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Seeing the movie 'San Andreas'? Drop, cover and laugh.

The Big One won't blow up LA like in the movie--but that doesn't mean buildings won't fall.

With the action movie “San Andreas” opening Friday, you’ve probably seen the trailer of buildings in downtown Los Angeles exploding apocalyptically. And you probably know that’s ridiculous.   Although I do get a kick out of seeing the letters in the Hollywood sign topple over like playing cards. Now, that would put an end to the tourism traffic through the canyon streets that neighborhood groups bitterly complain about. (No, wait, what am I thinking -- tourists would want selfies with the downed letters.)

In the movie, there’s a 9.1 magnitude quake on the San Andreas fault. But that won’t happen, because the Big One can’t be that big on that fault. (You’d need to go to Chile for that.)

“A 9.1 on the San Andreas is impossible,” Lucy Jones, the U.S. Geological Survey seismologist and consultant to Mayor Eric Garcetti on his earthquake safety campaign, told me last week. (And Jones is the guiding measure for all rational belief about earthquakes in Los Angeles.)

The possible magnitude of an earthquake is determined by the length and depth of the fault. “To get enough energy to get a 9.1, you’d need something longer than the state of California,” she says. The San Andreas is a mere 810 miles long from its northern end near Cape Mendocino to its southern end in the Salton Sea in the desert.   The worst the San Andreas could throw is an 8.3 temblor. (Which would, of course, still be really bad.)

As for that earthquake-generated tsunami that engulfs the Golden Gate Bridge in San Francisco in the movie? First of all, you’re not going to get a tsunami from the San Andreas, which runs on land. And second, even if you could get a tsunami there, the ocean isn’t deep enough to swell over the Golden Gate Bridge. You can’t have a wave taller than the ocean is deep, Jones says.

Fortunately, Jones has been all over the media cheerfully debunking the science of the movie.   She was even invited to the premiere at TCL Chinese Theatre, where she posed with its star, Dwayne “The Rock” Johnson and gushingly tweeted that it was her first red carpet. After that, she used Twitter to point out all the inaccurate geological moments—but she did give the film props for its “drop, cover, and hold on” mantra, which is useful advice in a real earthquake.

My guess is the movie has only gotten more publicity from Jones’ reviews of the science. And since our usual reaction to legitimate information about the possible Big One is to put our hands over our ears and go ‘lalalalala,’ it’s unlikely anyone would go to “San Andreas” and believe its depiction of the Big One.

Jones does estimate that we could have 1,500 buildings collapse across southern California—although many of those are the structures that potential mandatory retrofitting ordinances would target.  And it's time for all of us to be worried about earthquakes enough to urge that those kinds of laws be passed.

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