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What Is an Earthquake? - Windows to the Universe

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What Is an Earthquake?

The expression "on solid ground" is often used to describe something as stable. But sometimes the solid ground underfoot is not stable. It moves as Earth’s tectonic plates move. Sometimes it moves gradually. Sometimes it moves abruptly which causes the earth to shake - an earthquake!

"The ground seemed to twist under us like a top while it jerked this way and that, and up and down and every way," wrote a person describing the experience of being in the large 1906 earthquake in San Francisco, CA.

Earthquakes happen as large blocks of the Earth’s crust move suddenly past one another at a fault. When the force of plate tectonics causes the pieces of the Earth’s crust to move, sometimes the pieces do not slide smoothly past one another. There can be friction along the fault – jagged edges that snag the blocks of rock. This makes it difficult for them to move past each other. Sometimes they get stuck together temporarily. When the pieces of rock overcome the snags, energy is released. The release of energy causes shaking at the ground surface.

The location inside the Earth where an earthquake begins is called the focus (or hypocenter) of the earthquake. The point at the Earth’s surface directly above the focus is called the epicenter of the earthquake. At the epicenter, the strongest shaking occurs during an earthquake. Sometimes the ground surface breaks along the fault. Sometimes the movement is deep underground and the surface does not break.

Each year, more than a million earthquakes occur. Most of these are so small that people do not feel the Earth shaking. But a few are large enough that people feel them, and a few of those are so large that they cause significant damage.

Earthquakes cause damage to buildings and other built structures like bridges and roads. Earthquakes can also trigger landslides and mudslides. A large earthquake that happens under the ocean can form a tsunami – a giant ocean wave or series of waves that can cause massive destruction if it hits a populated coastal area.

Scientists can assess the probability that an earthquake will occur in an area over a number of years. These probabilities are often used to identify earthquake prone areas and the potential risk to people and buildings. However, they are not able to predict when an earthquake will occur. Unlike extreme weather events, earthquakes can not be forecast ahead of time.

Last modified January 19, 2010 by Randy Russell.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF