point of origin of an earthquake or a subsurface nuclear explosion. A synonym is the focus of an earthquake. Earthquakes An earthquake's hypocenter is the position where the strain energy s ...
at depth. It ruptured bilaterally along the CCF and displayed an almost pure right-lateral
focal mechanism The focal mechanism of an earthquake describes the deformation in the source region that generates the seismic waves. In the case of a fault-related event it refers to the orientation of the fault plane that slipped and the slip vector and is ...
. A focal mechanism analysis indicated the rupture
plane Plane(s) most often refers to: * Aero- or airplane, a powered, fixed-wing aircraft * Plane (geometry), a flat, 2-dimensional surface Plane or planes may also refer to: Biology * Plane (tree) or ''Platanus'', wetland native plant * Planes (gen ...
had a northwest strike and dipped steeply (80°) to the south.


Surface rupture

A -long
surface rupture In seismology, surface rupture (or ground rupture, or ground displacement) is the visible offset of the ground surface when an earthquake rupture along a fault affects the Earth's surface. Surface rupture is opposed by buried rupture, where the ...
through
Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...
alluvium Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluv ...
and
lake A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...
bed sediments was revealed. At its northern extreme, the rupture sliced through the Palm Spring Formation. The rupture zone consisted of two northwest-oriented segments, separated by a -wide discontinuity. Smaller, isolated ruptures were found away from the main trace. A maximum horizontal offset of was measured along the northern rupture located northwest of Ocotillo Wells, at the foothills of Borrego Mountain. Along the southern rupture, the maximum offset was , measured southeast of Ocotillo Wells. Vertical offsets of up to were also recorded. Unusually, there were left-lateral displacements from Ocotillo Badlands north of Highway 78 and at the northern base of Borrego Mountain. Whether these left-lateral offsets were part of the rupture mechanism or environmental changes unrelated to tectonic processes could not be determined.


Aftershocks

A one-year-long
aftershock In seismology, an aftershock is a smaller earthquake that follows a larger earthquake, in the same area of the main shock, caused as the displaced crust adjusts to the effects of the main shock. Large earthquakes can have hundreds to thousand ...
sequence followed; at least 135 aftershocks measuring 3.0 or greater was recorded. Most of the aftershocks were located away and subparallel to the northwest-southeast trending rupture. In the zone of aftershocks, the mainshock epicenter was located in the middle. The concentration of aftershocks was greater in the southeast of the mainshock than to the northwest. These aftershocks were predominantly right-lateral strike-slip events. Several recorded aftershocks had some sense of
dip-slip In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
faulting. There were aftershocks being reported close to the SMF and SHF at the southeastern extremity of the rupture. A magnitude 5.2 aftershock occurred at 19:03. Forty-five minutes later, a magnitude 4.7 aftershock caused minor damage in Calexico. Nearly a year later, a 5.8 aftershock was recorded. This shock had its own sequence of aftershocks.


Intensity

The maximum
peak ground acceleration Peak ground acceleration (PGA) is equal to the maximum ground acceleration that occurred during earthquake shaking at a location. PGA is equal to the amplitude of the largest absolute acceleration recorded on an wikt:accelerogram, accelerogram at a ...
(pga) recorded by a
seismometer A seismometer is an instrument that responds to ground noises and shaking such as caused by earthquakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The outpu ...
at El Centro was 0.14 ''g''. A maximum MMI of VII (''Very strong'') was assigned in the Borrego Mountain–Ocotillo Wells area (northeastern San Diego County) where surface rupturing occurred. Based on the study of ground effects alone, however, the MMI could be as high as IX (''Violent''). Severe damage was restricted to within a area, but the earthquake was felt for . In this zone, small offsets occurred along the Coyote Creek Fault and Highway 78 near Ocotillo Wells cracked. At
Anza-Borrego Desert State Park Anza-Borrego Desert State Park (, '' AN-zə bə-RAY-goh'') is a California State Park located within the Colorado Desert of southern California, United States. The park takes its name from 18th century Spanish explorer Juan Bautista de Anza and ...
, large boulders toppled. At Split Mountain, falling rocks damaged many vehicles belonging to campers. Large boulders also blocked the Montezuma-Borrego Highway.
Rockfall A rockfall or rock-fallWhittow, John (1984). ''Dictionary of Physical Geography''. London: Penguin, 1984. . is a quantity/sheets of rock that has fallen freely from a cliff face. The term is also used for collapse of rock from roof or walls of min ...
s, slumps and
liquefaction In materials science, liquefaction is a process that generates a liquid from a solid or a gas or that generates a non-liquid phase which behaves in accordance with fluid dynamics. It occurs both naturally and artificially. As an example of the ...
took place as a result of the
strong ground motion In seismology, strong ground motion is the strong earthquake shaking that occurs close to (less than about 50 km from) a causative fault. The strength of the shaking involved in strong ground motion usually overwhelms a seismometer, forci ...
. Heavier damage included cracked and fragmented concrete bridge piers. There was minor damage in Ocotillo Wells but a house had its walls split apart and bedroom detached from the main structure. A storage tank beside the house spilled 3,600 gallons of water over the porch, toppling its posts. Ground cracks appeared at the airport and roads. Residents were temporarily without water due to a damaged community
well A well is an excavation or structure created in the ground by digging, driving, or drilling to access liquid resources, usually water. The oldest and most common kind of well is a water well, to access groundwater in underground aquifers. The ...
pump. A motel west of Ocotillo Wells sustained broken water and sewerage pipes, cracked tiles and its lower floor was flooded by water from the swimming pool. Large
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s from Ocotillo Wells were displaced, snapping
anchor bolt Anchor bolts are used to connect structural and non-structural elements to concrete.. The connection can be made by a variety of different components: anchor bolts (also named fasteners), steel plates, or stiffeners. Anchor bolts transfer diffe ...
s and
X-bracing X-bracing is a structural engineering practice where the lateral load on a building is reduced by transferring the load into the exterior columns. X-bracing was used in the construction of the 1908 Singer Building, then the tallest building in t ...
s. A long rolling motion of up to thirty seconds was described. Swimming pools sloshed about for ten minutes. Plasters fell from the walls and ceilings on the second level of the Balboa Hotel in
El Centro El Centro (Spanish for "The Center") is a city and county seat of Imperial County, California, United States. El Centro is the largest city in the Imperial Valley, the east anchor of the Southern California Border Region, and the core urban are ...
. In
Calexico Calexico () is a city in southern Imperial County, California. Situated on the Mexican border, it is linked economically with the much larger city of Mexicali, the capital of the Mexican state of Baja California. It is about east of San Diego ...
, the ceiling of a
Safeway Safeway is an American supermarket chain founded by Marion Barton Skaggs in April 1915 in American Falls, Idaho. The chain provides grocery items, food and general merchandise and features a variety of specialty departments, such as bakery, d ...
supermarket partially collapsed. The upper brick wall of a
laundromat A self-service laundry, coin laundry, laundromat, or coin wash is a facility where clothes are washed and dried without much personalized professional help. They are known in the United Kingdom as launderettes or laundrettes, and in the Unit ...
in
Westmorland Westmorland (, formerly also spelt ''Westmoreland'';R. Wilkinson The British Isles, Sheet The British IslesVision of Britain/ref> is a historic county in North West England spanning the southern Lake District and the northern Dales. It had an ...
collapsed, while another building was cracked. MMI VI (''Strong'') shaking frightened many residents. Damage was minimal due to the area's sparse
urban planning Urban planning, also known as town planning, city planning, regional planning, or rural planning, is a technical and political process that is focused on the development and design of land use and the built environment, including air, water, ...
. Shifting furnitures; falling objects; rocking vehicles, trees and bushes; and minor cracks were reported. Minor rockslides occurred, including some at
Barrett Dam Barrett Dam is a concrete arch-gravity dam in southern San Diego County, California in the United States, forming Barrett Lake on Cottonwood Creek. The dam is part of the city of San Diego's local water supply system. Overview and operations Bar ...
. At
Borrego Springs Borrego Springs (''Borrego'', Spanish for "Sheep") is a census-designated place (CDP) in San Diego County, California. The population was 3,429 at the 2010 census, up from 2,535 at the 2000 census, made up of both seasonal and year-round reside ...
, minor cracks appeared in the facade of a church. Cracks also appeared in the ground and on windows. Furniture moved several inches while swimming pools sloshed. Goods and items fell off shelves in stores, forcing some to shut down. In
Mecca Mecca (; officially Makkah al-Mukarramah, commonly shortened to Makkah ()) is a city and administrative center of the Mecca Province of Saudi Arabia, and the Holiest sites in Islam, holiest city in Islam. It is inland from Jeddah on the Red ...
, several concrete pipelines ruptured and a truck nearly overturned. In
Anza Anza, Anzah, or de Anza might refer to: Communities United States * Anza, California, a town in Riverside County, California * Anza, Imperial County, California, a town in Imperial County, California, along California State Route 111 * Camp Anz ...
concrete floorings cracked and plasters fell from buildings. In a
public library A public library is a library that is accessible by the general public and is usually funded from public sources, such as taxes. It is operated by librarians and library paraprofessionals, who are also Civil service, civil servants. There are ...
at Imperial, 7,500 books fell from shelves. Cracked concrete pavements and driveways occurred in Yuma and Horn,
Arizona Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a state in the Southwestern United States. It is the 6th largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix. Arizona is part of the Fou ...
. At
San Diego San Diego ( , ; ) is a city on the Pacific Ocean coast of Southern California located immediately adjacent to the Mexico–United States border. With a 2020 population of 1,386,932, it is the List of United States cities by population, eigh ...
, a small crack appeared in a concrete
retaining wall Retaining walls are relatively rigid walls used for supporting soil laterally so that it can be retained at different levels on the two sides. Retaining walls are structures designed to restrain soil to a slope that it would not naturally keep to ...
. Roughly north of downtown San Diego, furnitures were displaced. Broken windows, severed powerlines, and sloshing of swimming pools were widely reported. Cracks appeared along Sunset Cliffs Boulevard and plasters detached from building. At
Mexicali Mexicali (; ) is the capital city of the Mexican state of Baja California. The city, seat of the Mexicali Municipality, has a population of 689,775, according to the 2010 census, while the Calexico–Mexicali metropolitan area is home to 1,000, ...
,
Baja California Baja California (; 'Lower California'), officially the Free and Sovereign State of Baja California ( es, Estado Libre y Soberano de Baja California), is a state in Mexico. It is the northernmost and westernmost of the 32 federal entities of Mex ...
, windows broke and lights went out at a sheriff office. Landslides occurred at
Sage Sage or SAGE may refer to: Plants * ''Salvia officinalis'', common sage, a small evergreen subshrub used as a culinary herb ** Lamiaceae, a family of flowering plants commonly known as the mint or deadnettle or sage family ** ''Salvia'', a large ...
. Grocery stores in Riverside suffered significant losses from fallen bottled products. The , which was docked at
Long Beach Long Beach is a city in Los Angeles County, California. It is the 42nd-most populous city in the United States, with a population of 466,742 as of 2020. A charter city, Long Beach is the seventh-most populous city in California. Incorporate ...
rocked for five minutes. In Los Angeles, two structures built before revised earthquake codes sustained minor damage such as the widening or reopening of plaster cracks formed by the
1933 Events January * January 11 – Sir Charles Kingsford Smith makes the first commercial flight between Australia and New Zealand. * January 17 – The United States Congress votes in favour of Philippines independence, against the wis ...
and 1952 earthquakes.
Plaster Plaster is a building material used for the protective or decorative coating of walls and ceilings and for Molding (decorative), moulding and casting decorative elements. In English, "plaster" usually means a material used for the interiors of ...
s fell off buildings in the city.


Post-earthquake slips

After the earthquake, the fault displayed a phenomenon known 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 ...
, observed only along the central and southernmost section of the rupture. It was discovered on June 9 by the manager of a
motel A motel, also known as a motor hotel, motor inn or motor lodge, is a hotel designed for motorists, usually having each room entered directly from the parking area for motor vehicles rather than through a central lobby. Entering dictionaries ...
at Ocotillo Wells. Aseismic creep increased the total horizontal displacement from to , and vertical displacement from to , two months after the mainshock. While the central section of the CCF rupture experienced aseismic creep, there was no movement along the southernmost rupture until from January 1969 to December 1970. There was no feasible way of measuring these new offsets as tire tracks, used for measuring, had disappeared. It is estimated that the post-earthquake slip produced of additional displacement, in addition to the during the earthquake. After a pair of earthquakes in 1987, new surface ruptures with of displacement were observed. These new surface ruptures measured .


Triggered slips

A remarkable feature was slip along faults located far from the epicenter area. The SAF,
Imperial Imperial is that which relates to an empire, emperor, or imperialism. Imperial or The Imperial may also refer to: Places United States * Imperial, California * Imperial, Missouri * Imperial, Nebraska * Imperial, Pennsylvania * Imperial, Texa ...
, and Superstition Hills faults displayed this phenomenon. Field observations revealed of right-lateral displacement occurred along these faults. Triggered slips were not observed on other prominent faults such as segments of the SJFZ north of the CCF, the SMF that lies parallel to the SHF, and the Elsinore Fault Zone.
Geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, althou ...
s postulated the slip was shaking-induced, ruling out stress transfer as a cause. Slip continued until 1972. Evidence of movement along the Imperial Fault ( from epicenter) was discovered on April 13 on
Interstate 8 Interstate 8 (I-8) is an Interstate Highway in the southwestern United States. It runs from the southern edge of Mission Bay at Sunset Cliffs Boulevard in San Diego, California, almost at the Pacific Ocean, to the junction with I-10, ...
when cracks appeared. However, the cracks were not well determined as there were already cracks to the road from an earthquake in March 1966 (the magnitude 3.6 earthquake is the smallest earthquake associated with a surface rupture). The Imperial Fault was the first to be discovered creeping which prompted checks on other faults. Creep occurred for , although its actual length is unknown as
dunes A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, fl ...
and urban developments obstructed any possible rupture trace. It produced an estimated of right-lateral slip. Along the SHF, of displacement was measured at Imler Road ( from epicenter). A section of the SHF moved. The SHF also produced creep after the 1987 earthquakes. Movement along the Southern California segment of the SAF ( northeast of the epicenter) was documented on April 24. Right-lateral displacement of and vertical scarps as high as were measured. Slip occurred for . The last major earthquake on this section of the SAF occurred in 1680, with an estimated magnitude of 7.8.


See also

*
List of earthquakes in 1968 This is a list of earthquakes in 1968. Only magnitude 6.0 or greater earthquakes appear on the list. Lower magnitude events are included if they have caused death, injury or damage. Events which occurred in remote areas will be excluded from the ...
*
List of earthquakes in California The earliest known California earthquake was documented in 1769 by the Spanish explorers and Catholic missionaries of the Portolá expedition as they traveled northward from San Diego along the Santa Ana River near the present site of Los Angeles ...
*
List of earthquakes in the United States The following is a list of notable earthquakes and tsunamis which had their epicenter in areas that are now part of the United States with the latter affecting areas of the United States. Those in ''italics'' were not part of the United States whe ...


References

Sources *


External links

* {{Earthquakes in the United States 1968 earthquakes Earthquakes in California 1968 natural disasters in the United States Geology of San Diego County, California Imperial Valley Natural history of Baja California 1968 in California Strike-slip earthquakes Anza-Borrego Desert State Park History of Imperial County, California