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As recently witnessed in China, an earthquake is one of the most terrifying phenomena that nature can whip up. We generally think of the ground we stand on as "rock-solid" and completely stable. An earthquake can shatter that perception instantly, and often with extreme violence.

Up until relatively recently, scientists only had unsubstantiated guesses as to what actually caused earthquakes. Even today there is still a certain amount of mystery surrounding them, but scientists have a much clearer understanding.

Earthquake Image Gallery

AFP/AFP/Getty Images
Family members gather at the collapsed Juyuan middle school, where six children died in southwest China Sichuan province on May 12, 2008, after an earthquake measuring 7.8 rocked the province. See more earthquake images.

There has been enormous progress in the past century: Scientists have identified the forces that cause earthquakes, and developed technology that can tell us an earthquake's magnitude and origin. The next hurdle is to find a way of predicting earthquakes, so they don't catch people by surprise.

In this article, we'll find out what causes earthquakes, and we'll also find out why they can have such a devastating effect on us.

Shaking Ground
An earthquake is a vibration that travels through the earth's crust. Technically, a large truck that rumbles down the street is causing a mini-earthquake, if you feel your house shaking as it goes by, but we tend to think of earthquakes as events that affect a fairly large area, such as an entire city. All kinds of things can cause earthquakes:

• volcanic eruptions
• meteor impacts
• underground explosions (an underground nuclear test, for example)
• collapsing structures (such as a collapsing mine)

But the majority of naturally-occurring earthquakes are caused by movements of the earth's plates, as we'll see in the next section.

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