A Seismologist Reviews 'San Andreas': What's Real and What's Preposterous

U.S. Geological Survey seismologist Dr. Lucy Jones breaks down the tumbling skyscrapers, tidal waves and more.

Documentaries educate, while movies entertain. Many documentaries explain facts, while most movies explore emotions. The new film San Andreas clearly is not a documentary. But it does capture some of the emotional realities of a disaster.

Nobody should confuse San Andreas with Seismology 101. Hollywood usually exaggerates for effect, and this movie is no exception; both the magnitude of the shaking and the damage displayed are exaggerated beyond reality. Magnitude 9 earthquakes only occur on “subduction zones,” places where tectonic plates collide, pushing one plate under another and deforming the sea floor to create tsunamis. It has been millions of years since there was an active subduction zone under Los Angeles or San Francisco. The modern day San Andreas Fault maxes out at about magnitude 8.3, and, being mostly on land, will never produce a big tsunami.

The level of destruction portrayed in the movie is over-the-top, collapsing high rises with heedless abandon. The gaping chasm we see rupturing the San Andreas in central California belongs to the realm of the completely impossible. If the fault could open up like that, there would be no friction — and without friction there would be no earthquake.

San Andreas makes a hero of the Caltech seismologist predicting the coming disaster. We seismologists sure wish we could do so, but we have yet to find any way to foresee the time of a particular earthquake. Magnetic and electric signals, strain meters and even animal behavior have been studied without success. The only time we know an earthquake becomes more likely is right after another one because of earthquake triggering, an important piece of seismology the movie gets right.

In San Andreas, a magnitude 7.1 in Nevada triggers a magnitude 9.1 in Los Angeles that triggers a magnitude 9.6 in San Francisco. If you adjust the magnitudes to what’s possible on the real San Andreas, the triggering pattern is actually plausible. In the day after the 1906 earthquake that devastated San Francisco, magnitude 5 to 6 earthquakes ruptured through Imperial Valley, Santa Monica Bay, Oregon and Nevada. At those distances, we call them “triggered earthquakes” instead of “aftershocks,” but they are common. The aftershocks that continue to rattle the characters in the movie are representative of what indeed could follow a real event.

The most plausible part of San Andreas is its portrayal of the emotions in a disaster. Loss of communication brings fear to families, while the knowledge of how to protect yourself and those around you reduces that fear and boosts the chances of survival. San Andreas nicely shows that knowing the fundamentals of first aid, how to “Drop, Cover, Hold On,” that tsunamis can be preceded by a draw-down of the ocean, that landlines work when cell phones are out and that having a “plan B” all can make life easier and safer after a big earthquake.

(Spoiler alert!) Even better, a young woman — Blake Gaines (played by Alexandra Daddario) — not only saves herself and those with her because of that knowledge and competence; she even wins the heart of the boy, because she is competent. That’s a message I can applaud.