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7/27/2019 Glossary of earthquake engineering,

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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 earthquakewhen the ground is shaking, it also experiences acceleration. The peak accelerationis the largest

increase in velocity recorded by a particular station during an earthquake.

accelerograph

An instrument that records the acceleration of the ground during an earthquake, also commonly

called an accelerometer.

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

Aftershocks are earthquakes that follow the largest shock of an earthquake sequence. They are

smaller than the mainshock and within 1-2 rupture lengths distance from the mainshock.

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.

alluvium

Alluvium is loose gravel, sand, silt, or clay deposited by current or past streams.

amplification

Shaking levels at a site may be increased, or amplified, 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 amplitude is the size of the wiggles on an earthquake recording.

arc

An arc is 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

aseismic

This term describes a fault on which no earthquakes have been observed.


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asperity

An asperity is an area on a fault that is stuck. The earthquake rupture usually begins at an

asperity.

asthenosphereThe asthenosphere is the ductile part of the earth just below the lithosphere, including the

uppermantle. 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. Seismic waves also become attenuated as they move away from the earthquake

source.

backarc

The backarc is the region landward of the volcanic chain on the other side from thesubduction

zone.

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 themantle. Also known as the Wadati-Benioff zone.

blind thrust fault

Athrust faultthat 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.
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bodywave

A body wave is 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 body waves. 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.

carbon14 age

An absolute age obtained for geologic materials containing bits or pieces of carbon using

measurements of the proportion of radioactive carbon (14 C) to daughter carbon (12 C). These

dates are independently calibrated with calendar dates. This is used to determine when past

earthquakes occurred on a fault.

compressionalstress

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.

P wave

A P wave, or compressionalwave, is 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.

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)

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
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The crust is 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)

deformation

Deformation is a change in the original shape of a material. When we are talking about

earthquakes, deformation is due to stress andstrain.

dip

Dip is the angle that a planar geologic surface (for example, a fault) is inclined from the

horizontal.

dip slip

Dip-slip faultsare 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.

directivity

Directivity is 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

Displacement is 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.

earthquake

Earthquake is a term 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.

earthquakehazard

Earthquake hazard is 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.

earthquakerisk
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Earthquake risk is 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 incorrectly used interchangeably.

epicenter

The epicenter is the point on the earth's surface vertically above the hypocenter (or focus), pointin the crust where a seismic rupture begins. See alsoEarthquake ABC.

fault

A fault is a fracture along which the blocks of crust on either side have moved relative to one

another parallel to the fracture.

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.

fault gouge

Fault gouge is crushed and ground-up rock produced by friction between the two sides when a

fault moves.

fault plane

The fault plane is the planar (flat) surface along which there is slip during an earthquake.

fault scarp

The fault scarp is the feature on the surface of the earth that looks like a step caused by slip onthe fault.

fault trace

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.

fault-planesolution

A fault plane solution is 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. Also calleda focal-mechanismsolution.

first motion

On a seismogram, the first motion is 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.
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focal depth

The focal depth refers to the depth of an earthquakehypocenter.

fault-planesolution

A fault plane solution is a way of showing the fault and the direction of slip on it from anearthquake, using circles with two intersecting curves that look like beach balls. Also called

a focal-mechanismsolution.

hypocenter

The hypocenter is the point within the earth where an earthquake rupture starts. The epicenter is

the point directly above it at the surface of the Earth. Also commonly termed the focus.

forearc

The forearc is the region between thesubduction 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.

frequency

The frequency is 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 is the acceleration of gravity 9.8 (m/s2) or the strength of the gravitational field (N/kg) (which it

turns out is equivalent). G is the proportionality constant 6.67x10-11 (N-m2/kg2) in Newton's law of

gravity. On the other hand, the force of gravity, or F = mg, at the surface of the earth, or F =

GMm/r2 at a distance r from the center of the earth (where r is greater than the radius of the

earth). When there is an earthquake, the forces caused by the shaking can be measured as a

percentage of gravity, or percent g.

geodesy

Geodesy is the science of determining the size and shape of the earth and the precise location of

points on its surface.

geodetic

Geodetic refers to the use of geodesyfor measurements.

geologictime scale
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a chronological sequence of geologic events usually represented in the form of a chart showing

names of various rock layers and indicating the estimated duration of each geologic unit. See

also: Geologic Time(Fact Sheet 2007-3015)

geology

Geology is 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.

geophysics

Geophysics is the branch of earth science which employs physical measurements and

mathematical models to explore and analyze the structure and dynamics of the solid Earth and

similar bodies and their fluid envelopes.

geomorphology

Geomorphology is the study of the character and origin of landforms, such as mountains, valleys,

etc.

geotechnical

Geotechnical means referring to the use of scientific methods and engineering principles to

acquire, interpret, and apply knowledge of earth materials for solving engineering problems.

graben

A graben is a down-dropped block of the earth's crust resulting from extension, or pulling, of the

crust.

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.

great circle

The shortest path between two points on the surface of a sphere lies along a great circle.

groundfailure
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The term ground failure is a general reference tolandslides,liquefaction, lateral spreads,

and any other consequence of shaking that affects the stability of the ground.

groundmotion

Ground motion is 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.

halfspace

A half space is a mathematical model used to approximate the earth when performing some

calculations in seismology. The model is much simpler than the real earth.

harmonictremor

Harmonic tremor describes continuous rhythmic earthquakes that can be detected by

seismographs. Harmonic tremors often precede or accompany volcanic eruptions.

arthquakehazard

Earthquake hazard is anything associated with an earthquake that may affect the normalactivities of people. This includes surface faulting,ground

shaking,landslides, liquefaction,tectonic deformation, tsunamis, and seiches.

hertz (Hz)

Hertz (Hz) is a unit of measurement named in honor of Heinrich Hertz, the German physicist who

in 1887 first produced electromagnetic waves. Expresses thefrequency in cycles per second; 1

Hz = 1 cycle of rise and fall of a wave per second .

Holocene

The Holocene denotes the past 10,000 years. It includes most of the time since the end of themost recent ice age. If slip has occurred on a fault during the Holocene, the fault is commonly

considered active.

horst

A horst is found together with a graben in an extensional environment. The graben are the

downdropped blocks and the horst are the upthrown blocks that lie next to the graben.
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hypocenter

The hypocenter is the point within the earth where an earthquake rupture starts. The epicenter is

the point directly above it at the surface of the Earth. Also commonly termed the focus.

intensity

The intensity is 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.

interplate

Interplate pertains to processes betweenthe earth's crustal plates.

interplatecoupling

Interplate coupling is 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 accumulatingstress,

whereas weak coupling means that the fault is unlocked or only capable of accumulating low

stress.

intraplate

Intraplate pertains to processes withinthe plates.

isoseismal(line)

An isoseismal (line) is a contour or line on a map bounding points of equal intensityfor a

particular earthquake.

kinematic

Kinematic refers to the general movement patterns and directions of the earth's rocks that

produce rock deformation.

landslide

A landslide is a movement of surface material down a slope.

Late Quaternary

The late Quaternary refers informally to the past 0.5-1.0 million years. Faults that have slipped

during this time are sometimes considered active.
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lateral spreador flow

Lateral spread or flow are terms referring to landslides that commonly form on gentle slopes and

that have rapid fluid-like flow movement, like water.

least-squaresfit

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.

lifelines

Lifelines are 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.

lithology

Lithology is the description of rock composition (what it is made of) and texture.

lithosphere

The lithosphere is 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.

lockedfault

A locked fault is 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 strainfor extended periods

that is eventually released in an earthquake when frictional resistance is overcome.

Lovewave

A Love wave is a surface wave having a horizontal motion that is transverse (or perpendicular) to

the direction the wave is traveling.

Ma
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Ma is an abbreviation for one million years ago (Megannum).

magneticpolarity reversal

A magnetic polarity reversal is a change of the earth's magnetic field to the opposite polarity. This

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 toestimate 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

The magnitude is a number that characterizes the relative size of an earthquake. Magnitude is

based on measurement of the maximum motion recorded by aseismograph. 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 donot satisfactorily measure the size of the largest earthquakes. The moment magnitude (Mw)

scale, based on the concept ofseismic 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.

mainshock

The mainshock is the largest earthquake in a sequence, sometimes preceded by one or

moreforeshocks, and almost always followed by many aftershocks.

mantle

The mantle is the part of the earth's interior between the metallic outer core and thecrust.

microzonation

Microzonation is the identification of separate individual areas having different potentials for

hazardous earthquake effects.

Moho

The Moho is the boundary between the crustand the mantlein the earth. This is a depth where

seismic waves change velocity and there is also a change in chemical composition. Also termed

the Mohorovicic' discontinuityafter the Croatian seismologist Andrija Mohorovicic' (1857-1936) who

discovered it. The boundary is between 25 and 60 km deep beneath the continents and between5 and 8 km deep beneath the ocean floor.

momenttensor

A mathematical representation of the movement on a fault during an earthquake, comprising of

nine generalized couples, or nine sets of two vectors. The tensor depends of the source strength

and fault orientation.
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natural frequency

The natural frequency is 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.

normalfault

Seedip slip.

normalstress

The normal stress is 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. Normal stress can either be

compressional or tensional.

oceanicspreadingridge

An oceanic spreading ridge is the 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.

oceanictrench

An oceanic trench is a linear depression of the sea floor caused by the subduction of one plate

under another.

P wave

A P wave, or compressionalwave, is 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.

paleoseismicity

Paleoseismicity refers to earthquakes recorded geologically, most of them unknown from human

descriptions or seismograms. Geologic records of past earthquakes can include faulted layers of

sediment and rock, injections of liquefied sand, landslides, abruptly raised or lowered shorelines,

and tsunami deposits.

peak accelerationSeeacceleration.

pedogenic

Pedogenic means pertaining to processes that add, transfer, transform, or remove soil

constituents.
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percent

SeeG or g.

period

The period is the time interval required for one full cycle of a wave.

plate tectonics

Plate Tectonics is the theory supported by a wide range of evidence that considers the

earth'scrust and upper mantleto 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 ridgesalong which new

crustal material is produced, andtransform faults that accommodate horizontal slip (strike slip)

between adjoining plates. (See also "This Dynamic Earth: The Story of Plate Tectonics".)

Pleistocene

The Pleistocene is 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.

Poissondistribution

A Poisson distribution is a probability distribution that characterizes discrete events occurring

independently of one another in time.

Q

Q is the inverse ofattenuation. A high Q means a low attenuation.

Quaternary

The Quaternary is the geologic time period comprising about the last 1.65 million years.

radiometric

Radiometric is pertaining to the measurement of geologic time by the analysis of certain

radioisotopes in rocks and their known rates of decay.

Rayleighwave

A Rayleigh wave is a seismic surface wave causing the ground to shake in an elliptical motion,

with no transverse, or perpendicular, motion.

recurrenceinterval
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The recurrence interval, or returnperiod, is the average time span between earthquake

occurrences on a fault or in a source zone.

reflection

The reflection is 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

Refraction is (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

atsunami wave front owing to variations in the water depth along a coastline.

regressionanalysis

A regression analysis is 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.

residual

The residual is the difference between the measured and predicted values of some quantity.

returnperiod

Seerecurrence interval.

Richter scale

The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California

Institute of Technology as a mathematical device to compare the size of earthquakes. The

magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded

by seismographs. Adjustments are included for the variation in the distance between the various

seismographs and the epicenter of the earthquakes. On the Richter Scale, magnitude is

expressed in whole numbers and decimal fractions. For example, a magnitude 5.3 might be

computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3.

Because of the logarithmic basis of the scale, each whole number increase in magnitude

represents a tenfold increase in measured amplitude; as an estimate of energy, each whole

number step in the magnitude scale corresponds to the release of about 31 times more energy

than the amount associated with the preceding whole number value.

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.
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Ring of Fire

The "Ring of Fire", also called the Circum-Pacific belt, is the zone of earthquakes surrounding the

Pacific Ocean- 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.

rupturefront

The rupture front is 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.

rupturevelocity

The speed at which a rupturefrontmoves across the surface of the fault during an earthquake.

S wave

An S wave, or shear wave, is a seismic body wave that shakes the ground back and forth

perpendicular to the direction the wave is moving.

sandboil

A sand boil is sand and water that come out onto the ground surface during an earthquake as a

result ofliquefaction at shallow depth.

sea-floor spreading

Sea-floor spreading is what happens at the mid-oceanic ridge where a divergent boundary is

causing two plates to move away from one another resulting in spreading of the sea floor. As the

plates move apart, new material wells up and cools onto the edge of the plates.

secular

Secular refers 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

Segmentation is 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
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A seiche is the sloshing of a closed body of water from earthquake shaking. Swimming pools

often have seiches during earthquakes.

seismicgap

A seismic gap is 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.

seismichazard

Seeearthqauke hazard.

seismicmoment

The seismic moment is 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 theamplitude spectraof seismic waves.

seismicreflectionor refractionline

A seismic refraction or seismic reflection line is 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.

seismicreflectionor refractionline

A seismic refraction or seismic reflection line is 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.

seismicwave

A seismic wave is 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 (PandS waves).

seismiczone

A seismic zone is an area of seismicity probably sharing a common cause. Example: "The New

Madrid Seismic Zone."

seismicity

Seismicity refers to the geographic and historical distribution of earthquakes.
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seismogenic

Seismogenic means capable of generating earthquakes. The base of the seismogenic zone is the

top of the more ductileasthenosphere.

seismogram

A seismogram is a record written by a seismograph in response to ground motions produced by

an earthquake, explosion, or other ground-motion sources.

seismograph

A seismograph, or 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. Seismographgenerally refers to the seismometer and its recording device as a singleunit. ( See alsoEarthquake ABC and Seismographs - Keeping Track of Earthquakes.)

seismology

Seismology is the study of earthquakes and the structure of the earth, by both naturally and

artificially generated seismic waves.

seismometer

Seeseismograph.

shadowzone

The shadow zone is the area of the earth from angular distances of 104 to 140 degrees from a

given earthquake that does not receive any direct P waves. The shadow zone results from S

waves being stopped entirely by the liquid core and P waves being bent (refracted) by the liquid

core.

shear stress

Shear stress is the stress component parallel to a given surface, such as a fault plane, that

results from forces applied parallel to the surface or from remote forces transmitted through the

surrounding rock.

shear wave

SeeS wave.

slab

A slab is the oceanic crustal plate that underthrusts the continental plate in a subduction

zone and is consumed by the earth'smantle.

slickensides
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Slickensides are polished striated rock surfaces caused by one rock mass moving across another

on a fault.

slip

Slip is the relative displacement of formerly adjacent points on opposite sides of a fault,

measured on the fault surface.

slip model

A slip model is akinematic model that describes the amount, distribution, and timing of slip

associated with an earthquake.

slip rate

The slip rate is how fast the two sides of a fault are slipping relative to one another, as

determined fromgeodetic 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

Soil is (1) In engineering, all unconsolidated material abovebedrock. (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 pedogenicprocesses (3) In other words, dirt.

soil profile

The soil profile is the vertical arrangement of layers of soil down to thebedrock.

source

The source is the term for the released forces that generate acoustic or seismic waves, also

called the earthquake source.

spectral acceleration(SA)

PGA (peak acceleration) is what is experienced by a particle on the ground. SA (spectral

acceleration)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

The spectrum is a curve showing amplitudeand phase as a function offrequency or period,

or how much of each type of shaking there is from an earthquake.

standarddeviation

The standard deviation is 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
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The station refers to the place where a geophysical instrument is located.

stick-slip

Stick slip refers to the fast movement that occurs between two sides of a fault when the two sides

of the fault become unstuck. The rock becomes distorted, or bent, but holds its position until the

earthquake occurs. When the rock snaps back into an unstrained position it is called elastic

rebound. Stick-slip displacement on a fault radiates energy in the form of seismic waves, creating

an earthquake.

stochastic

Stochastic is a term applied to processes that have random characteristics.

Strain is the small changes in length and volume associated with deformation of the earth

bytectonic stresses or by the passage ofseismic waves.

strain rate

The strain rate is how fast the lithosphereis being deformed from plate tectonic movement.

stress

Stress is the force per unit area acting on a plane within a body. Six values are required to

characterize completely the stress at a point: three normalcomponents and

three shearcomponents.

stressdropThe stress drop is the difference between the stress across a fault before and after an

earthquake.

strike

The strike is the 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

Strike-slip faultsare 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.

strongmotion

Strong motion is ground motion of sufficient amplitude and duration to be potentially

damaging to a building or other structure.
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subduction

Subduction is the process of the oceaniclithosphere colliding with and descending beneath the

continental lithosphere.

subductionzone

The subduction zone is the place where twolithospheric plates come together, one riding overthe other. Most volcanoes on land occur parallel to and inland from the boundary between the two

plates.

surfacefaulting

Surface faulting is displacement that reaches the earth's surface during slip along a fault.

Commonly occurs with shallow earthquakes, those with an epicenterless than 20 km. Surface

faulting also may accompany aseismiccreep or natural or man-induced subsidence.

surfacewaveA surface wave is a seismicseismic wavethat is trapped near the surface of the earth.

surfacewavemagnitude

Seemagnitude.

tectonic

Tectonic refers to rock-deforming processes and resulting structures that occur over large

sections of the lithosphere.

tectonicplates

The tectonic plates are the large, thin, relatively rigid plates that move relative to one another on

the outer surface of the Earth.

teleseismic

Teleseismic is pertaining to earthquakes at distances greater than 1,000 km from the

measurement site.

tensionalstress

Tensional stress is 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

Seedip slip.

time history
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The time history is 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.

transformfault

A transform fault is a special variety of strike-slip fault that accommodates relative horizontal slip

between other tectonicelements, such as oceanic crustal plates. Often extend from oceanicridges.

traveltimecurve

A traveltime curve is a graph of arrival times, commonlyPor 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 tsunami is 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.

tsunamimagnitude(Mt)

The tsunami magnitude, or Mt, is a number used to compare sizes oftsunamis 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.

tsunamigenic

Tsunamigenic is referring to those earthquakes, commonly along majorsubduction-zone plate

boundaries such as those bordering the Pacific Ocean, that can generate tsunamis.

turbidites

Turbidites are 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

Velocity is how fast a point on the ground is shaking as a result of an earthquake.

velocity structure

A velocity structure is a generalized regional model of the earth's crust that represents crustal

structure using layers having different assumed seismic velocities.

wavefront
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The wave front is the instantaneous boundary between the seismic waves in the earth material,

and the material that the seismic energy has not yet reached.

wavelength

The wavelength is the distance between successive points of equal amplitude and phase on a

wave (for example, crest to crest or trough to trough).

YBP

YBP is an abbreviation for "yearsbeforepresent".
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