Dear California: You Can't Get The Largest Earthquakes In The World
Mika McKinnon
I love disaster movies. I even love cheesy disaster movies, ones where the speed of cold is a thing, and violent volcanoes pop up in the middle of a continental plate. I'll be watching San Andreas when it comes out next spring. I just expect to be entirely befuddled by its earthquakes.
The iconic Hollywood sign only exists to be symbolically destroyed in movies. Screenshot from San Andreas
The just-released trailer features somber music, the earth gaping open, lots of crumbling buildings that don't sway so much as dissolve, and other classic features of a killer-earthquake sequence. You can watch it here:
It also features what I think is a seismologist-character talking about how even though the earthquake is in California, it's going to be felt on the east coast. I was happy to take that as a metaphorical/logistical thing — if a major earthquake hits the port of Long Beach, the ramifications will be felt across the entire country — but then I read an interview with the director and now I'm not feeling as confident in my scientific generosity.
On US Today, director Brad Peyton gushes:
We're going to show what the largest earthquake ever recorded would look like, all in a highly populated area.
The only trouble? California is on a transform seismic zone, where tectonic plates move past each other. Massive megaquakes only happen on subduction zones, where oceanic plates dive under continental or oceanic plates. California is geologically incapable of producing anything near the largest earthquakes ever recorded.
Tectonic plates move centimeters per year, at about the same rate your fingernails grow. As they gets stuck and locked, stress builds up until eventually overcoming friction and snapping, suddenly releasing a massive amount of energy in an earthquake. This builds up even more stress on the ends of the fault rupture zones where the earth didn't move, increasing the probability of an earthquake at those locations in the future.
The San Andreas Fault runs along the boundary between the North American plate and the Pacific plate. The fault roughly traces the coastline, heading offshore and acquiring a new name north of San Francisco. The Pacific plate is slowly crawling north, creaking past the North American plate.
Tectonic map of the west coast. Image credit: USGS
Transform zones, where tectonic plates move past each other horizontally in a transverse motion, produce shallow, frequent earthquakes, intensely shaking a relatively small (city-sized) area. It can absolutely be devastating, particularly if local geology like sedimentary basins amplify shaking, and the secondary effects of fires and landslides can amplify the disaster. This is the type of motion responsible for earthquakes in Los Angeles and San Francisco.
What's an earthquake disaster movie without giant gaping cracks to the center of the Earth? Screenshot from San Andreas
The very largest earthquake ever recorded in California was a magnitude 7.9 in 1857, vaguely near Santa Barbara. This is no where near the largest earthquake ever recorded, a magnitude 9.5 earthquake in Chile in 1960. That earthquake released over 250 times more energy than California's Biggest One. If seismograms for both earthquakes were laid side-by-side, the Chilean earthquake would have an amplitude nearly 40 times greater. We don't even need to stack California up against the biggest of all time for its earthquakes to be outdone: since 1900, at least 89 earthquakes around the world have been larger than anything ever experienced in California. That's not even including earthquakes in the past few years, like the magnitude 8.2 in Chile last spring.
Technically, California may have the capacity for a major subduction zone megaquake, but only in the northern reaches where the fault darts offshore, transitions into the Cascadia Subduction Zone, and the San Andreas fault doesn't exist anymore. Subduction zone earthquakes are in a class of their own because they really are just incomparable: The Big One for a major Californian city will be destructive and miserable and very likely deadly, but in terms of seismology, it will have nothing on when The Big One hits the Cascadia subduction zone in the Pacific Northwest. Even if the very southern portion of the fault managed to produce a megaquake in the very north of coastal California, that's a sparsely-populated portion of the state full of forests, fog, and pot farms, not iconic cities awaiting cinematic destruction.
The only up-side for poor doomed souls living in subduction zones like me is that the largest of the subduction earthquakes are typically deeper, spreading out the shaking over a wider area. That means more people and places are impacted, but the intensity can sometimes be less than for a shallower earthquake. Of course, the vertical displacement of a subduction earthquake can also produce the nastiest of secondary effects: tsunami. I suspect that's what the wave wiping out a building of squishy humans is partway through the trailer, although I haven't yet been establish a plausible explanation for how the transverse motion of a transform zone could trigger a major tsunami.
Everywhere has disasters. In the United States, this ranges from earthquakes on the west coast,… Read more
California, I love you dearly, and I love seeing anything that will increase public awareness to increase personal preparation or public pressure to seismically upgrade buildings and infrastructure, but your earthquakes just aren't in the same league as the major earth-movers. Be happy about that. Embrace it. Sleep a little better at night knowing megaquakes aren't in your future. At least, after you secure your bookcases to the wall, store sturdy shoes under the edge of the bed, and teach your neighbours where your bed is in case they need to dig you out of a collapsed building. You don't need the biggest earthquake around to be plenty scary to the people who live through your geotectonic restlessness.
Is the Hoover Dam more likely to crumble under stress from seismic waves, or be overtopped and collapse by earthquake-induced seiche? Screenshot from San Andreas.
San Andreas is based on science by Tom Jordan, director of the Southern California Earthquake Center. He may be its science consultant, or maybe they just set a script researcher reading up on things he's written. If he is a consultant, that leaves me hopeful that I'll spot scientific Easter Eggs when we see the whole movie. Unfortunately, even if San Andreas hired a science consultant, that doesn't mean he was actually actively involved or had any influence beyond answering a few questions or trying to teach an eager director-student without much time to do it.
All I'm really hoping for is a bit of scientific details interspersed throughout a lot of flashy special effects of destruction. A few references to moment magnitudes with a rant about the idiocy of the Richter scale, maybe differentiation between P-, S- and surface wave arrival times, swimming pool seiche, or even a comment on how aftershocks drop off exponentially over time: that's all it would take for me to be happy. I'm starting from low expectations for the science in this now we're apparently working with a megaquake in LA: I just want something to tickle my disaster-researcher geophysicist heart.
Most of my students have never heard of The Core anymore: it's about time I get a new "Hire a science consultant; still come up with totally nonsensical science" movie example. I'm looking forward to seeing this in May.
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