The 1915 Imperial Valley quake, which was responsible for at least 6 deaths, numerous injuries, and almost one million dollars of damage to property, actually came as a set of two strong shocks separated by about one hour. The first, lighter quake came at 8 p.m. It was strong enough to stop clocks and weaken structures across the epicentral region. The second quake, slightly stronger, caused many weakened structures to collapse. Both shocks were apparently quite short in duration, each lasting only about 11 seconds.

Because of the first shock, many people fled into the streets, and were still there when the second quake struck. Thus, many residents of the Imperial Valley escaped severe injury, even though a great many buildings (mostly unreinforced masonry) partially collapsed. However, in Mexicali, Mexico, gambling halls and dance halls were at peak business hours, and despite the fact that most people fled the buildings during the first quake, many of them returned soon after, to continue with the evening's entertainment. Thus, when the second shock hit, many inside were seriously injured by falling material or pinned underneath collapsed walls. All six deaths reported occurred at such a location.