Faults are fractures in Earth's crust where rocks on either side of the crack have slid past each other.
Sometimes the cracks are tiny, as thin as hair, with barely noticeable movement between the rock layers. But faults can also be hundreds of miles long, such as the San Andreas Fault in California and the Anatolian Fault in Turkey, both of which are visible from space.
Fault lines are usually much thinner than their length or depth. Earthquakes that occur on faults are generally about 375 miles (600 kilometers) deep. Below that, rocks are probably too warm for faults to generate enough friction to create earthquakes.
Three types of faults
There are three kinds of faults: strike-slip, normal and reverse faults. Each type is the outcome of different forces pushing or pulling on the crust, causing rocks to slide up, down or past each other.
Strike-slip faults indicate rocks are sliding past each other, with little to no vertical movement. Both the San Andreas and Anatolian Faults are strike-slip.
Normal faults create space. Two blocks of crust pull apart, extending the crust. The Basin and Range Province in North America and the East African Rift Zone are two well-known regions where normal faults are spreading apart Earth's crust.
Reverse faults, also called thrust faults, squeeze the crust, pushing two blocks of crust on top of each other. These faults are commonly found in mountain ranges such as the Himalayas and the Rocky Mountains.
Strike-slip faults are usually vertical, while normal and reverse faults are often at an angle to the surface of the Earth. The different styles of faulting can also combine in a single earthquake, with one fault moving in a vertical and strike-slip motion.
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