M 6.9 - The 1940 Imperial Valley, California Earthquake

The main earthquake [May 18 at 8:36 PM local time] took nine lives and caused property damage estimated at $6 million [equivalent to approximately $130.6 million in 2023 dollars¹]. Damage from a strong aftershock near Brawley at 05:51 UTC [May 18 at 9:51 PM local time] 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 [at 05:51 UTC], 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 [also known as Cocopah]. 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 km long, extending from the epicenter of the main shock southeast, about 5 km past Cocopar. Rupture of the northwest section of the fault may have occurred during the damaging aftershock at 05:51 UTC. Where the surface fracture crosses the All American Canal east of Calexico, the largest displacement of 4.5 m occurred. At one point on the Solfatara Canal, the slip was as much as 3.7 m.

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

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. (Ref. 13, 292, 381, 521, 533, 546, 547.)

Maximum observed Modified Mercalli Intensity (MMI) X

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.

¹Based on US Bureau of Labor Statistics Consumer Price Index and calculated in "Inflation Calculator", U.S. Official Inflation Data, Alioth Finance (https://www.officialdata.org/), retrieved 15 Feb 2024.

In addition to the estimated $6 million direct damage from this earthquake, the damage to the irrigation canals caused an indefinite amount of indirect damage to crops due to lack of water. In total, about 60 miles [about 95 km] of canal banks were damaged or destroyed in the Imperial Irrigation District.

At least 20 people were seriously injured by the earthquake.

Abridged from The Imperial Valley Earthquakes of 1940, by Franklin P Ulrich (1941), Bulletin of the Seismological Society of America, 31(1), p. 13-31.