Imperial Valley Earthquake – California – May 19, 1940

The Imperial Valley experienced an earthquake of magnitude 7.1. The population of the Valley was only 40,000 at the time, so casualties were few and most of the damage occurred in buildings and irrigation canals.

The Imperial Valley of California, near El Centro, located about ten miles from the Mexican border, was hit with a magnitude 7.1 earthquake on May 19, 1940. The earthquake took nine lives from a region that, at the time, had a population of about 40,000, and caused $6 million worth of property damage, including the effects of a strong aftershock near Brawley. Damage to other centers such as El Centro and Holtville was much less. In addition, there was extensive destruction to the structures and canals of the Imperial Irrigation District (IID) with ground displacements as large as 10–14 feet.

The Imperial Valley earthquake of 1940 is particularly important to the history of California because it was the first earthquake to provide documented evidence of surface rupturing since the 1906 San Francisco quake. Furthermore, it provided detailed local accounts of the kind of shaking motion that accompanied the event. Not until the San Fernando earthquake of 1971 were comparable records available.

Soon after the Imperial Valley earthquake occurred, all available manpower and machinery were brought in to repair the Alamo and Solfatara canals of the IID. Although the loss of water would have led to the loss of only one crop, cantaloupes, the damage done to this crop would have cost the industry a million and a half dollars. Two weeks’ worth of water loss would have been catastrophic for this crop. Fortunately, water was restored within four days.

Because the water table was high, numerous sand boils appeared all over the region—in fields, roads, and under houses—creating three-inch wide craters and sending water and sand several feet into the air. As a result of the earthquake, a 40,000-gallon water tank at Holtville collapsed as did a 100,000-gallon tank at Imperial. Miles of drainage ditches were obstructed by elevation of the ground with, in several locations, water escaping on to adjacent land. Bridges and flumes buckled as a result of repeated up and down earth movements. Reconstruction of banks became necessary as ground subsided.

The initial estimate of the strength of the earthquake was 6.4 but, as surface destruction was examined in greater detail, it became clear that the present figure of 7.1 is more appropriate; putting the Imperial Valley earthquake comparable to the San Francisco 1906 quake as far as magnitude is concerned. This reassessment is also related to the unusual character of the earthquake. It consisted of numerous quakes, all of them occurring in the first fifteen seconds of the event, followed by several shakings within the next five minutes. Normally these would be called aftershocks but, because they were comparable in magnitude to the main event, they must all be regarded as part of a single earthquake.

The Imperial Valley is a large, flat, crop-growing area in the very south of California, between the Salton Sea which lies to its north and the Mexican border which lies to its south. The valley extends south into Mexico where it is called the Mexicali Valley. The Valley is a working agricultural area, one of the most productive agricultural areas of the world and the largest year-round irrigated agricultural area in North America. It is the seventh largest food producing area in the world with such crops as melons, citrus fruits, alfalfa, barley, lettuce, and other vegetables.

The Valley was formed in 1907 from the eastern half of San Diego County. The county took its name from Imperial Valley. The Valley was named for the Imperial Land Company—a subsidiary of the California Development Company—which at the beginning of the twentieth century had reclaimed the southern portion of the Colorado Desert for agriculture. Irrigation water is supplied from the Colorado River via the All-American Canal. The population of the region grew from 13,000 in 1910, to 63,000 in 1950, and to 142,000 by the year 2000.