1940 El Centro earthquake

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, and 1987 have impacted the region in varying degrees.

Earthquake

The earthquake was the result of a rupture along the Imperial Fault, with its epicenter 5 mi 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 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 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 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 It was felt as far afield as Los Angeles and Tucson, Arizona.

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 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 427 m 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.

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 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 40 -, with a maximum recorded displacement of 4.5 m close to the border. The sense of movement along the rupture was almost pure strike-slip, with no vertical displacement seen.

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.

References

Sources

References

1. {{harvnb. Stover. Coffman. 1993
2. Hough, S.E.. (2004). "Finding fault in California: an earthquake tourist's guide". Mountain Press Publishing.
3. {{harvnb. Trifunac. Brune. 1970
4. Mueller, K.J.. (1995). "Late quaternary activity of the Laguna Salada fault in northern Baja California, Mexico". Geological Society of America Bulletin.
5. Elizabeth Varin. (April 2, 2011). "Historic Imperial County earthquakes". [[Imperial Valley Press]].
6. "Imperial Valley Earthquake". Southern California Earthquake Data Center.
7. (May 20, 1940). "Shock Lighter Than in 1933; Scientist Lays Temblor to Earth Movement Along San Jacinto Fault". Los Angeles Times.
8. (1982). "The Imperial Valley, California, Earthquake of October 15, 1979". [[United States Government Printing Office]].
9. {{harvnb. Trifunac. Brune. 1970
10. (May 19, 1940). "Quake Razes Imperial; Three Killed; Brawley and El Centro Damaged by Temblor; All Southland Jarred". Los Angeles Times.
11. (May 21, 1940). "Quake Zone Acts to Solve Water Crisis; Imperial Valley Rationed as Crews Start Repairs on Nine Canal Breaks". Los Angeles Times.
12. {{harvnb. Stover. Coffman. 1993
13. (1970). "Earthquake History of the United States – Publication 41-1". [[United States Department of Commerce]] / [[United States Department of the Interior]].
14. "Seismology and Earthquake Engineering".
15. {{harvnb. Trifunac. Brune. 1970
16. {{harvnb. Trifunac. Brune. 1970
17. Sieh, K.. (1996). "The repetition of large-earthquake ruptures". Proceedings of the National Academy of Sciences.
18. Gunn, A.M.. (2007). "Encyclopedia of Disasters: Environmental Catastrophes and Human Tragedies, Volume 1". Greenwood Publishing Group.
19. "Historic World Earthquakes". U.S. Geological Survey Earthquake Hazards Program.