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The 1940 El Centro earthquake (or 1940 Imperial Valley earthquake) occurred at 21:35 Pacific Standard Time on May 18 (05:35 UTC on May 19) in the Imperial Valley in southeastern
Southern California Southern California (commonly shortened to SoCal) is a geographic and Cultural area, cultural List of regions of California, region that generally comprises the southern portion of the U.S. state of California. Its densely populated coastal reg ...
near the international border of the United States and
Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...
. It had a moment magnitude of 6.9 and a maximum perceived intensity of X (''Extreme'') on the
Mercalli intensity scale The Modified Mercalli intensity scale (MM, MMI, or MCS) measures the effects of an earthquake at a given location. This is in contrast with the seismic magnitude usually reported for an earthquake. Magnitude scales measure the inherent force or ...
. 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 in the Imperial Valley, causing widespread damage to irrigation systems and killing nine people.


Tectonic setting

The Salton Trough is part of the complex plate boundary between the Pacific plate and the North American plate where it undergoes a transition from the continental transform of the San Andreas Fault system to the series of short spreading centers of the East Pacific Rise linked by oceanic transforms in the Gulf of California. The two main right lateral strike-slip fault strands that extend across the southern part of the trough are the Elsinore Fault Zone/ Laguna Salada Fault to the western side of the trough and the Imperial Fault to the east. The Imperial Fault is linked to the San Andreas Fault through the Brawley seismic zone, which is a spreading center beneath the southern end of the Salton Sea. With the San Jacinto Fault Zone 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, 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 song ...
, and
1987 Events January * January 1 – Bolivia reintroduces the Boliviano currency. * January 2 – Chadian–Libyan conflict – Battle of Fada: The Military of Chad, Chadian army destroys a Libyan armoured brigade. * January 3 – Afghan leader ...
have impacted the region in varying degrees.


Earthquake

The earthquake was the result of a rupture along the Imperial Fault, with its epicenter north of Calexico, California. A strong secondary earthquake with a magnitude of 5.5 followed a little more than an hour after the mainshock and was centered near Brawley. In an interview with the ''
Los Angeles Times The ''Los Angeles Times'' is an American Newspaper#Daily, daily newspaper that began publishing in Los Angeles, California, in 1881. Based in the Greater Los Angeles city of El Segundo, California, El Segundo since 2018, it is the List of new ...
'' shortly after the earthquake, Dr. Beno Gutenberg, a geophysicist and professor at the Caltech Seismological Laboratory in Pasadena, prematurely reported that the source of the earthquake 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 ...
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 States of Mexico, Mexican state of Baja California. The city, which is the seat of the Mexicali Municipality, has a population of 689,775, according to the 2010 census, while the Calexico–Mexicali, Cale ...
and was responsible for nine fatalities. Imperial appeared to receive the most damage and was described as "razed" with at least 80 percent of its buildings damaged and at least two people killed. It was felt as far afield as
Los Angeles Los Angeles, often referred to by its initials L.A., is the List of municipalities in California, most populous city in the U.S. state of California, and the commercial, Financial District, Los Angeles, financial, and Culture of Los Angeles, ...
and
Tucson, Arizona Tucson (; ; ) is a city in Pima County, Arizona, United States, and its county seat. It is the second-most populous city in Arizona, behind Phoenix, Arizona, Phoenix, with a population of 542,630 in the 2020 United States census. The Tucson ...
. 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 and a large portion of the eastern and southern Coachella Valley in the Colorado Desert region of Southern California. Established under the Stat ...
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 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 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 (geologist), Thomas Pratt (U. S. Geological Survey ...
'' 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

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


See also

* 1892 Laguna Salada earthquake * List of earthquakes in 1940 *
List of earthquakes in California A list is a set of discrete items of information collected and set forth in some format for utility, entertainment, or other purposes. A list may be memorialized in any number of ways, including existing only in the mind of the list-maker, bu ...
* List of earthquakes in Mexico * List of earthquakes in the United States


References

Sources * * * *


External links

* {{Earthquakes in the United States 1940 1940 El C
Earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...
El C El C El C
Earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...
Earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...
Geology of Imperial County, California Natural history of Baja California
Earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...
Earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...
Earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...
Earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...
Earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...
El Centro earthquake El Centro earthquake