A giant earthquake will strike California this summer. Skyscrapers will topple, the Hoover Dam will crumble and a massive tsunami will wash across the Golden Gate Bridge. Or at least, that’s the scenario that will play out on the big screen in San Andreas.

The moviemakers consulted Thomas Jordan, director of the Southern California Earthquake Center, before they started filming, but “they probably didn’t take much of my advice,” he says. While the actual threats from the Big One are pretty terrifying, they are nowhere near the devastation witnessed by Dwayne "The Rock" Johnson and his onscreen companions. Even the largest of San Andreas' quakes can’t produce a massive tsunami like the one that swells over San Francisco in the movie. “The really big tsunamis, like the one that hit Japan, are caused by earthquakes that generate a major displacement of the ocean floor,” Jordan says. The San Andreas fault sits far inland, and the land slips past on either side. For that reason, a quake also can’t cause the fault to split apart into a giant chasm as it does in the film. And despite the warnings of distraught movie scientists, even the largest of California's quakes won’t be felt by anything but seismometers on the East Coast.

That doesn’t mean California is off the hook, though. While the movie may be more fantasy than reality, the Big One is coming, and it will produce plenty of destruction. “We think Southern California is locked and loaded, that the stresses have really built up, and when things start unleashing, they could unleash for years,” says U.S. Geological Survey seismologist Ned Field.

California sits at the border between two major tectonic plates—the Pacific plate, which is moving northwest, and the North American plate, which is sliding past it to the southeast. The two plates don’t just meet at a single line, and the state is crisscrossed with dozens of earthquake faults. The San Andreas is the most worrisome, because it generates the quakes that are really dangerous to California residents, Jordan notes.

The northern San Andreas leveled San Francisco in 1906, but it’s been a lot longer since the southern part of the fault ruptured. On average, Southern California has seen big quakes every 110 to 140 years, based on records of past earthquakes and studies of earthquake faults. The last big quake near Los Angeles, a magnitude 7.9, struck Fort Tejon in 1857. Farther south, near Palm Springs, the fault hasn’t ruptured in over 300 years. “Eventually the fault will have to break,” Jordan says.

While seismologists can’t predict exactly when that will happen, every few years they release a forecast for the likelihood of such an event. The latest forecast, published earlier this year by the USGS, estimates a 7 percent chance that a magnitude 8 quake will occur in California within the next 30 years. That’s about as big as earthquakes can get in California, notes Jordan—a magnitude 8.3 quake might be possible if the entire San Andreas fault were to rupture from the Mexico border up to northern California. “We don’t think that’s likely,” he says.

To figure out what could realistically happen when the Big One finally strikes, a team of earthquake experts sat down sat down several years ago and created the ShakeOut scenario. Seismologists modeled how the ground would shake and then other experts, including engineers and social scientists, used that information to estimate the resulting damage and impacts. The detailed report examines the effects of a hypothetical 7.8 quake that strikes the Coachella Valley at 10 a.m. on November 13, 2008. In the following minutes, the earthquake waves travel across California, leveling older buildings, disrupting roads and severing electric, telephone and water lines.

But the quake is only the beginning.

Hundreds of fires start, and with roads blocked and the water system damaged, emergency personnel aren’t be able to put them all out. Smaller fires merge into larger ones, taking out whole sections of Los Angeles. The lines that bring water, electricity and gas to Los Angeles all cross the San Andreas fault—they break during the quake and won’t be fixed for months. Though most modern buildings survive the shaking, many are rendered structurally unusable. Aftershocks shake the state in the following days, continuing the destruction.

The scenario is actually somewhat of an underestimate, notes one scientist behind the ShakeOut, USGS seismologist Lucy Jones. The report’s team was surprised by the extent of the fire damage from the quake, Jones says, but it could be worse if the Santa Ana winds are blowing when the event happens. These seasonal winds blow dusty, dry air from inland toward the coast, increasing risks of wildfires. And while Los Angeles keeps a supply of water on its side of the San Andreas, the reservoirs have been drained by the current drought—if the quake struck today, water reserves wouldn't last the maximum of six months that they would when full, she notes.

Overall, such a quake would cause some $200 billion in damage, 50,000 injuries and 2,000 deaths, the researchers estimated. But “it’s not so much about dying in the earthquake. It’s about being miserable after the earthquake and people giving up on Southern California,” says Jones. Everything a city relies on to function—water, electricity, sewage systems, telecommunications, roads—would be damaged and possibly not repaired for more than a year. Without functioning infrastructure, the local economy could easily collapse, and people would abandon Los Angeles.

“Imagine America without Los Angeles,” Jones posits. While the fictional disaster in San Andreas could be an additional wake-up call for Californians, Jones worries that its unrealistic scenario could lead people to believe that there’s nothing to worry about or nothing they can do about it. Moviegoers may think that scientists will be able to give them fair warning of the Big One, even though earthquake prediction is currently an impossibility.

But Californians can prepare for what will come. Jones spent most of 2014 working with the LA mayor's office to identify vulnerabilities and better prepare the city for the inevitable. The task force reported that building codes could be changed to require retrofitting of older structures so that they would withstand powerful shaking. The Los Angeles aqueduct could be fortified so that it won’t break when the San Andreas ruptures. Power, telecommunications and internet systems could be strengthened or have backup systems to ensure that people would be able to communicate. The plan would take billions of dollars and several decades to implement—and would have to overcome many obstacles—but it would improve the city’s ability to survive a quake catastrophe.

On an individual level, homeowners can retrofit their property to better hold up against shaking. People can include fire extinguishers in their earthquake kits to put out little flames before they get out of hand. And schools, businesses and families can participate in ShakeOut drills—the next one is on October 15—to practice what they’ll need to do on earthquake day.

“Everyone should live every day like it could be the day of the Big One,” says Field. Because any day, even today, could be that day.