The movie opens with a small tremor occurring in the hills outside Los Angeles near a United States Geological Survey (USGS) research post, cutting to a scene of a teenage girl on a date with her boyfriend at the Earthquake Ride at Universal Studios Hollywood. The girl is later revealed to be Heather, daughter of Clare Winslow (Joanna Kerns), a seismologist with the USGS.

Clare and her staff, among whom is her assistant Jerry Soloway (Ed Begley Jr.), have been studying a series of tremors near Los Angeles. The With this information, she concludes that there is a better-than-average chance that a massive earthquake will strike along the San Andreas Fault and cause severe damage to Los Angeles, and such an earthquake appears imminent. Against her will, she conducts an interview with Kevin Conrad (Richard Masur), a sensationalist television reporter who prematurely airs it after altering the interview to shock instead of inform, creating a political firestorm and causing tension between Clare and her husband Steve (Dan Lauria).

Steve works closely with high-powered and wealthy real estate developer Wendell Cates (Robert Ginty) who faces losing money and his socio-political reputation from public fear of the possibility of the earthquake. Wendell threatens Clare and attempts to have her fired from her job. Nevertheless, Clare tries to alert the more skeptical city and state government officials including Chad Spaulding (Joe Spano) of the Office of Emergency Management. Fearing political fallout and possible panic, they decide to ignore her warnings.

As this unfolds, Clare's family dynamic is further explored through her strained relationship with teenage daughter Heather (Holly Fields), which is mirrored by the relationship of Clare's mother Anita Parker (Bonnie Bartlett) and Clare's sister Laurie (Lindsay Frost), who are estranged from each other due to Anita's open resentment of Laurie's romantic relationship with LAPD officer Matt (Alan Autry).

When tremors are detected along the smaller, lesser-known Newport-Inglewood fault, city officials call a news conference to alert citizens of the threat. For most residents, however, it will already be too late. Not long after preparations and evacuations begin, the long-feared earthquake strikes, reaching 8.0 in magnitude and quickly followed by a 7.2 aftershock, causing massive damage and killing thousands; among the victims is Anita, who had been trapped in a high-rise condominium elevator with Laurie during the quake. During the time that they are trapped they reconcile shortly before Laurie is rescued by other survivors, but Anita is less fortunate and dies when the elevator crashes to the bottom of the shaft. Another victim is Miguel, son of Clare's housekeeper Sonia, who is fatally injured during the collapse of his high school gymnasium at his graduation rehearsal. Steve Winslow is thought to be dead after being crushed by a wall at the airport, but he is revealed to have survived the quake at the end of the film.

Other deaths include those of Wendell Cates, who is thrown to his death from his skyscraper window, and Chad Spaulding, who is electrocuted while attempting to escape from the USGS safety bunker beneath City Hall. A more redeeming storyline is that of Kevin Conrad, who is transformed from a cut-thr…

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