A fault is a fracture or a collection of fractures in the Earth’s surface where movement has occurred. Most faults are inactive, but some, like California’s San Andreas fault, are quite active. Geologists have not discovered all of the Earth’s faults, and sometimes earthquakes occur that take the world by surprise, like the one in Northridge, California, in 1994. When earthquakes occur on faults that were previously unknown, they are called blind faults.