Earth's crust is very hard, and it is cracked in
many places. In fact, it is broken into large pieces called plates.
Even the smallest plate is bigger than the whole United States. These
plates move very slowly, but constantly. (Most plates are moving about
as fast as your fingernails are growing—not very fast!)
Some plates are moving apart, and the space between them is being
filled by new rock from deep within the Earth. Other plates are
slowly jamming together. California's famous San Andreas Fault is
the result of the Pacific Plate (which underlies most of the Pacific
Ocean) sliding past the North American Plate (which carries North
America, Greenland, a piece of northeastern Asia, and the Arctic
and northern Atlantic Oceans).
Eventually—perhaps over hundreds of years—the stress
on the rocks becomes too great, and they break or move suddenly.
This results in an earthquake.