What to Know About Earthquakes

If you just felt the ground shaking, you might be wondering what happened or how to react the next time an earthquake strikes. Here are the answers to some common questions about earthquakes.

What causes them?

To understand earthquakes, imagine the Earth as an egg, said Mark Benthien, the communications director for the Statewide California Earthquake Center, a research organization.

The egg’s shell represents the Earth’s crust, and “if you look at it from a worldwide view, there are 12 or so major egg pieces of the crust that are called plates,” Mr. Benthien said. Pieces of the egg’s shell — tectonic plates — move around slowly, about as quickly as your fingernails grow, building up pressure between them.

Most earthquakes occur when the force of the moving tectonic plates exceeds the friction between them. When this happens, the pressure releases suddenly and plates move into, past or away from one another. The pressure is released as seismic waves that pass through the earth, causing the ground to shake.

In rare cases, an earthquake can also occur in the interior of a tectonic plate.

Earthquakes can also be caused by human action, such as the disposal of waste fluids as part of the process of oil production.

Can earthquakes be predicted?

No. It is not possible to predict where or when an earthquake might happen.

However, national seismic hazard maps from the U.S. Geological Survey describe how many strong earthquakes are likely to happen in the next 10,000 years in the United States. More earthquakes with damaging shaking are likely to occur along the West Coast, along Alaska’s south coast and in parts of Hawaii, Puerto Rico and the Virgin Islands.

Globally, earthquakes are most likely near the “Ring of Fire,” which spans the Pacific Ocean at the edges of tectonic plates.

You can prepare ahead of time.

Should people worry about earthquakes before they happen? “It’s not that they should be worried, but they should act,” said Mr. Benthien, who leads the Earthquake Country Alliance, an earthquake coordination organization.

The group suggests securing items in your home ahead of time. Fasten down bookcases and other furniture, as well as your television and even your hot water heater. “As our buildings are built better and better, these aspects are really the cause of the most injuries — not the buildings collapsing, but just all this stuff flying around,” Mr. Benthien said.

You should also prepare at least one kit of supplies to use in the aftermath of an earthquake. Among other things, your emergency kit should include a fire extinguisher, extra doses of medications you may be taking, first-aid supplies, food and water, according to the U.S.G.S.

Earthquake early warning systems such as ShakeAlert can also sometimes provide precious seconds of advance warning before shaking strikes your area, said Robert-Michael de Groot, a coordinator for the program, which is run by the U.S.G.S. This gives you more time to react and avoid injury. “ShakeAlert asks you to do what you already do, but do it sooner,” he said. (Here’s how to sign up for U.S.G.S. early warning systems.)

What should I do if an earthquake happens?

The next time you feel the ground shaking, follow these three steps: Drop to the ground, cover your body to prevent injuries — by crawling under a table, for example — and hold on, according to the Earthquake Country Alliance. If you use a wheelchair or walker, remain seated, bend over and cover your head and neck.

Experts used to advise people to stand under a doorway during an earthquake, but they don’t do that anymore, Dr. de Groot said. “There was a time when the doorway was structurally more stable, stronger than the rest of other parts of the house,” he said. “That really isn’t the case anymore.”

No matter where you are when an earthquake strikes, Dr. de Groot said, stay alert and take protective measures. “If you’re driving on the highway at 70 miles an hour, you may not be able to drop, cover and hold on,” he said. “But there’s an idea about having that situational awareness and knowing how to protect yourself no matter where you are.”

“So staying in your place, dropping, is actually the most important part,” he said, advising people to stay low on the ground, and find a way to protect themselves, wherever they are.