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By Linda Carroll

If only the screenwriters for 'San Andreas' had done their science homework.

They could have kept their apocalyptic vision intact and still been scientifically accurate—just by moving the blockbuster movie’s mega earthquakes and massive tsunami about a thousand miles north to Portland and Seattle.

But they opted for California and the San Andreas, which, scientists say, can’t produce a magnitude 9 event, let alone a city-inundating wave of water. (Moviegoers beware: Spoilers ahead!)

“Even if the entire San Andreas were to break all at once from north to south you wouldn’t get anything larger than an 8-ish earthquake,” said Jean Paul Ampuero, an assistant professor at Caltech -- the institution from which the movie’s scientist-hero, played by Paul Giamatti, is supposed to hail.

The disasters start early in the film, when a swarm of earthquakes on an undiscovered Nevada fault shakes the Hoover Dam to pieces.

Not too likely, said Graham Kent, director of the Nevada Seismological Laboratory at the University of Nevada and Nevada’s state seismologist.

The engineers who built the dam knew there were faults in the area and designed it with earthquakes in mind, Kent says.

And the movie’s magnitude 9-plus events on the San Andreas are not possible, experts say.

In simulations designed to estimate the magnitude of the largest possible earthquake on the San Andreas, the most anyone could come up with is a magnitude 8.3, Kent said.

And then there’s the issue of the monster tsunami that drowns San Francisco, skyscrapers and all. Also impossible experts say, because of the orientation and location—inland, for the most part—of the San Andreas. Smaller surges are possible along California beaches. Those would be caused by underwater landslides, but there’s no way this type of earthquake fault can spawn a huge tsunami.

Kent says he was surprised that the movie makers didn’t choose to set the action in the Pacific Northwest, which could generate a magnitude 9 event. Earthquake magnitude is related to the area of the rock surfaces that grind by one another. In subduction zones, like the Cascadia Fault off the coasts of Seattle and Portland where the ocean crust is diving under the North American Plate, the faults are oriented at a shallow angle instead of the near 90 degrees of the San Andreas.

That geometry allows for a much larger fault surface. And, in fact, geologists have determined that the Cascadia Subduction Zone has experienced both mega earthquakes and monster tsunamis in the past.

“The good news,” Kent said, “is that the movie, no matter how bad it is in terms of scientific content, is getting people starting to talk about the inevitable, which is a large earthquake in the West. That is the silver lining.”

There’s no doubt that both Northern and Southern California are overdue for the “big one,” the magnitude 7 to 8 event that will shake San Francisco and Los Angeles to their cores. But the aftermath will look a lot different from what is portrayed in 'San Andreas.'

The movie’s computer-generated scenes showing tall buildings crumbling and toppling in both cities leave real-life geophysicist Jack Boatwright concerned that viewers will get the wrong impression about the safety of modern skyscrapers.

“Those buildings are designed not to collapse in an earthquake,” said Boatwright, the Northern California coordinator for the Earthquake Hazards Program at the U.S. Geological Survey. “The concern I have is that we don’t want to get people panicked about how buildings are going to function. You’re more likely to be hit by something falling in your office than by a building falling down.”

A quake doesn’t have to be nearly as bad as what’s in ‘San Andreas’ to have serious consequences for cities.

San Francisco got a glimpse of what could happen during the 1989 Loma Prieta earthquake, which the USGS pegged at 7.1.

While skyscrapers did fine, certain areas of the city, like the Marina District, did not. City planners learned from that, said Patrick Otellini, chief resilience officer and director of the Earthquake Safety Implementation Program for the City and County of San Francisco.

“After Loma Prieta we noticed all the failures in unreinforced masonry buildings,” Otellini said. “About 2,000 of those have been retrofitted.”

Another issue was buildings that collapsed because they had too little structural support on their bottom floors, which were serving as either garages or shops. Legislation passed in 2013 mandated that some 5,000 of these buildings, which house about 125,000 San Franciscans, would need to be retrofitted by 2020, Otellini said.

And unlike the movie scenario, where residents of the city are encouraged to leave, city officials are hoping most people will be able to stay in their homes when the big one comes, Otellini said.

A big focus these days “is preparedness on the individual level,” said Kate Long, deputy program manager for the California Emergency Management Agency’s Earthquake and Tsunami Program. “In the movie The Rock comes from Los Angeles to San Francisco to save his daughter. In reality if you’re rescued it’s going to be by your neighbor. How prepared you and your co-workers and neighbors are is what will make all the difference in how well you survive.”