El Centro earthquake [ May 18-1940]

El Centro earthquake [ May 18-1940] 

The 1940 El Centro earthquake (or 1940 Imperial Valley earthquake) occurred in the Imperial Valley in southeastern Southern California near the international border of the United States and Mexico. It had a moment magnitude of 6.9 and a maximum perceived intensity of X (Extreme) on the Mercalli intensity scale. 

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 to hit the Imperial Valley, and caused widespread damage to irrigation systems and led to the deaths of nine people.

A surface rupture was formed during the earthquake of 40–60 km (25–37 mi), with a maximum, recorded displacement of 4.5 m (15 ft) 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.