blind thrust fault

Diagram of blind thrust fault. Image credit: Stephen Nelson, Tulane University).

A fault is a fracture or zone of fractures in a planet's crust, accompanied by displacement of the opposing sides parallel to the fracture. There are three basic types of fault – strike-slip, normal, and thrust – as shown in the diagram. These range in size from a few tens of meters to hundreds of kilometers in dimension.


Types of fault


Strike-slip faults are vertical (or nearly vertical) fractures where the blocks have mostly moved horizontally. If the block opposite an observer looking across the fault moves to the right, the slip style is termed right lateral; if the block moves to the left, the motion is termed left lateral. A transform fault is a special variety of strike-slip fault that accommodates relative horizontal slip between other tectonic elements, such as oceanic crustal plates. Transform faults often extend from mid-ocean ridges.


Dip-slip faults are inclined fractures where the blocks have mostly shifted vertically. If the rock mass above an inclined fault moves down, the fault is termed normal, whereas if the rock above the fault moves up, the fault is termed reverse. A thrust fault is a reverse fault with a dip of 45° or less. Oblique-slip faults have significant components of different slip styles. A blind thrust fault is a thrust fault that does not rupture all the way up to the surface so there is no evidence of it on the ground. It is "buried" under the uppermost layers of rock in the crust.


types of fault


Other aspects of faults


The fault plane is the planar (flat) surface along which there is slip during an earthquake. The fault trace is the intersection of a fault with the ground surface; also, the line commonly plotted on geologic maps to represent a fault.


A fault scarp is a cliff or steep slope formed directly by movement along a fault and representing the exposed surface of the fault before modification by erosion and weathering. Fault gouge is crushed and ground-up rock produced by friction between the two sides when a fault moves.


fault scarp
More than 6 ft was added to this fault scarp by vertical movement in the 1983 Borah Peak, Idaho, earthquake. Credit: US Geological Survey.


Slip is the relative displacement of formerly adjacent points on opposite sides of a fault, measured on the fault surface. The slip rate is how fast the two sides of a fault are slipping relative to one another, as determined from geodetic measurements, from offset man-made structures, or from offset geologic features whose age can be estimated. It is measured parallel to the predominant slip direction or estimated from the vertical or horizontal offset of geologic markers.