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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 Hollywood fantasy is not California reality.

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.

Here is some science-based information on earthquakes.

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.

2014 U.S. Seismic Hazard Map
2014 U.S. seismic hazard map showing the earthquake peak ground acceleration (PGA) that has a 2% chance of being exceeded in 50 years. Red indicates the highest hazard, and gray indicates the lowest hazard. (Public domain.)

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, 42 of the 50 states could experience damaging ground shaking from an earthquake in 50 years (which is the typical lifetime of a building), and 16 states have a relatively high likelihood of experiencing damaging ground shaking. A 2014 map of seismic hazards in the United States is shown (left), A larger hi-res version of the 2014 US Hazard Map 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. There are simple actions we all should take to get prepared to survive and recover - visit Seven Steps to Earthquake Safety (Earthquake Country Alliance) to learn more.

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.” The San Andreas fault is not long and deep enough to have a magnitude 9 or larger earthquake as depicted in the movie. The largest historical earthquake on the northern San Andreas was the 1906 magnitude 7.9 earthquake. In 1857 the Fort Tejon earthquake occurred on the southern San Andreas fault; it is believed to have had a magnitude of about 7.9 as well. Computer models show that the San Andreas fault is capable of producing earthquakes up to about magnitude 8.3, but anything larger is extremely unlikely. Shaking from even the largest possible San Andreas fault events will not be felt on the east coast.

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.

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