The 1940 El Centro earthquake (or 1940 Imperial Valley earthquake) occurred at 21:35

Pacific Standard Time The Pacific Time Zone (PT) is a time zone encompassing parts of western Canada, the western United States, and western Mexico. Places in this zone observe standard time by subtracting eight hours from Coordinated Universal Time ( UTC−08:00 ...

on May 18 (05:35 UTC on May 19) in the

Imperial Valley , photo = Salton Sea from Space.jpg , photo_caption = The Imperial Valley below the Salton Sea. The US-Mexican border runs diagonally across the lower left of the image. , map_image = Newriverwatershed-1-.jpg , map_caption = Map of Imperial ...

in southeastern

Southern California Southern California (commonly shortened to SoCal) is a geographic and cultural region that generally comprises the southern portion of the U.S. state of California. It includes the Los Angeles metropolitan area, the second most populous urban a ...

near the

international border Borders are usually defined as geographical boundaries, imposed either by features such as oceans and terrain, or by political entities such as governments, sovereign states, federated states, and other subnational entities. Political borders c ...

of the United States and

Mexico Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...

. It had a moment magnitude of 6.9 and a maximum perceived intensity of X (''Extreme'') on the Mercalli intensity scale. It was the first major earthquake to be recorded by a strong-motion seismograph located next to a fault rupture. The earthquake was characterized as a typical moderate-sized destructive event with a complex energy release signature. It was the strongest recorded earthquake to hit the Imperial Valley, and caused widespread damage to irrigation systems and led to the deaths of nine people.

Tectonic setting

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 stat ...

is part of the complex plate boundary between the Pacific Plate and 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 Pacif ...

where it undergoes a transition from the continental transform of the

San Andreas Fault The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizonta ...

system to the series of short

spreading center A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diver ...

s of 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 alon ...

linked by oceanic transforms in the

Gulf of California The Gulf of California ( es, Golfo de California), also known as the Sea of Cortés (''Mar de Cortés'') or Sea of Cortez, or less commonly as the Vermilion Sea (''Mar Bermejo''), is a marginal sea of the Pacific Ocean that separates the Baja C ...

. The two main right lateral strike-slip fault strands that extend across the southern part of the trough are the

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 ...

/ Laguna Salada Fault to the western side of the trough and 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 I ...

to the east. The Imperial Fault is linked to the San Andreas Fault through the

Brawley Seismic Zone The Brawley Seismic Zone (BSZ), also known as the Brawley fault zone, is a predominantly extensional tectonic zone that connects the southern terminus of the San Andreas Fault with the Imperial Fault in Southern California. The BSZ is named for ...

, which is a spreading center beneath the southern end of 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 ...

. With 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 i ...

to the west, the Elsinore fault to the south-southwest, and the Imperial fault centered directly under the Imperial Valley, the area frequently encounters seismic activity, including moderate and damaging earthquakes. Other events in 1852, 1892,

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 ...

, 1942,

1979 Events January * January 1 ** United Nations Secretary-General Kurt Waldheim heralds the start of the '' International Year of the Child''. Many musicians donate to the '' Music for UNICEF Concert'' fund, among them ABBA, who write the so ...

, and

1987 File:1987 Events Collage.png, From top left, clockwise: The MS Herald of Free Enterprise capsizes after leaving the Port of Zeebrugge in Belgium, killing 193; Northwest Airlines Flight 255 crashes after takeoff from Detroit Metropolitan Airport, ...

have impacted the region in varying degrees.

Earthquake

The earthquake was the result of a rupture along the Imperial Fault, with its

epicenter The epicenter, epicentre () or epicentrum in seismology is the point on the Earth's surface directly above a hypocenter or focus, the point where an earthquake or an underground explosion originates. Surface damage Before the instrumental pe ...

north of

Calexico, California 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 ...

. A strong secondary quake with a magnitude of 5.5 followed a little more than an hour after the main shock and was centered near Brawley. In an interview with the ''

Los Angeles Times The ''Los Angeles Times'' (abbreviated as ''LA Times'') is a daily newspaper that started publishing in Los Angeles in 1881. Based in the LA-adjacent suburb of El Segundo since 2018, it is the sixth-largest newspaper by circulation in the U ...

'' shortly after the earthquake, Dr.

Beno Gutenberg Beno Gutenberg (; June 4, 1889 – January 25, 1960) was a German-American seismologist who made several important contributions to the science. He was a colleague and mentor of Charles Francis Richter at the California Institute of Technolog ...

, a geophysicist and professor at the Caltech Seismological Laboratory in Pasadena, prematurely reported that the source of the quake was the San Jacinto Fault and compared the earthquake to the

1933 Long Beach earthquake The 1933 Long Beach earthquake took place on March 10 at south of downtown Los Angeles. The epicenter was offshore, southeast of Long Beach, California, on the Newport–Inglewood Fault. The earthquake had a magnitude estimated at 6.4 , and a ...

saying "The energy of the Imperial Vally shock was less, and it should have done less damage, than the one of 1933, even had it occurred in an area of greater population." The Imperial Fault had not been previously differentiated from the other faults in the area by geologists until after the earthquake occurred.

Damage

The event caused significant damage in the towns of Brawley, Imperial, El Centro, Calexico and

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 1,000,0 ...

and was responsible for the deaths of nine people. Imperial appeared to receive the most damage and was described as "razed" with at least 80% of the buildings in that city being damaged, and at least two people being killed there. It was felt as far afield as

Los Angeles Los Angeles ( ; es, Los Ángeles, link=no , ), often referred to by its initials L.A., is the List of municipalities in California, largest city in the U.S. state, state of California and the List of United States cities by population, sec ...

and

Tucson, Arizona , "(at the) base of the black ill , nicknames = "The Old Pueblo", "Optics Valley", "America's biggest small town" , image_map = , mapsize = 260px , map_caption = Interactive map ...

The earthquake also caused substantial damage to irrigation systems over a very wide area of the valley, with the canals of the

Imperial Irrigation District The Imperial Irrigation District (IID) is an irrigation district that serves the Imperial Valley in Southern California. Established under the State Water Code, the IID supplies roughly of Imperial Valley farmland with raw Colorado River water ...

being breached in multiple locations. With ninety percent of the damage done to the canals south of the international border, water rationing became necessary in Brawley and Imperial during the days following the earthquake spanning the restoration period. The canals brought water from the Mexican side of the border to the United States and supplied 3000 miles of irrigation waterways and ditches in the Imperial Valley. The primary trunkline was the

Alamo Canal The Alamo Canal ( es, Canal del Álamo) was a long waterway that connected the Colorado River to the head of the Alamo River. The canal was constructed to provide irrigation to the Imperial Valley. A small portion of the canal was located in th ...

and it sustained nine breaks, each several hundred feet in length, along a twenty-mile stretch between Sharp's heading and Cudahy check. In Mexico, a long wooden

flume A flume is a human-made channel for water, in the form of an open declined gravity chute whose walls are raised above the surrounding terrain, in contrast to a trench or ditch. Flumes are not to be confused with aqueducts, which are built to t ...

was completely destroyed south of Mexicali where the Solfatara canal crossed the New River, and water storage tanks were destroyed at Imperial and Holtville. The railroad system also suffered damage, with bent rails and damage to several bridges. The heaviest damage in Mexicali, the capital of the State of Baja California, was a fire which destroyed a hotel there. The fire was blamed on a short circuit.

Strong motion

A strong-motion seismograph at El Centro recorded the earthquake and provided the first example of such a recording made very close to a fault rupture in a major earthquake. This gave a detailed record of the different types of shaking associated with the earthquake. It is often used in design of earthquake-proof structures today, particularly for the time history analysis method. The recording showed that the earthquake consisted of several sub-events, with a total of 13 being recorded in just over five minutes. A 1970 report on the earthquake in the ''

Bulletin of the Seismological Society of America ''Bulletin of the Seismological Society of America'' (BSSA) is a bimonthly peer reviewed scientific journal published by the Seismological Society of America. The editor-in-chief is Thomas Pratt (U. S. Geological Survey). The journal covers seis ...

'' defined an event as "any occurrence of energy release which generates seismic phases that can be resolved and identified" then stated that the size of those discrete events were abnormally strong for regular aftershocks. The majority of the energy released during the earthquake occurred in the first fifteen seconds, although significant energy was released as late as the last of those events, near the 5:20 mark on the seismograph record.

Surface rupture

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 th ...

was formed during the earthquake of , with a maximum recorded displacement of close to the border. The sense of movement along the rupture was almost pure strike-slip, with no vertical displacement seen. During the

1979 Imperial Valley earthquake The 1979 Imperial Valley earthquake occurred at 16:16 Pacific Daylight Time (23:16 UTC) on 15 October just south of the Mexico–United States border. It affected Imperial Valley in Southern California and Mexicali Valley in northern Baja Calif ...

the same section of the fault ruptured on the US side of the border, but this time there was no sign of rupture on the Mexican side. The displacement pattern of the two earthquakes was very similar on the US side, suggesting that the Imperial Fault slips in discrete patches. Two of these patches are thought to have ruptured in 1940 but only the northern one in 1979.

Aftermath

Soon after the earthquake, reconstruction efforts were concentrated on repairing the Alamo and Solfatara irrigation canals to save the

cantaloupe The cantaloupe, rockmelon (Australia and New Zealand, although cantaloupe is used in some states of Australia), sweet melon, or spanspek (Southern Africa) is a melon that is a variety of the muskmelon species (''Cucumis melo'') from the fami ...

crop, and the water supply was restored in less than four days.

References

Sources * * * *

External links

* {{Earthquakes in the United States

1940 A calendar from 1940 according to the Gregorian calendar, factoring in the dates of Easter and related holidays, cannot be used again until the year 5280. Events Below, the events of World War II have the "WWII" prefix. January * Januar ...

El C EL, El or el may refer to: Religion * El (deity), a Semitic word for "God" People * EL (rapper) (born 1983), stage name of Elorm Adablah, a Ghanaian rapper and sound engineer * El DeBarge, music artist * El Franco Lee (1949–2016), American po ...

Earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...

Geology of Imperial County, California Natural history of Baja California

1940 disasters in North America