A
B C D
E F G
H I J K
L M N
O P
Q R S
T U V W
X Y Z
acceleration.
When you step on the accelerator in the car or put on the brakes,
the car goes faster or slower. When it is changing from one speed
to another, it is accelerating (faster) or decelerating (slower).
This change from one speed, or velocity, to another is called
acceleration. During an earthquake when the ground is shaking,
it also experiences acceleration. The peak acceleration
is the largest acceleration recorded by a particular station during
an earthquake.
accelerogram.
The recording of the acceleration
of the ground during an earthquake.
accelerograph.
An instrument that records the acceleration
of the ground during an earthquake, also commonly called an accelerometer.
accretionary
wedge. Sediments, the top layer of material on a tectonic
plate, that accumulate and deform where oceanic and continental
plates collide. These sediments are scraped off the top of the
downgoing oceanic crustal plate and are appended to the edge of
the continental plate.
active
fault. A fault
that is likely to have another earthquake sometime in the future.
Faults are commonly considered to be active if they have moved
one or more times in the last 10,000 years.
aftershocks.
Earthquakes that follow the largest shock of an earthquake sequence.
They are smaller than the mainshock and within 1-2 fault lengths
distance from the mainshock fault. Aftershocks can continue over
a period of weeks, months, or years. In general, the larger the
mainshock, the larger and more numerous the aftershocks, and the
longer they will continue.
(See also Earthquake
ABC's)
alluvium.
Loose gravel, sand, silt, or clay deposited by streams.
amplification.
Most earthquakes are relatively small, in fact, so small that
no one feels them. In order for seismologists to see the recording
of the movement of the ground from the smaller earthquakes, the
recording has to be made larger. It's like looking at the recording
through a magnifying glass, and the amount that it is magnified
is the amplification. Shaking levels at a site may also be increased
by focusing of seismic energy caused by the geometry of the sediment
velocity structure, such as basin subsurface topography, or by
surface topography.
amplitude.
The size of the wiggles on an earthquake recording.
arc.
A chain of volcanoes (volcanic arc) that sometimes forms on the
land when an oceanic plate collides with a continental plate and
then slides down underneath it (subduction).
aseismic.
This term describes a fault on which no earthquakes have been
observed.
asperity.
An area on a fault that is stuck.
asthenosphere.
The ductile part of the earth just below the lithosphere, including
the lower mantle. The
asthenosphere is about 180 km thick.
attenuation.
When you throw a pebble in a pond, it makes waves on the surface
that move out from the place where the pebble entered the water.
The waves are largest where they are formed and gradually get
smaller as they move away. This decrease in size, or amplitude,
of the waves is called attenuation.
backarc.
The region landward of the chain of volcanoes (volcanic arc),
in a subduction system.
basement.
Harder and usually older igneous and metamorphic rocks that underlie
the main sedimentary rock sequences (softer and usually younger)
of a region and extend downward to the base of the crust.
bedrock.
Relatively hard, solid rock that commonly underlies softer rock,
sediment, or soil; a subset of the basement.
Benioff
zone. A dipping planar (flat) zone of earthquakes that
is produced by the interaction of a downgoing oceanic crustal
plate with a continental plate. These earthquakes can be produced
by slip along the subduction thrust fault or by slip on faults
within the downgoing plate as a result of bending and extension
as the plate is pulled into the mantle.
Also known as the Wadati-Benioff zone.
blind
thrust fault. 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.
body
wave. A seismic wave that moves through the interior
of the earth, as opposed to surface
waves that travel near the earth's surface.
P and S waves are examples. Each type of wave shakes the ground
in different ways.
brittle-ductile
boundary. The depth in the crust where the crust changes
from being brittle (tending to break) above, to being ductile
(tending to bend) below. Most earthquakes occur in the brittle
portion of the crust above the brittle-ductile boundary.
14C
age date. 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.
compressional
stress. The stress that squeezes something. It is the
stress component perpendicular to a given surface, such as a fault
plane, that results from forces applied perpendicular to the surface
or from remote forces transmitted through the surrounding rock.
compressional
wave. See P
wave.
core.
The innermost part of the earth. The outer core extends from 2500
to 3500 miles below the earth's surface and is liquid metal. The
inner core is the central 500 miles and is solid metal. (See also
Earthquake ABC's)
creep.
Slow, more or less continuous movement occurring on faults due
to ongoing tectonic deformation. Faults that are creeping do not
tend to have large earthquakes.
crust.
The outermost major layer of the earth, ranging from about 10
to 65 km in thickness worldwide. The uppermost 15-35 km of crust
is brittle enough to produce earthquakes. (See also Earthquake
ABC's)
dip.
The angle that a planar geologic surface (for example, a fault)
is inclined from the horizontal.
dip
slip. See fault.
directivity.
An effect of a fault rupturing whereby earthquake ground
motion in the direction of rupture propagation is more severe
than that in other directions from the earthquake source.
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.
deformation.
A change in the original shape of a material. When we are talking
about earthquakes, deformation is due to stress
and strain.
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, or by volcanic or magmatic activity, or other sudden
stress changes in the earth.
earthquake
hazard. Anything associated with an earthquake that
may affect the normal activities of people. This includes surface
faulting, ground
shaking, landslides,
liquefaction, tectonic
deformation, tsunamis,
and seiches.
earthquake
risk. The probable building damage, and number of people
that are expected to be hurt or killed if a likely earthquake
on a particular fault occurs. Earthquake risk and earthquake hazard
are occasionally used interchangeably.
epicenter.
The point on the earth's surface vertically above the point in
the crust where a seismic rupture begins. (See also Earthquake
ABC's)
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.
fault
creep. See creep.
fault
gouge. Crushed and ground-up rock produced by friction
between the two sides when a fault moves.
fault
plane. The planar (flat) surface along which there
is slip during an earthquake.
fault-plane
solution. A way of showing the fault and the direction
of slip on it from an earthquake, using circles with two intersecting
curves that look like beach balls.
fault
scarp. A feature on the surface of the earth that looks
like a step caused by slip on the fault.
fault
trace. Intersection of a fault with the ground surface;
also, the line commonly plotted on geologic maps to represent
a fault.
first
motion. On a seismogram, the direction of ground motion
as the P wave arrives
at the seismometer.
Upward ground motion indicates an expansion in the source region;
downward motion indicates a contraction.
focal
depth. A term that refers to the depth of an earthquake
hypocenter.
focal-mechanism
solution. See fault-plane
solution.
focus.
See hypocenter.
forearc.
The region between the subduction
zone and the volcanic chain (volcanic arc).
foreshocks.
Foreshocks are relatively smaller earthquakes that precede
the largest earthquake in a series, which is termed the mainshock.
Not all mainshocks have foreshocks. (See also Earthquake
ABC's)
frequency.
The number of times something happens in a certain period of time,
such as the ground shaking up and down or back and forth during
an earthquake.
G
or g. G
or g is the force of gravity (an acceleration of 9.78 meters/second2).
When there is an earthquake, the forces caused by the shaking
can be measured as a percentage of the force of gravity, or percent
g.
geodesy.
The science of determining the size and shape of the earth and
the precise location of points on its surface.
geodetic.
Referring to the determination of the size and shape of the earth
and the precise location of points on its surface.
geology.
The study of the planet earth -- the materials it is made of,
the processes that act on those materials, the products formed,
and the history of the planet and its life forms since its origin.
geomorphology.
The study of the character and origin of landforms, such as mountains,
valleys, etc.
geophysics.
The study of the earth by physical methods.
geotechnical.
Referring to the use of scientific methods and engineering principles
to acquire, interpret, and apply knowledge of earth materials
for solving engineering problems.
gravity.
The attraction between two masses, such as the earth and an
object on its surface. Commonly referred to as the acceleration
of gravity. Changes in the gravity field can be used to infer
information about the structure of the earth's lithosphere and
upper mantle.
graben.
a down-dropped block of the earth's crust resulting from extension,
or pulling, of the crust. See also horst.
ground
failure. A general reference to
landslides, liquefaction,
lateral spreads,
and any other consequence of shaking that affects the stability
of the ground.
ground
motion (shaking). The movement of the earth's surface
from earthquakes or explosions. Ground motion is produced by waves
that are generated by sudden slip on a fault or sudden pressure
at the explosive source and travel through the earth and along
its surface. (See also Earthquake
FAQ discussion.)
halfspace.
A mathematical model used to approximate the earth when performing
some calculations in seismology. The model is much simpler than
the real earth.
harmonic
tremor. Continuous rhythmic earthquakes that can be
detected by seimographs. Harmonic tremors often precede or accompany
volcanic eruptions.
hazard.
See earthquake hazard.
Hertz
(Hz). A unit of frequency.
Expressed in cycles per second.
Holocene.
Refers to a period of time between the present and 10,000 years
before present. Applied to rocks or faults, this term indicates
the period of rock formation or the time of most recent fault
slip. Faults of this age are commonly considered active.
horst.
An upthrown block lying between two steep-angled fault blocks.
See also graben.
hypocenter.
The point within the earth where an earthquake rupture starts.
Also commonly termed the focus.
intensity.
A number (written as a Roman numeral) describing the severity
of an earthquake in terms of its effects on the earth's surface
and on humans and their structures. Several scales exist, but
the ones most commonly used in the United States are the Modified
Mercalli scale and the Rossi-Forel scale. There are many intensities
for an earthquake, depending on where you are, unlike the magnitude,
which is one number for each earthquake. ( See also Earthquake
ABC's & FAQ discussion.)
intraplate
and interplate. Intraplate pertains to processes
within the earth's crustal plates. Interplate pertains
to processes between the plates.
interplate
coupling. The ability of a fault
between two plates to lock and accumulate stress. Strong interplate
coupling means that the fault is locked and capable of accumulating
stress, whereas weak coupling means that the fault is unlocked
or only capable of accumulating low stress.
isoseismal.
A contour or line on a map bounding points of equal intensity
for a particular earthquake.
kinematic.
Referring to the general movement patterns and directions
of the earth's rocks that produce rock deformation.
landslide.
The downslope movement of soil and/or rock.
Late
Quaternary. The age between the present and 500,000
years before the present. Faults of this age are sometimes considered
active based on the observation of historical activity on faults
of this age in some locales.
lateral
spread and flow. Terms referring to landslides
that commonly form on gentle slopes and that have rapid fluid-like
flow movement, like water.
least-squares
fit. When plotting data points on a graph, the least-squares-fit
is the line or curve that comes closest to going through all the
points.
left-lateral.
If you were to stand on the fault and look along its length, this
is a type of strike-slip fault where the left block moves toward
you and the right block moves away. (See also right-lateral.)
lifelines.
Structures that are important or critical for a community
to function, such as roadways, pipelines, powerlines, sewers,
communications, and port facilities.
liquefaction.
A process by which water-saturated sediment temporarily loses
strength and acts as a fluid, like when you wiggle your toes in
the wet sand near the water at the beach. This effect can be caused
by earthquake shaking. (See also Earthquake
FAQ discussion.)
lithology.
The description of rock composition (what it's made of) and texture.
lithosphere.
The outer solid part of the earth, including the crust
and uppermost mantle.
The lithosphere is about 100 km thick, although its thickness
is age dependent (older lithosphere is thicker).The lithosphere
below the crust is brittle enough at some locations to produce
earthquakes by faulting, such as within a subducted oceanic plate.
locked
fault. A fault that is not slipping because frictional
resistance on the fault is greater than the shear stress across
the fault (it is stuck). Such faults may store strain
for extended periods that is eventually released in an earthquake
when frictional resistance is overcome.
Love
wave. A type of seismic surface wave having a horizontal
motion that is transverse (or perpendicular) to the direction
the wave is traveling.
Ma.
An abbreviation for one million years ago (Megannum).
magnetic
polarity reversal. A change of the earth's magnetic
field to the opposite polarity that has occurred at irregular
intervals during geologic time. Polarity reversals can be preserved
in sequences of magnetized rocks and compared with standard polarity-change
time scales to estimate geologic ages of the rocks. Rocks created
along the oceanic
spreading ridges commonly preserve this pattern of polarity
reversals as they cool, and this pattern can be used to determine
the rate of ocean ridge spreading. The reversal patterns recorded
in the rocks are termed sea-floor magnetic lineaments.
magnitude.
A number that characterizes the relative size of an earthquake.
Magnitude is based on measurement of the maximum motion recorded
by a seismograph.
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, based on
the concept of seismic
moment, is uniformly applicable to all sizes of earthquakes
but is more difficult to compute than the other types. All magnitude
scales should yield approximately the same value for any given
earthquake. ( See also Earthquake
ABC's & FAQ discussion.)
mainshock.
The largest earthquake in a sequence, sometimes preceded by one
or more foreshocks,
and almost always followed by many aftershocks.
mantle.
The part of the earth's interior between the metallic outer core
and the crust.
microzonation.
The identification of separate individual areas having different
potentials for hazardous earthquake effects.
Moho.
The boundary between the crust
and the mantle in the
earth. This is a depth where seismic waves change velocity and
there is also a change in chemical composition. Also termed the
Mohorovicic' discontinuity after the Croatian seismologist Andrija
Mohorovicic' (1857-1936) who discovered it. The boundary is between
25 and 60 km deep beneath the continents and between 5 and 8 km
deep beneath the ocean floor.
moment
magnitude. See magnitude.
natural
frequency. The frequency at which a particular object
or system vibrates when pushed by a single force or impulse, and
not influenced by other external forces or by damping. If you
hold a slinky by one end and let it hang down and then give it
one push up from the bottom, the rate of up-and-down motion is
its natural frequency.
normal
stress. That stress
component perpendicular to a given plane. If you lean against
a door after you close it, you are applying normal stress to the
door. ( See also shear stress.)
normal
fault. (See fault.)
oceanic
spreading ridge. A fracture zone along the ocean bottom
where molten mantle material comes to the surface, thus creating
new crust. This fracture can be seen beneath the ocean as a line
of ridges that form as molten rock reaches the ocean bottom and
solidifies.
oceanic
trench. A linear depression of the sea floor caused
by the subduction of one plate under another.
P
wave. A seismic body
wave that shakes the ground back and forth in the same direction
and the opposite direction as the direction the wave is moving.
( See also Earthquake
ABC's.)
paleoseismic.
Referring to the history of seismic events that is determined
by looking at the layers of rock beneath the surface and how they
have been shifted by earthquakes in the past.
peak
acceleration. See acceleration
or Earthquake FAQ.
pedogenic.
Pertaining to processes that add, transfer, transform, or remove
soil constituents.
period.
The time interval required for one full cycle of a wave.
plate
tectonics. A theory supported by a wide range of evidence
that considers the earth's crust
and upper mantle to
be composed of several large, thin, relatively rigid plates that
move relative to one another. Slip on faults that define the plate
boundaries commonly results in earthquakes. Several styles of
faults bound the plates, including thrust
faults along which plate material is subducted or consumed
in the mantle,
oceanic spreading ridges along which new crustal material
is produced, and transform
faults that accommodate horizontal slip (strike slip) between
adjoining plates. (See also "This
Dynamic Earth: The Story of Plate Tectonics".)
Pleistocene.
The time period between about 10,000 years before present
and about 1,650,000 years before present. As a descriptive term
applied to rocks or faults, it marks the period of rock formation
or the time of most recent fault slip, respectively. Faults of
Pleistocene age may be considered active though their activity
rates are commonly lower than younger faults.
Poisson
distribution. A probability distribution that characterizes
discrete events occurring independently of one another in time.
Q.
See attenuation.
Quaternary.
The geologic time period comprising about the last 1.65 million
years.
radiometric.
Pertaining to the measurement of geologic time by the analysis
of certain radioisotopes in rocks and their known rates of decay.
Rayleigh
wave. A seismic surface
wave causing the ground to shake in an elliptical motion,
with no transverse, or perpendicular, motion.
recurrence
interval. The average time span between large earthquakes
at a particular site. Also termed "return period".
reflection.
The energy or wave from an earthquake that has been returned (reflected)
from an boundary between two different materials within the earth,
just as a mirror reflects light.
refraction.
(1) The deflection, or bending, of the ray path of a seismic wave
caused by its passage from one material to another having different
elastic properties. (2) Bending of a tsunami
wave front owing to variations in the water depth along a coastline.
regression
analysis. A statistical technique applied to data to
determine, for predictive purposes, the degree of correlation
of a dependent variable with one or more independent variables,
in other words, to see if there is a strong or weak cause and
effect relationship between to things. See least-squares
fit.
residual.
The difference between the measured and predicted values of some
quantity.
return
period. See recurrence
interval.
right-lateral.
If you were to stand on the fault and look along its length, this
is a type of strike-slip fault where the right block moves toward
you and the left block moves away. (See also left-lateral.)
Ring
of Fire. The zone of earthquakes
surrounding the Pacific Ocean which is called the Circum-Pacific
belt--about 90% of the world's earthquakes occur there. The next
most seismic region (5-6% of earthquakes) is the Alpide belt (extends
from Mediterranean region, eastward through Turkey, Iran, and
northern India.
rupture
front. The instantaneous boundary between the slipping
and locked parts of a fault during an earthquake. Rupture in one
direction on the fault is referred to as unilateral. Rupture may
radiate outward in a circular manner or it may radiate toward
the two ends of the fault from an interior point, behavior referred
to as bilateral.
rupture
velocity. The speed at which a rupture
front moves across the surface of the fault during an earthquake.
S
wave. A seismic body
wave that shakes the ground back and forth perpendicular to
the direction the wave is moving, also called a shear wave.
sand
boil. Sand and water that
come out onto the ground surface during an earthquake as a result
of liquefaction
at shallow depth.
sea-floor
spreading. See magnetic
polarity reversals.
secular.
Referring to long-term changes that take place slowly and imperceptibly.
Commonly used to describe changes in elevation, tilt, and stress
or strain rates that are related to long-term tectonic
deformation. For example, a mountain that is growing is getting
taller so slowly that we cannot see it happen, but if we were
to measure the elevation one year and then the next, we could
see that it has grown taller.
segmentation.
The breaking up of a fault along its length into several smaller
faults. This can happen as a result of other faults crossing it,
topography changes, or bends in the strike
of the faults. Segmentation can limit the length of faulting
in a single earthquake to some fraction of the total fault length,
thus also limiting the size of the earthquake.
seiche.
The sloshing of a closed body of water from earthquake shaking.
Swimming pools often have seiches during earthquakes.
seismic
gap. A section of a fault that has produced earthquakes
in the past but is now quiet. For some seismic gaps, no earthquakes
have been observed historically, but it is believed that the fault
segment is capable of producing earthquakes on some other basis,
such as plate-motion information or strain measurements.
seismic
hazard. See earthquake
hazard.
seismicity.
The geographic and historical distribution of earthquakes.
seismic
moment. A measure of the size of an earthquake based
on the area of fault rupture, the average amount of slip,
and the force that was required to overcome the friction sticking
the rocks together that were offset by faulting. Seismic moment
can also be calculated from the amplitude spectra
of seismic waves.
seismic
reflection or refraction line. A set of seismographs
usually lined up along the earth's surface to record seismic waves
generated by an explosion for the purpose of recording reflections
and refractions of these waves from velocity discontinuities within
the earth. The data collected can be used to infer the internal
structure of the earth.
seismic
wave. An elastic wave generated by an impulse such
as an earthquake or an explosion. Seismic waves may travel either
along or near the earth's surface ( Rayleigh
and Love waves)
or through the earth's interior (P
and S waves).
seismic
zone. An area of seismicity
probably sharing a common cause. Example: "The New Madrid Seismic
Zone."
seismogenic.
Capable of generating earthquakes.
seismogram.
A record written by a seismograph in response to ground motions
produced by an earthquake, explosion, or other ground-motion sources.
( See also FAQ discussion.)
seismograph
seismology.
The study of earthquakes and the structure of the earth, by both
naturally and artificially generated seismic
waves.
seismometer
or seismograph. A seismometer is an instrument used
to detect and record earthquakes. Generally, it consists of a
mass attached to a fixed base. During an earthquake, the base
moves and the mass does not. The motion of the base with respect
to the mass is commonly transformed into an electrical voltage.
The electrical voltage is recorded on paper, magnetic tape, or
another recording medium. This record is proportional to the motion
of the seismometer mass relative to the earth, but it can be mathematically
converted to a record of the absolute motion of the ground. Seismograph
is a term that refers to the seismometer and its recording device
as a single unit. ( See also Earthquake
ABC's and FAQ discussion.)
shear
stress. The stress component parallel to a given surface,
such as a fault plane, that results from forces applied parallel
to the surface or from reinote forces transmitted through the
surrounding rock. If you lean against the edge of the door where
the latch is, you are applying shear stress to the door.
shear
wave. See S
wave.
slab.
The oceanic crustal plate that underthrusts the continental
plate in a subduction
zone and is consumed by the earth's mantle.
slickensides.
Polished striated rock surfaces caused by one rock mass moving
across another on a fault.
slip.
The relative displacement
of formerly adjacent points on opposite sides of a fault, measured
on the fault surface.
slip
model. A kinematic
model that describes the amount, distribution, and timing of slip
associated with an earthquake.
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.
soil.
(1) In engineering, all unconsolidated material above bedrock.
(2) In soil science, naturally occurring layers of mineral and
(or) organic constituents that differ from the underlying parent
material in their physical, chemical, mineralogical, and morphological
character because of pedogenic
processes (3) In other words, dirt.
soil
profile. The vertical arrangement of layers of soil
down to the bedrock.
source.
The released forces that generate acoustic or seismic waves, also
called the earthquake source.
spectral
acceleration or SA. PGA (peak acceleration) is what
is experienced by a particle on the ground. SA is approximately
what is experienced by a building, as modeled by a particle on
a massless vertical rod having the same natural period of vibration
as the building.
spectrum.
A curve showing amplitude
and phase as a function of frequency
or period, or how much
of each type of shaking there is from an earthquake.
standard
deviation. How much a set of data is different
from the curve it should make when plotted on a graph. Or, the
square root of the average of the squares of deviations about
the mean of a set of data. Standard deviation is a statistical
measure of spread or variability.
station.
Refers to the place where a geophysical instrument is located.
stick-slip
The fast movement that occurs between two sides of a fault when
the two sides of the fault become unstuck. Stick-slip displacement
on a fault radiates energy in the form of seismic waves, creating
an earthquake.
stochastic.
Applied to processes that have random characteristics.
strain.
Small changes in length and volume associated with deformation
of the earth by tectonic
stresses or by the passage of seismic
waves.
strain
rate. How fast the lithosphere
is being deformed from plate tectonic movement.
stress.
Force per unit area acting on a plane within a body. Six values
are required to characterize completely the stress at a point:
three normal components
and three shear components.
stress
drop. The difference between the stress
across a fault before and after an earthquake.
strike.
Trend or bearing, relative to north, of the line defined by the
intersection of a planar geologic surface (for example, a fault
or a bed) and a horizontal surface such as the ground.
strike-slip.
See fault. (See also
Earthquake ABC's.)
strong
motion. Ground
motion of sufficient amplitude
and duration to be potentially damaging to a building or other
structure.
subduction.
The process of the oceanic lithosphere
colliding with and descending beneath the continental lithosphere.
subduction
zone. The place where two lithosphere plates come together,
one riding over the other. Most volcanoes on land occur parallel
to and inland from the boundary between the two plates.
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.
surface
wave. Seismic wave that travels along the earth's surface.
Love and Rayleigh
waves are the most common.
Surface-wave
magnitude. See magnitude.
tectonic.
Refers to rock-deforming processes and resulting structures that
occur over large sections of the lithosphere.
tectonic
plates. The large, thin, relatively rigid plates that
move relative to one another on the outer surface of the Earth.
teleseismic.
Pertaining to earthquakes at distances greater than 1,000 km from
the measurement site.
tensional
stress. The stress that tends to pull something apart.
It is the stress component perpendicular to a given surface, such
as a fault plane, that results from forces applied perpendicular
to the surface or from remote forces transmitted through the surrounding
rock.
thrust
fault. See fault.
(Also see Earthquake
ABC's.)
time
history. The sequence of values of any time-varying
quantity (such as a ground motion measurement) measured at a set
of fixed times. Also termed time series.
transform
fault. A special variety of strike-slip fault that
accommodates relative horizontal slip between other tectonic
elements, such as oceanic crustal plates. Often extend from oceanic
ridges.
traveltime
curve. A graph of arrival times, commonly P
or S waves, recorded
at different points as a function of distance from the seismic
source. Seismic velocities
within the earth can be computed from the slopes of the resulting
curves.
tsunami.
A sea wave of local or distant origin that results from large-scale
seafloor displacements associated with large earthquakes, major
submarine slides, or exploding volcanic islands. (See also Earthquake
ABC's.)
tsunamigenic.
Referring to those earthquakes, commonly along major subduction-zone
plate boundaries such as those bordering the Pacific Ocean, that
can generate tsunamis.
tsunami
magnitude (Mt). A number used to compare sizes of tsunamis
generated by different earthquakes and calculated from the logarithm
of the maximum amplitude of the tsunami
wave measured by a tide gauge distant from the tsunami source.
turbidites.
Sea-bottom deposits formed by massive slope failures where rivers
have deposited large deltas. These slopes fail in response to
earthquake shaking or excessive sedimentation load. The temporal
correlation of turbidite occurrence for some deltas of the Pacific
Northwest suggests that these deposits have been formed by earthquakes.
velocity.
How fast a point on the ground is shaking as a result of an earthquake.
velocity
structure. A generalized regional model of the earth's
crust that represents
crustal structure using layers having different assumed seismic
velocities.
wave
front. See rupture
front.
wavelength.
The distance between successive points of equal amplitude
and phase on a wave (for example, crest to crest or trough to
trough).
YBP.
An abbreviation for years before present.
For more technical
definitions of these terms, see:
Montana Bureau of Mines &
Geology
See also:
Volcano
Hazards Program Photo Glossary
Volcano
World's Volcanic Glossary