Now, consider this: if we hold the foot wall stationary, gravity will normally want to pull the hanging wall down, right? Faults that move the way you would expect gravity to move them normally are called normal faults!
Compare the image to the right with the normal fault above. Along a reverse fault one rocky block is pushed up relative to rock on the other side.
Can you see the foot-shaped foot wall and the hanging wall resting or hanging above it? Think about this: if we hold the foot wall stationary, where would the hanging wall go if we reversed gravity? The hanging wall will slide upwards, right? When movement along a fault is the reverse of what you would expect with normal gravity we call them reverse faults!
Strike-slip faults have a different type of movement than normal and reverse faults. You probably noticed that the blocks that move on either side of a reverse or normal fault slide up or down along a dipping fault surface.
All the stress and strain produced by moving plates builds up in the Earth's rocky crust until it simply can't take it any more. All at once, CRACK!, the rock breaks and the two rocky blocks move in opposite directions along a more or less planar fracture surface called a fault.
We classify faults by how the two rocky blocks on either side of a fault move relative to each other. The one you see here is a normal fault. A normal fault drops rock on one side of the fault down relative to the other side. Take a look at the side that shows the fault and arrows indicating movement. See the block farthest to the right that looks kind of like a foot? That's the foot wall. Now look at the block on the other side of the fault. See how it's resting or hanging on top of the foot wall block? That's the hanging wall.
The Keweenaw Fault is a thrust fault, the name we give to prominent reverse faults. They are driven by significant tectonic events that affect large areas, like continental collisions.
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How does rock respond to stress?
[updated 2021] A fault is a rock fracture where the two sides have been displaced relative to each other. Faults are categorized into three general groups based on the sense of slip or movement.
SEE TABS ABOVE for stand-alone versions of each fault type.
This clip includes selected excerpts from the more-in-depth animation, "Earthquake Faults, Plate Boundaries, & Stress"
Normal fault—the block above the inclined fault moves down relative to the block below the fault. This fault motion is caused by extensional forces and results in extension. [Other names: normal-slip fault, tensional fault or gravity fault] Examples include Basin & Range faults.
Reverse fault—the block above the inclined fault moves up relative to the block below the fault. This fault motion is caused by compressional forces and results in shortening. A reverse fault is called a thrust fault if the dip of the fault plane is small. [Other names: reverse-slip fault or compressional fault.] Examples include the Rocky Mountains and the Himalayan Mountains.
Strike-slip fault—movement of blocks along a fault is horizontal and the fault plane is nearly vertical. If the block on the far side of the fault moves to the left, as shown in this animation, the fault is called left-lateral (Figure 2). If it moves to the right, the fault is called right-lateral. The fault motion of a strike-slip fault is caused by shearing forces. [Other names: trans current fault, lateral fault, tear fault or wrench fault.] Examples include the San Andreas Fault, California; Anatolian Fault, Turkey.
Two broad categories of faults exist - faults that exhibit horizontal movement and faults that exhibit vertical movement. Faults showing vertical movement include tensional (normal) and compressional (reverse) faults. Tensional faults are produced through tension (extension or pulling apart) of the crust causing the hanging wall to move down relative to the footwall. A fault plane is a relatively flat surface where rocks break due to displacement. The hanging wall is the block of rock that sits above the fault plane; whereas, the footwall is the block of rock that sits below the fault plane. Compressional faults are produced through compression (shortening or pushing together) of the crust causing the hanging wall to move up relative to the footwall. Note that in the animation the rocks are layered. The older rocks are found at the bottom layers and become younger as you move up toward the surface. Examine a point along the fault plane.
fault, in geology, a planar or gently curved fracture in the rocks of Earth’s crust, where compressional or tensional forces cause relative displacement of the rocks on the opposite sides of the fracture. Faults range in length from a few centimetres to many hundreds of kilometres, and displacement likewise may range from less than a centimetre to several hundred kilometres along the fracture surface (the fault plane). In some instances, the movement is distributed over a fault zone composed of many individual faults that occupy a belt hundreds of metres wide. The geographic distribution of faults varies; some large areas have almost none, others are cut by innumerable faults. Faults may be vertical, horizontal, or inclined at any angle. Although the angle of inclination of a specific fault plane tends to be relatively uniform, it may differ considerably along its length from place to place. When rocks slip past each other in faulting, the upper or overlying block along the fault plane is called the hanging wall, or headwall; the block below is called the footwall. The fault strike is the direction of the line of intersection between the fault plane and Earth’s surface. The dip of a fault plane is its angle of inclination measured from the horizontal. types of faulting in tectonic earthquakes
Faults are classified according to their angle of dip and their relative displacement. Normal dip-slip faults are produced by vertical compression as Earth’s crust lengthens. The hanging wall slides down relative to the footwall. Normal faults are common; they bound many of the mountain ranges of the world and many of the rift valleys found along spreading margins of tectonic plates. Rift valleys are formed by the sliding of the hanging walls downward many thousands of metres, where they then become the valley floors.
A block that has dropped relatively downward between two normal faults dipping toward each other is called a graben. A block that has been relatively uplifted between two normal faults that dip away from each other is called a horst. A tilted block that lies between two normal faults dipping in the same direction is a tilted fault block.
Reverse dip-slip faults result from horizontal compressional forces caused by a shortening, or contraction, of Earth’s crust. The hanging wall moves up and over the footwall. Thrust faults are reverse faults that dip less than 45°. Thrust faults with a very low angle of dip and a very large total displacement are called overthrusts or detachments; these are often found in intensely deformed mountain belts. Large thrust faults are characteristic of compressive tectonic plate boundaries, such as those that have created the Himalayas and the subduction zones along the west coast of South America.
San Andreas Fault
Strike-slip (also called transcurrent, wrench, or lateral) faults are similarly caused by horizontal compression, but they release their energy by rock displacement in a horizontal direction almost parallel to the compressional force. The fault plane is essentially vertical, and the relative slip is lateral along the plane. These faults are widespread. Many are found at the boundary between obliquely converging oceanic and continental tectonic plates. Well-known terrestrial examples include the San Andreas Fault, which, during the San Francisco earthquake of 1906, had a maximum movement of 6 metres (20 feet), and the Anatolian Fault, which, during the İzmit earthquake of 1999, moved more than 2.5 metres (8.1 feet).
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Oblique-slip faults have simultaneous displacement up or down the dip and along the strike. The displacement of the blocks on the opposite sides of the fault plane usually is measured in relation to sedimentary strata or other stratigraphic markers, such as veins and dikes. The movement along a fault may be rotational, with the offset blocks rotating relative to one another.
Fault slip may polish smooth the walls of the fault plane, marking them with striations called slickensides, or it may crush them to a fine-grained, claylike substance known as fault gouge; when the crushed rock is relatively coarse-grained, it is referred to as fault breccia. Occasionally, the beds adjacent to the fault plane fold or bend as they resist slippage because of friction. Areas of deep sedimentary rock cover often show no surface indications of the faulting below.
Movement of rock along a fault may occur as a continuous creep or as a series of spasmodic jumps of a few metres during a few seconds. Such jumps are separated by intervals during which stress builds up until it overcomes the frictional forces along the fault plane and causes another slip. Most, if not all, earthquakes are caused by rapid slip along faults.