We have earthquakes mainly because the earth's crust is constantly being shoved and squished and pulled apart by forces created by the earth's plates drifting around on the earth's surface. Think of the earth as an egg with a broken shell, and all the pieces move around in different directions. We live on a piece of the shell that includes the part of California west of the San Andreas Fault and which also includes most of the Pacific Ocean. The rest of the United States is on another piece of "shell" called the North American Plate. The San Andreas Fault is the boundary between the two plates. Every year we here on the Pacific plate move northwest about 5 centimeters (2 inches), but we don't slide smoothly past the rest of North America along the San Andreas Fault. Instead, we get stuck against the fault, and it isn't until after many years that enough energy builds up along the fault for it to break again. When the fault breaks all the energy of the plates moving since the last earthquake gets released at once. That makes a large earthquake! So, a lot of really big earthquakes occur on boundaries between plates. Can you look at a map of the earth's plates and predict where else there are probably a lot of earthquakes?
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