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.