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.
Three types of faults
There are three kinds of faults: strike-slip, normal and thrust (reverse) faults, said Nicholas van der Elst, a seismologist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York. Each type is the outcome of different forces pushing or pulling on the crust, causing rocks to slide up, down or past each other.
"Each describes a different kind of relative motion," van der Elst said.
Strike-slip faults indicate rocks are sliding past each other horizontally, 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, stretching the crust into a valley. 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, slide one block of crust on top of another. These faults are commonly found in collisions zones, where tectonic plates push up 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 event, with one fault moving in both a vertical and strike-slip motion during an earthquake. [Countdown: 13 Crazy Earthquake Facts]
All faults are related to the movement of Earth's tectonic plates. The biggest faults mark the boundary between two plates. Seen from above, these appear as broad zones of deformation, with many faults braided together. "Plate boundaries are always growing and changing, so these faults develop kinks and bends as they slide past each other, which generates more faults," van der Elst said.
Individual fault lines are usually narrower than their length or depth. Most earthquakes strike less than 50 miles (80 kilometers) below the Earth’s surface. The deepest earthquakes occur on reverse faults at about 375 miles (600 km) below the surface. Below these depths, rocks are probably too warm for faults to generate enough friction to create earthquakes, van der Elst said.
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