STEPHANIE WILSON, SAVANNAH, GEORGIA: I'm Stephanie Wilson from Savannah, Georgia. And my question for CNN is: What makes an earthquake occur?</s>LIND GEE, SEISMOLOGIST, BERKELEY SEISMOLOGICAL LABORATORY: The surface of the Earth is broken into seven to 11 major plates. These are cold, stronger pieces of rock that float on top of softer material that makes up the interior of the Earth. The interior of the Earth is moving very slowly. It convects, just like coffee in your coffee cup when it's really hot. These plates move with respect to each other, and they kind of bump and grind during this process over millions and millions of years. And that bumping and grinding process leads to earthquakes. In the San Francisco Bay area, we're on the boundary of two major plates: the North American and the Pacific plate. They are divided by the San Andreas fault zone. And we experience a large number of earthquakes associated with those faults.
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