UNIDENTIFIED FEMALE: Viewer Linda Barnhart asks</s>CNN: "What is the difference between an earthquake and an aftershock?"</s>ANN KELLAN, CNN CORRESPONDENT: Think of an earthquake is a way the earth releases stress. Usually, there's a movement or shift along fractures or faults within layers of rock. The layers of rock are called plates within the earth. Now, the quake in the Seattle area occurred when one of earth's plates shifted under another plate, causing a crack in one of the plates. Now, aftershocks usually occur after a big earthquake. The bigger the quake, the more likely there will be an aftershock. They occur geographically close to the main earthquake. That's what makes it an aftershock. And that's what happened with a recent aftershock in the Seattle area. It doesn't necessarily have to be smaller in magnitude, but they tend to be smaller. And earthquakes, like the one in Seattle that occur 30 to 180 miles below the surface, have fewer aftershocks and usually don't cause damage. Compared to earthquakes like the one in Los Angeles back in 1994, that occurred closer to the surface. Think of an aftershock as a way the earth readjusts to the shifts in the plates and the new stresses created from the main earthquake. If there are going to be aftershocks, odds are higher they will occur in the first hour and diminish over time.
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