A sequel, "Earthquake II", was planned, and a first draft of the script was written by George Fox, who wrote the script for "Earthquake", but it never made it into development. The sequel follows several of the surviving characters of the original: George Kennedy; Victoria Principal; Richard Roundtree and Gabriel Dell, as they settle in San Francisco. The multi-tiered plot centers on a group of scientists trying to predict future earthquakes on the west coast, a corrupt builder constructing high rise apartments on unstable land, and the original characters adjusting to new relationships (Kennedy and Principal) and new business ventures (Roundtree and Dell). An unexpected, massive earthquake hits off the coast of San Francisco, leveling the city, as a tsunami threatens to wash the Bay Area off the map. Completed in late-1975, the script went through channels at Universal (up to Sid Sheinberg) and the project was active up until early-1977 ("EQII" and Rollercoaster (1977) were in pre-production simultaneously) but "EQII" project was killed. This original script was newly discovered in 2005 and details are available on "www.earthquakemovie.com".
When the movie played at Graumann's Chinese Theater in Hollywood in 1974 it was shown in "Sensurround" with heavy bass speakers set on the floor around the theater. Very soon after the preview performances, a giant net had to be rigged above the patrons because of fear that the ornate ceiling decorations might break loose and fall on the audience below due to the low bass rumble of earthquake sequences. During the film's initial preview performances at the theatre, the Sensurround vibrations actually caused an occasional, very small and harmless piece of plaster or paint to crack and fall down onto the audience. Worried that continuous periods of strong sound waves throughout the film's exclusive run might loosen elements of the ceiling's ornamentation and light fixtures, the theatre management ordered the safety net to be slung below the entire auditorium ceiling, just below the ornamentation. This action was publicized in the local papers. Whether or not the ceiling's very visible "safety net" was an actual, workable safety measure or whether it was merely a publicity gimmick, it did serve to heighten audience anticipation of the film's effects.
The "Hollywood Dam" featured in the film is actually known as the "Mulholland Dam", named after famed engineer William Mulholland. The nearly-identical St. Francis Dam, near present-day Valencia, California, actually collapsed due to a geological fault on 12 March 1928. This disaster killed over 450 people and ranks second highest in terms of loss of life (behind the 1906 San Francisco earthquake) in the history of California.
Shots of the "Black Tower" (a 12-story office building on the movie lot) swaying was accomplished by shooting its reflection in a mirror, and then warping the mirror back and forth. This sequence was actually shot as a test prior to production, and wound up in the final film as an "in" joke at Universal Studios' expense: at the time, the "Black Tower" actually housed Universal's top executives.
Many scenes shot for the movie were left on the cutting room floor (over 30 minutes' worth). Notable scenes include: additional footage of Charlton Heston, Ava Gardner and Lloyd Nolan (specifically, a sub plot involving a previous abortion by Gardner's character); George Kennedy (after his character, Lew Slade, is suspended by the police captain); Barry Sullivan and Kip Niven (whose seismologist characters survive the quake, and discuss the magnitude); and Geneviève Bujold (her character, Denise Marshall, shows up to a movie studio for her "bit part" in a movie, only to be turned away due to quake damage on the set). Additional stunt sequences during the earthquake were also deleted.
The crash that motorcycle daredevil Miles Quade has coming out of the loop on the stunt track was not scripted, but an actual spill taken by the stuntman Bud Ekins. Ekins was uninjured and the crash was worked into the final cut.
The producer, Jennings Lang, offered a cameo role to his friend Walter Matthau, which Matthau accepted without compensation, on the condition that he be billed under his "real name" (which its not), "Walter Matuschanskyasky". Matthau's role was originally scripted as "a drunk sits at the end of the bar", which was expanded by writer George Fox, giving the character lines of dialogue (involving toasts to celebrities). When the film was completed, as agreed by Lang and Matthau, "The Drunk" was credited as "Walter Matuschanskayasky". This lead to a long-standing, but false, rumor that "Matuschanskayasky" was Matthau's real name.
In a bizarre coincidence, the location on the first day of shooting was rocked by an earthquake. In an even more bizarre coincidence, an earthquake also struck the location where the last day of shooting occurred.
As part of a new marketing gimmick to promote action and disaster movies in the 70's, theaters were asked to install a new audio system called Sensurround. Sensurround produced a low frequency sound vibration along theater seats giving an audience the feeling of being in the movie. For "Earthquake", when there was an earthquake, Sensurround would vibrated the seats like an actual earthquake. Unfortunately the speaker system was a custom job that often required removing a couple of rows of seats and it was expensive. It was used for a few more films throughout the rest of the '70s, but after theaters received structural damage, patrons got ill from the experience and nearby businesses complained of noise pollution, Sensurround was basically halted.
Producers traveled to Europe to interest Audrey Hepburn for the role of Remy Graff in the film-a role which eventually went to Ava Gardner. Audrey had been retired from films for nearly 10 years at this point and did not like the dialogue of the character nor the physical requirements of the film role and turned the producers down. The part of Remy Graff was filled at the last minute by Ava Gardner after filming had already started.
The trivia item below may give away important plot points.
The original shooting script had Charlton Heston's character survive at the end of the film, while Ava Gardner and George Kennedy's characters are killed (separately) in the storm drain tunnel. However, Heston was dissatisfied with the script as written, since his character survives to rebuild the city with his mistress by his side (which he felt was not morally sound). Since Heston had script approval, he insisted his character die while trying to save his drowning wife. The change was made, Kennedy's character survives, and says the final lines of the film originally intended for Heston.