Historic Earthquakes

Imperial Valley, California
1940 05 19 04:36 UTC (local 05/18)
Magnitude 7.1
Intensity X

Imperial Valley, California
1940 05 19 05:51 UTC (local 05/18)
Magnitude 5.5
Intensity IX

The main earthquake took nine lives and caused property damage estimated at $6 million. Damage from a strong aftershock near Brawley at 05 51 UTC is included in this estimate.

The first shock damaged about 80 percent of the buildings in Imperial. Many buildings in the business district were condemned, and older residences sustained severe damage. Four people were killed in the collapse of a grocery store. Damage to a lesser extent occurred at El Centro and Holtville. Elevated water tanks at Holtville and Imperial collapsed, and a water tank at Brawley was damaged.

The downtown business area at Brawley was damaged severely by the second shock, and about 25 percent of the houses in the residential area were damaged. About half of the business structures had to be condemned. Many breaks in water mains occurred and water pipes were broken.

Damage to the structures and canals of the Imperial Irrigation District in the United States and Mexico was widespread. Breaks occurred over almost the entire length of the Ash Canal, from Holtville to the Mexico border. The Alamo Canal, the main feeder for the entire system, had eight major breaks; a section of the Solfatara Canal in Baja California was destroyed south of Cocopar. The earthquake demolished the New River flume, a 427-meter-long timber structure on the West Side Main Canal south of Mexicali.

Right-lateral offset occurred along the Imperial fault. The pattern of offset indicates that the main part of the offsets occurred along a surface fracture about 20 to 25 kilometers long, extending from the epicenter of the main shock southeast, about 5 kilometers past Cocopar. Rupture of the northwest section of the fault may have occurred during a damaging aftershock at 05 51 UTC. Where the surface fracture crosses the All American Canal east of Calexico, the largest displacement of 4.5 meters occurred. At one point on the Solfatara Canal, the slip was as much as 3.7 meters.

In Baja California, the Inter-California Railroad track was displaced at Grape, and about 300 meters of railroad track settled north of Grape. At Cocopar, the track shifted 2 meters, and at Meloland, it shifted about 46 centimeters.

Many sand boils were observed near Gadsden on the Yuma Project in Baja California. Geysers spouting water several meters high also were reported. Canals, drainage channels, flumes, and bridges were damaged near Gadsden. The main earthquake was felt over much of southern California, southwest Arizona, and northern Baja California. About 48 aftershocks occurred through the end of 1940. Those on May 23 caused more damage at Brawley.

Abridged from Seismicity of the United States, 1568-1989 (Revised), by Carl W. Stover and Jerry L. Coffman, U.S. Geological Survey Professional Paper 1527, United States Government Printing Office, Washington: 1993.

See also: 1940 Imperial Valley Earthquake from SCECDC.