Fault Lines: Facts About Cracks in the Earth

San Andreas Fault. (Image credit: USGS.)

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.

Faults are categorized into three general groups based on the sense of slip or movement. (Image credit: IRIS)

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.

Earth’s biggest exposed fault

For nearly a century, scientists have been aware of a 4.47 mile-deep (7.2 km) oceanic abyss — known as the Weber Deep — located off the coast of eastern Indonesia in the Banda Sea. But until recently, they had been unable to explain how it got so deep.

The Weber Deep is the deepest point in the ocean that is not in a trench; trenches are formed during the subduction of two tectonic plates — when one slides under the other. However, the Weber Deep is a forearc basin, which is essentially a depression located in front of the Banda arc (curved chain of volcanic islands), according to New Atlas. So the question remained: Why is the Weber Deep as deep as a trench?

Based on studies of the sea bed and knowledge of geology, one hypothesis stated that the abyss was the result of an extension along a potential low-angle fault — but this theory had remained unproven. Now, researchers at Australian National University (ANU) and Royal Holloway University of London have confirmed this theory. Lead researcher Jonathan Pownall came upon extensions of the fault line on the mountains of the Banda arc islands while on a boat trip.

“I was stunned to see the hypothesized fault plane, this time not on a computer screen, but poking above the waves,” said Pownall in a Science Daily press release. Indeed, the huge abyss had been formed by “extension along what might be Earth’s largest-identified exposed fault plane,” he said.

Through further analysis of high-resolution maps of the sea floor, the geologists discovered that the bottom-level rocks were cut by hundreds of straight parallel scars. These cuts reveal that a piece of the Earth’s crust “bigger than Belgium or Tasmania” must have been torn apart by 74.5-mile (120 km) of extension along a low-angle, crack — or detachment fault — to form the depression, according to the press release.

This Banda Detachment fault represents a rip in the ocean floor that is exposed for more than 23,166 square miles (60,000 square km). In fact, in some areas, the amount of extension was so severe that there was no longer any trace of oceanic crust, according to New Atlas.

The new find will help geologists assess the dangers of future tsunamis stemming from this area which is located in the Ring of Fire, a hotbed of earthquake and volcanic activity in the Pacific Ocean.

Email Becky Oskin or follow her @beckyoskin. Follow us @livescience, Facebook & Google+.

Additional reporting by Traci Pedersen, Live Science contributor.

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Contributing Writer
Becky Oskin covers Earth science, climate change and space, as well as general science topics. Becky was a science reporter at Live Science and The Pasadena Star-News; she has freelanced for New Scientist and the American Institute of Physics. She earned a master's degree in geology from Caltech, a bachelor's degree from Washington State University, and a graduate certificate in science writing from the University of California, Santa Cruz.