Earthquakes, Megaquakes, and the Movies
Lights! Cameras! Disaster!
Throughout the history of Hollywood, disaster films have been sure-fire winners for moviemakers. Beginning with “The Wind” in 1928, Americans have been plagued by a “Twister” and “The Perfect Storm”. We’ve survived “Volcano” and “Earthquake” and “The Swarm” all followed by “Armageddon”. That’s not even mentioning us getting through “The Towering Inferno” and finally making it to “ The Day After Tomorrow”.
With amazing special effects, it’s easy to get caught up in the fantasy disaster epic. But real-world science is often at odds with Hollywood. What makes a great science fantasy film often bears no relation to real facts or the hazards people truly face.
The U.S. Geological Survey is the lead federal agency responsible for researching, monitoring and forecasting geologic hazards such as earthquakes, volcanoes and landslides. And we have the further responsibility to educate Americans about the real hazards they face and to separate science fact from science fantasy.
Since earthquakes are featured in the most recent offering in the made-for-television disaster film genre, let’s start with some science-based information on them.
Big earthquakes are naturally occurring events well outside the powers of humans to create or stop. An earthquake is caused by a sudden slip on a fault, much like what happens when you snap your fingers. Before the snap, you push your fingers together and sideways. Because you are pushing them together, friction keeps them from slipping. When you apply enough stress to overcome this friction, your fingers move suddenly, releasing energy. The same “stick-slip” process goes on in the earth. Stresses in the earth's outer layer push the side of the fault together. The friction across the surface of the fault holds the rocks together so they do not slip immediately when pushed sideways. Eventually enough stress builds up and the rocks slip suddenly, releasing energy in waves that travel through the rock to cause the shaking that we feel during an earthquake.
Unlike finger-snaps, earthquakes typically originate several to tens of miles below the surface of the Earth. It takes many years – decades to centuries – to build up enough stress to make a large earthquake, and the fault may be tens to hundreds of miles long. The scale and force necessary to produce earthquakes are well beyond our daily lives. Likewise, people cannot prevent earthquakes from happening or stop them once they’ve started – giant nuclear explosions at shallow depths, like those in some movies, won’t actually stop an earthquake.
It’s well known that California, the Pacific Northwest and Alaska all have frequent earthquakes, some of which are quite damaging. Some areas of the country are more at risk than others, but, in fact, 38 states have an earthquake risk. A map of seismic hazards in the United States is shown (left), larger versions can be found here.
The two most important variables affecting earthquake damage are the intensity of ground shaking caused by the quake and the quality of the engineering of structures in the region. The level of shaking, in turn, is controlled by the proximity of the earthquake source to the affected region and the types of rocks that seismic waves pass through en route (particularly those at or near the ground surface). Generally, the bigger and closer the earthquake, the stronger the shaking. But there have been large earthquakes with very little damage either because they caused little shaking or because the buildings were built to withstand that shaking. In other cases, moderate earthquakes have caused significant damage either because the shaking was locally amplified, or more likely because the structures were poorly engineered.
People can’t stop earthquakes from happening. People can significantly mitigate their effects by identifying hazards, building safer structures, and learning about earthquake safety.
Meanwhile, let’s clear up a few other things:
The idea of a “Mega-Quake” – an earthquake of magnitude 10 or larger – while theoretically possible—is very highly unlikely. Earthquake magnitude is based in part on the length of faults -- the longer the fault, the larger the earthquake. The simple truth is that there are no known faults capable of generating a magnitude 10 or larger “mega-quake.”
Then there’s this business of California falling off into the ocean. NOT TRUE! The ocean is not a great hole into which California can fall, but it is itself land at a somewhat lower elevation with water above it. It’s absolutely impossible that California will be swept out to sea. Instead, southwestern California is moving ever so slowly (2 inches per year) towards Alaska as it slides past central and eastern California. 15 million years (and many earthquakes) from now, Los Angeles and San Francisco will be next-door neighbors.
Another popular cinematic and literary device is a fault that opens during an earthquake to swallow up an inconvenient character. But unfortunately for principled writers, gaping faults exist only in movies and novels. The ground moves parallel to a fault during an earthquake, not away from it. If the fault could open, there would be no friction. Without friction, there would be no earthquake. Shallow crevasses can form during earthquake induced landslides, lateral spreads, or other types of ground failures. Faults, however, do not gape open during an earthquake.
So when you see the next big disaster film, rest assured that movies are just entertainment. Enjoy them! And then go learn about the real-world science behind disasters and if you live in an area where hazards exist, take the suggested steps to protect you and your family.