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A - B - C - D - E - F - G - H - I - J - K - L - M
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beheaded channel

    Portion of a channel downstream from a fault, which is separated from its headwaters by strike-slip movement along the fault. In the process of abandoning the older downstream portion of the channel (and thereby "beheading" the downstream portion of the channel), the headwaters carve a new path straight across the fault. In subsequent earthquakes, the new channel will be offset, and it will eventually become beheaded as well. Thus the cycle will continue.

carbon-14 (14C or C-14) age

    An absolute age obtained for geologic materials containing bits or pieces of carbon using measurements of the proportion of radioactive carbon (14C) to daughter carbon (12C). These dates are independently calibrated with calendar dates. This is used to determine when past earthquakes occurred on a fault.

carbon-14 (14C or C-14) dating

    The application of specific procedures to determine the carbon-14 age of a sample.


    The bed of a creek or river; gully.


    Slow, more or less continuous movement along a fault. It is sometimes called "seismic creep" to distinguish it from the slumping of rock or soil on slopes (which is also known as creep), and sometimes called "aseismic creep", since it does not trigger events greater than microearthquakes. Faults that are creeping do not tend to have large earthquakes.

creeping segment

    A portion of a fault characterized by aseismic creep.


    The thin outer layer of the Earth's surface, averaging about 10 kilometers thick under the oceans and up to about 50 kilometers thick on the continents. This is the only layer of the Earth that humans have actually seen.


    The angle between a geologic surface -- for example, a fault plane -- and the horizontal. The direction of dip can be thought of as the direction a ball, if placed upon the tilted surface, would roll. Thus, a ball placed on a north-dipping fault plane would roll northward. The dip of a surface is always perpendicular to the strike of that surface.

dip slip

    Fault movement (slip) that is parallel to the dip of the fault. This can describe both normal slip and reverse slip.


    The difference between the initial position of a reference point and any later position. The amount any point affected by an earthquake has moved from where it was before the earthquake.


    A channel through which water can flow out of a particular region.


    This term is used to describe both sudden slip on a fault, and the resulting ground shaking and radiated seismic energy caused by the slip.


    The point on the Earth's surface directly above the (subterranean) point of origin (hypocenter) of an earthquake; only two measurements, latitude and longitude, are needed to locate it.



    A fracture along which the blocks of crust on either side have moved relative to one another parallel to the fracture. 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. 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 (or thrust). Oblique-slip faults have significant components of both slip styles. Faults are caused by earthquakes, and earthquakes are likely to reoccur on pre-existing faults.

fault creep

fault line

    A commonly used term that is synonymous with the surface trace of a fault (or the fault trace). It should never be used to describe the fault itself; faults are planar, not linear, features.

fault rupture

fault scarp

fault strand

    An individual fault of a set of closely-spaced parallel or subparallel faults of a fault system.

fault trace

    Intersection of a fault with the ground surface; also, the line commonly plotted on geologic maps to represent a fault.


    That point within the Earth from which originates the first motion of an earthquake and its elastic waves, i.e., the point of origin of an earthquake. (See hypocenter.)


    Of the two sides of a non-vertical fault, the side below the fault plane. It is called the footwall because where inactive faults have been "filled in" with mineral deposits and then mined, this is the side on which miners walk. (See hanging wall.)


    The study of the character and origin of landforms, such as mountains, valleys, etc.

hanging wall

    Of the two sides of a fault, the side above the fault plane. It is called the hanging wall because where inactive faults have been "filled in" with mineral deposits and then mined, this is the side on which miners can hang their lanterns. (See footwall.)


    The calculated location of the focus of an earthquake. It can be expressed with no fewer than three measurements: latitude, longitude, and depth.


    A measure of the effects of an earthquake at a particular place on humans, structures and/or the land itself. The intensity at a point depends not only upon the strength of the earthquake (magnitude) but also upon the distance from the earthquake to the point and the local geology at that point. It is usually estimated from descriptions of shaking or damage to buildings or terrain. Note that there are many intensities for an earthquake, depending on where you are, unlike the magnitude, which is one number for each earthquake.

lateral fault

    A fault in which slips in such a way that the two sides move with a predominantly lateral motion (with respect to each other). There are two kinds of lateral slip: right-lateral and left-lateral. They can be distinguished by standing on one side of the fault, facing the fault (and, of course, the other side), and noting which way the objects across the fault have moved with respect to you. If the objects across the fault have moved to your right, the fault is right-lateral. If the motion is to the left, then the fault is left-lateral. (Also called strike-slip fault.)

linear ridge


    A number that characterizes the relative size of an earthquake, as determined from seismographic information. Several scales have been defined, but the most commonly used are (1) local magnitude (ML), commonly referred to as "Richter magnitude"; (2) surface-wave magnitude (MS); (3) body-wave magnitude (mb); and (4) moment magnitude (MW). Scales 1-3 have limited range and applicability and do not satisfactorily measure the size of the largest earthquakes. The moment magnitude (MW) scale is uniformly applicable to all sizes of earthquakes but is more difficult to compute than the other types. All magnitude scales are calibrated to yield approximately the same value for any given earthquake; however, since the moment magnitude scale is not based on the same measurements as Richter (local or surface-wave) magnitudes, the different magnitudes do not always agree, particularly for very large quakes.

mb (body-wave magnitude)

    Defined by Gutenberg and Richter in 1956, body-wave magnitude uses only short-period P waves to arrive at a numerical magnitude rating. This rating is useful for judging the size of explosions (including nuclear bomb tests!), since they tend to produce smaller S waves than natural earthquakes.

ML (local magnitude)

    Based upon the Richter's original magnitude scale, this is a measure of the amplitude of the maximum trace deflection (i.e. the distance between the resting position of the seismogram needle and the crest of the largest squiggle it records) versus distance from the source. Large earthquakes can produce so much shaking that seismograph traces go "off-scale," leading to a "saturation" in the maximum amplitude of deflection. Consequently, local magnitude tends to be used only for earthquakes smaller than about magnitude 6.

MS (surface-wave magnitude)

    Surface-wave magnitude is calculated using the amplitude, on a long-period vertical seismometer, of surface waves with a 20-second period.

MW (moment magnitude)

    The moment magnitude scale is a way of rating the seismic moment of an earthquake with a simple, logarithmic numerical scale similar to the original Richter magnitude scale. Because it relates directly to the energy released by an earthquake, and because it does not "saturate" the way local magnitude does, it has become the standard in modern seismology.


    A term used to describe earthquakes under Richter magnitude 2, and occasionally, slightly larger quakes, especially those not felt by people nearby.

Modified Mercalli Intensity Scale

    Mercalli intensity scale modified for North American conditions. A scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, that is designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects.

normal fault

    A fault characterized by predominantly vertical displacement in which the hanging wall is moved downward with respect to the footwall of the fault. Generally, this kind of fault is a sign of tectonic extension.

oblique (fault or slip)

    Describing motion that is a combination of movement both perpendicular and parallel to the strike of a fault -- a combination of strike-slip and dip-slip (whether normal or reverse).


    v. To displace (relative to each other) the two halves of an object or set of objects (as a channel, fence, road, canal, row of telephone poles, etc.) that crosses a lateral fault, by motion along that fault. Something that runs directly across the fault will be separated and displaced (i.e., offset) by an earthquaken. Something that has been offset by motion along a lateral fault,  or the amount by which something has been offset.  adj. The quality of having been offset by motion along a lateral fault. Offsets observed in the field may have accumulated from one or more separate earthquakes.

offset channel

    A channel that has been offset by motion along a lateral fault. Although the upstream portion of the channel will not line up with or project into the downstream portion of the channel, the term offset channel implies that the two portions are still connected, although the channel may bend sharply at the fault.


    The study of past earthquakes on a fault, as determined by looking at the layers of rock beneath the surface and how they have been shifted by earthquakes in the past.


    The time between two successive wave crests. Period is inversely related to frequency; that is, long-period waves have low frequency, whereas short-period waves have high frequency. The higher frequencies (short-period waves) die off with distance more quickly than the low frequencies (long-period waves), just like you can hear a low pitched noise at a greater distance than a high pitched noise. This is also why nearby earthquakes feel jerky while distant earthquakes produce a rolling motion.


    One of the huge sections which make up the Earth's crust. The plates are continuously moving.

plate boundary

    The place where two or more plates in the Earth's crust meet.

plate tectonics

    The widely accepted theory that the Earth's crust and upper mantle (the lithosphere) is broken into a number of more or less rigid, but constantly moving, segments or plates. The theory predicts that most earthquakes occur when plates move past one another.

ponded sediments

    Sediments which have ponded, or collected, behind a barrier which impedes the outward flow of those sediments.

pressure ridge

pull-apart basin

radiocarbon age (or radiocarbon dating)

reverse fault

    A fault in which the displacement is predominantly vertical, and the hanging wall is moved upward with respect to the footwall. Some amount of reverse slip is often seen in predominantly lateral faults. If a reverse fault has a dip angle of less than 45 degrees, it is called a thrust fault.

Richter scale

    Introduced in 1935 by Charles F. Richter, the Richter scale is a numerical scale for quantifying earthquake magnitude -- typically it refers to local magnitude, but for larger quakes, it often refers to surface-wave magnitude. (Currently, large quakes are generally assigned a moment magnitude, which is scaled to be similar, but is based on seismic moment, a better measure of the energy of an earthquake.) Since the Richter scale is logarithmic, very small earthquakes (microearthquakes) can have a negative magnitudes. While the scale has no theoretical upper limit, the practical upper limit, given the strength of materials in the crust, is just below 9 for local or surface-wave magnitudes (and just below 10 for moment magnitudes).

rupture zone

    The area of the Earth through which faulting occurred during an earthquake. For very small earthquakes, this zone could be the size of a pinhead, but in the case of a great earthquake, the rupture zone may extend several hundred kilometers in length and tens of kilometers in width. (See surface rupture.)

sag pond


    A roughly linear, cliff-like slope or face that breaks the continuity of a surface into distinct levels. Scarps are often produced by faulting, especially that which involves a significant amount of dip slip.


    A standing wave in a closed body of water such as a lake or bay. It can be characterized as the sloshing of water in the enclosing basin. Seiches can be initiated by local atmospheric changes, tidal currents, or earthquakes. Similar to water sloshing in a bathtub. Swimming pools often have seiches during earthquakes.

seismic creep

seismic moment

    A measure of the strength of an earthquake, equal to the product of the force and the moment arm of the double-couple system of forces that produces ground displacements equivalent to that produced by the actual earthquake slip. It is also equal to the product of the rigidity modulus of the Earth material, the fault surface area, and the average slip along the fault. Therefore, both seismological and geological observations can produce the same result.

sense of slip

    The sense with which one side of a fault slips relative to the other side, in a reference frame defined by a horizontal (level) plane oriented with the pull of Earth's gravity pointed "down" (perpendicular to the plane).

shutter ridge

    A linear hill or scarp sloping in a direction opposite to the overall local topographic gradient, formed by strike-slip or oblique-slip offset of pre-existing irregular topography. A shutter ridge tends to block the flow of water and debris across the fault, as shutters block the flow of light or air through a window.


    The relative displacement of formerly adjacent points on opposite sides of a fault, measured on the fault surface.

slip rate

    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.



    The direction, or trend, of the line marking the intersection of a fault plane (or another planar geologic feature) with the horizontal. Strike is always at a right angle to dip.

strike-slip fault

    A fault along which the slip motion is parallel to the strike of the fault. (Also called lateral fault.)

surface faulting

    Displacement that reaches the Earth's surface during slip along a fault. Commonly occurs with shallow earthquakes, those with an epicenter less than 20 km. Surface faulting also may accompany aseismic creep or natural or man-induced subsidence. Synonymous with surface rupture.

surface rupture

    The breakage of ground along the surface trace of a fault caused by the intersection of the ground surface with the fault surface that ruptured in an earthquake. Synonymous with surface faulting.

surface trace

    The intersection of a fault plane with the ground surface. It is sometimes, but not always, expressed at the surface by geomorphic evidence (ridges, valleys, saddles, etc.).

tectonic plate

transform fault

    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 oceanic ridges.

thrust fault

    A specific kind of reverse fault in which the dip of the fault is less than 45 degrees over much if not all of its length. It is characterized not so much by vertical displacement, but by horizontal compression. Thrust faults are an obvious sign of compressional tectonics.

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For more comprehensive lists of earthquake-related definitions, please visit:
the USGS Earthquake Glossary
the SCEC Seismology/Geology Glossary
and NEIC's Common Terms in Seismology