The outer layer, or crust, of the earth is made up of plates that are constantly moving past one another in very slow side-to-side and up-and-down movements. These movements cause the earth’s rocks to rub against and slide past each other at their outer edges, creating a break in the crust.
This break is called a fault. Faults can lie far below the surface of the earth, or they may be at the top and visible.
As the plates’ movements travel away from the fault, they cause vibrations in the rocks for many miles around. These vibrations cause the ground to split open. This splitting is called an earthquake. Earthquakes are capable of destroying roads, buildings, and human life.
The San Andreas fault, which caused the great San Francisco earthquake in 1906, is 600 miles long!