Tag Archives: San Andreas Fault

“San Andreas” – The Scientific Reality

San Andreas—a Hollywood action-adventure film set in California amid not one, but two magnitude 9+ earthquakes in quick succession and the destruction that follows—was released worldwide today. As the movie trailers made clear, this spectacle is meant to be a blockbuster: death-defying heroics, eye-popping explosions, and a sentimental father-daughter relationship. What the movie doesn’t have is a basis in scientific reality.

Are magnitude 9+ earthquakes possible on the San Andreas Fault?

Thanks to the recent publication of the third Uniform California Earthquake Rupture Forecast (UCERF3), which represents the latest model from the Working Group on California Earthquake Probabilities, an answer is readily available: no. The consensus among earth scientists is that the largest magnitude events expected on the San Andreas Fault system are around M8.3, forecast in UCERF3 to occur less frequently than about once every 1 million years. To put this in context, an asteroid with a diameter of 1,000 meters is expected to strike the Earth about once every 440,000 years. Magnitude 9+ earthquakes on the San Andreas are essentially impossible because the crustal fault zone isn’t long or deep enough to accumulate and release such enormous levels of energy.

My colleague Delphine Fitzenz, an earthquake scientist, in her work exploring UCERF3, has found that, ironically, the largest loss-causing event in California isn’t even on the San Andreas Fault, which passes about 50 km east of Los Angeles. Instead, the largest loss-causing event in California is one that spans the Elsinore Fault and runs up one of the blind thrusts, like the Compton or Puente Hills faults, that cuts directly below Los Angeles. But the title Elsinore + Puente Hills doesn’t evoke fear to the same degree as San Andreas.

Will skyscrapers disintegrate and topple over from very strong shaking?

Source: San Andreas Official Trailer 2

Short answer: No.

In a major California earthquake, some older buildings, such as those made of non-ductile reinforced concrete, that weren’t designed to modern building codes and that haven’t been retrofitted might collapse and many buildings (even newer ones) would be significantly damaged. But buildings would not disintegrate and topple over in the dramatic and sensational fashion seen in the movie trailers. California has one of the world’s strictest seismic building codes, with the first version published in the early part of the 20th century following the 1925 Santa Barbara Earthquake. The trailers’ collapse scenes are good examples of what happens when Hollywood drinks too much coffee.

A character played by Paul Giamatti says that people will feel shaking on the East Coast of the U.S. Is this possible?

First off, why is the movie’s scientist played by a goofy Paul Giamatti while the search-and-rescue character is played by the muscle-ridden actor Dwayne “The Rock” Johnson? I know earth scientists. A whole pack of them sit not far from my desk, and I promise you that besides big brains, these people have panache.

As to the question: even if we pretend that a M9+ earthquake were to occur in California, the shaking would not be felt on the East Coast, more than 4000 km away. California’s geologic features are such that they attenuate earthquake shaking over short distances. For example, the 1906 M7.8 San Francisco Earthquake, which ruptured 477 km of the San Andreas Fault, was only felt as far east as central Nevada.

Do earthquakes cause enormous cracks in the earth’s surface? 

Source: San Andreas Official Trailer 2

I think my colleague Emel Seyhan, a geotechnical engineer who specializes in engineering seismology, summed it up well when she described this crater from a trailer as “too long, too wide, and too deep” to be caused by an earthquake on the San Andreas Fault and like nothing she had ever seen in nature. San Andreas is a strike-slip fault; so shearing forces cause slip during an earthquake. One side of the fault grinds horizontally past the other side. But in this photo, the two sides have pulled apart, as if the Earth’s crust were in a tug-of-war and one side had just lost. This type of ground failure, where the cracks open at the surface, has been observed in earthquakes but is shallow and often due to the complexity of the fault system underneath. The magnitude of the ground failure in real instances, while impressive, is much less dramatic and typically less than a few meters wide. Tamer images would not have been so good for ticket sales.

Will a San Andreas earthquake cause a tsunami to strike San Francisco?

Source: San Andreas Official Trailer 2

San Andreas is a strike-slip fault, and the horizontal motion of these fault systems does not produce large tsunami. Instead, most destructive tsunami are generated by offshore subduction zones that displace huge amounts of water as a result of deformation of the sea floor when they rupture. That said, tsunami have been observed along California’s coast, triggered mostly by distant earthquakes and limited to a few meters or less. For example, the 2011 M9 Tohoku, Japan, earthquake was strong enough to generate tsunami waves that caused one death and more than $100 million in damages to 27 harbors statewide.

One of the largest tsunami threats to California’s northern coastline is from the Cascadia Subduction Zone, stretching from Cape Mendocino in northern California to Vancouver Island in British Colombia. In 1700, a massive Cascadia quake likely caused a 50-foot tsunami in parts of northern California, and scientists believe that the fault has produced 19 earthquakes in the 8.7-9.2 magnitude range over the past 10,000 years. Because Cascadia is just offshore California, many residents would have little warning time to evacuate.

I hope San Andreas prompts some viewers in earthquake-prone regions to take steps to prepare themselves, their families, and their communities for disasters. It wouldn’t be the first time that cinema has spurred social action. But any positive impact will likely be tempered because the movie’s producers played so fast and loose with reality. Viewers will figure this out. I wonder how much more powerful the movie would have been had it been based on a more realistic earthquake scenario, like the M7.8 rupture along the southernmost section of the San Andreas Fault developed for the Great Southern California ShakeOut. Were such an earthquake to occur, RMS estimates that it would cause close to 2,000 fatalities and some $150 billion in direct damage, as well as significant disruption due to fault offsets and secondary perils, including fire following, liquefaction, and landslide impacts. Now that’s truly frightening and should motivate Californians to prepare.