What is a fault and what are the different

types?

A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to

move relative to each other. This movement may occur rapidly, in the form of an earthquake - or may occur slowly, in the form of creep. Faults may range in length from a few millimeters to thousands of kilometers. Most faults produce repeated displacements over geologic time. During

an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary angle in between.

Earth scientists use the angle of the fault with respect to the surface (known as the dip) and the direction of slipslip along the fault to classify faults. Faults which move along the direction of the

dip plane are dip-slip faults and described as either normal or reverse (thrust), depending on their motion. Faults which move horizontally are known as strike-slip faults and are classified as either

right-lateral or left-lateral. Faults which show both dip-slip and strike-slip motion are known as oblique-slip faults.

The following definitions are adapted from The Earth by Press and Siever.

normal fault - a dip-slip fault in which the block above the fault has moved downward relative to the block below. This type of faulting occurs in response to extension and is often observed in the Western United States Basin and Range Province and along oceanic ridge systems.

thrust fault - a dip-slip fault in which the upper block, above the fault plane, moves up and over

the lower block. This type of faulting is common in areas of compression, such as regions where one plate is being subducted under another as in Japan. When the dip angle is shallow, a reverse fault is often described as a thrust fault.

strike-slip fault - a fault on which the two blocks slide past one another. The San Andreas Fault is

an example of a right lateral fault. left-lateral strike-slip fault - a fault on which the displacement of the far block is to the left when

viewed from either side.

right-lateral strike-slip fault - a fault on which the displacement of the far block is to the right when viewed from either side.

Different types of Faults

A close look at faults helps geologists to understand how the tectonic plates have moved relative to

one another.

Types of movement of crustal blocks that can occur along faults during an earthquake:

1. Where the crust is being pulled apart, normal faulting occurs, in which the overlying (hanging-wall) block moves down with respect to the lower (foot wall) block.

2. Where the crust is being compressed, reverse faulting occurs, in which the hanging-wall block

moves up and over the footwall block – reverse slip on a gently inclined plane is referred to as thrust faulting.

3. Crustal blocks may also move sideways past each other, usually along nearly-vertical faults. This ‘strike-slip’ movement is described as sinistral when the far side moves to the left, and

dextral, when the far side moves to the right.

4. An oblique slip involves various combinations of these basic movements, as in the 1855 Wairarapa Fault rupture, which included both reverse and dextral movement. (COM pg. 100).

Faults can be as short as a few metres and as long as 1000km. The fault rupture from an

earthquake isn’t always a straight or continuous line. Sometimes there can be short offsets between parts of the fault, and even major faults can have large bends in them.

The epicenter is the point on the Earth's surface that is directly above the hypocenter or focus, the point where an earthquake or underground explosion originates. The word derives from the New

Latin noun epicentrum,[2] the latinisation of the ancient Greek adjective ἐπίκεντρος (epikentros),

"occupying a cardinal point, situated on a centre",[3] from ἐπί (epi) "on, upon, at"[4] and κέντρον

(kentron) "centre".[5] The term was coined by the Irish seismologist Robert Mallet.[6]

The epicenter is directly above the earthquake's hypocenter (focus).

In the case of earthquakes, the epicenter is directly above the point where the fault begins to rupture, and in most cases, it is the area of greatest damage. However, in larger events, the length

of the fault rupture is much longer, and damage can be spread across the rupture zone. For example, in the magnitude 7.9, 2002 Denali earthquake in Alaska, the epicenter was at the western end of the rupture, but the greatest damage occurred about 330 km away at the eastern end of the

rupture zone.[7]

A fault line is the surface trace of a fault, the line of intersection between the fault plane and the Earth's

surface.

The fault plane is the planar (flat) surface along which there is slip during an earthquake.

Fault plane diagram. (Image courtesy of Kian H. Chong, Univ. of California, Davis)