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Glossary
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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.
channel
The bed of a creek or river; gully.
creep
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.
crust
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.
dip
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.
displacement
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.
drainage
A channel through which water can flow out of a particular region.
earthquake
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.
epicenter
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.
escarpment
fault
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.
focus
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.)
footwall
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.)
geomorphology
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.)
hypocenter
The calculated location of the focus of an earthquake.
It can be expressed with no fewer than three
measurements: latitude, longitude, and depth.
intensity
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
magnitude
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.
microearthquake
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).
offset
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
earthquake. n. 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.
paleoseismology
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.
period
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.
plate
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 topographic depression produced by extensional bends or stepovers
along a strike-slip fault. It may or may
not contain water year-round. Synonymous with pull-apart basin.
(Compare pressure ridge.)
scarp
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.
seiche
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.
slip
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.
strand
strike
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.
Return to the top of the page.
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
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