This not the most comprehensive list and the definitions may not all be the best but I put this together very fast. Any suggestions on new entries or revision of current entries is gladly welcomed. Some of these terms are not well defined anywhere but are used in the literature frequently, which is why I did this. It would be nice to have as many seismological terms as possible in one place if you need to look something up.

## A Return to Top

**acceleration:** The rate of change of velocity of a reference point. Commonly expressed as a fraction or percentage of the acceleration due to gravity (g) where g = 980 cm/s2.

**accelerogram:** The record from an accelerograph showing ground acceleration as a function of time.

**accelerograph:** A compact, rugged, and relatively inexpensive instrument that records the signal from an accelerometer. Film is the most common recording medium.

**accelerometer:** A sensor whose output is almost directly proportional to ground acceleration. The conventional strong-motion accelerometer is a simple, nearly critically damped oscillator having a natural frequency of about 20 Hz.

**accretionary wedge:** A prism of deformed sediments in the upper plate of a subduction zone near the deformation front, formed in part by accretion of sediments of the downgoing plate, but also including sediments derived from the interior of the upper plate.

**active fault:** a fault along which slip has occurred.

**active margin:** continental margin characterized by volcanic activity and earthquakes (i.e., location of transform fault or subduction)

**aftershocks:** smaller earthquakes following the largest earthquake of a series in a restricted crustal volume. Originates within less than one fault length of the main rupture, and is part of a flurry of earthquakes that occurs at a higher than regional rate of seismicity before the mainshock.

**Airy phase:** a local minimum at 20 sec and 200 sec on group velocity curves that result from a large amount of energy arriving at the same time.

**aliasing:** a phenomena where frequencies are overlapping each other, and the the individual contributions from those frequencies are unable to be deciphered. This is avoided by decreasing the sampling interval such that it is higher than the highest angular frequency content of the signal (see **Nyquist frequency**).

**alluvium:** Loosely compacted gravel, sand, silt, or clay deposited by streams.

**amplification:** An increase in seismic-signal amplitude within some range of frequency as waves propagate through different earth materials. The signal is both amplified and deamplified at the same site in a manner that is dependent on the frequency band. The degree of amplification is also a complex function of the level of shaking such that, as the level of shaking increases, the amount of amplification may decrease. 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 (wave):** the maximum height of a wave crest or depth of a trough.

**anisotropy:** a material where the stress-strain behavior depends on the orientation of the sample.

**antipode:** the point exactly opposite of the epicenter where the wavefront (assuming a homogeneous Earth) should arrive simultaneously at the receiver and produce strong amplifications.

**arc** Commonly refers to the chain of volcanoes (volcanic arc) that sometimes form inland and that are produced by subduction.

**arias intensity:** A ground-motion parameter derived from an accelerogram and proportional to the integral over time of the acceleration squared. Expressed in units of velocity (meters per second or centimeters per second).

**aseismic front:** In a subduction zone, the boundary between the seismogenic foerearc basin and the aseismic volcanic arc.

**aseismic region:** one that is almost free of earthquakes.

**asperity:** A region on a fault of high strength produced by one or more of the following conditions: increased normal stress, high friction, low pore pressure, or geometric changes in the fault such as fault bends, offsets, or roughness. This term is used in two contexts: it may refer to sections of a fault that radiate uncommon seismic energy or it may refer to locked sections of the fault that cause fault segmentation.

**asthenosphere:** the layer below the lithosphere that is marked by low seismic-wave velocities and high seismic-wave attenuation. It is a soft layer, probably partially molten.

**attenuation:**A decrease in seismic-signal amplitude as waves propagate from the seismic source. Attenuation is caused by geometric spreading of seismic-wave energy and by the absorption and scattering of seismic energy in different earth materials (termed anelastic attenuation). Q and kappa are attenuation parameters used in modeling the attenuation of ground motions.

**auxiliary fault-plane:** a plane orthogonal to the fault plane.

**azimuth:** the angle measured clockwise from north a ray must travel to arrive at a given station.

## B Return to Top

**"b" value:** A coefficient describing the ratio of small to large earthquakes within a given area and time period, often shown to be the same over wide range of magnitudes. It is the slope of the curve in the Gutenberg-Richter recurrence relationship.

**backarc**: The region landward of the chain of volcanoes (volcanic arc) in a subduction system.

**back-azimuth:** the angle measured from north to the direction from which the energy arrives at a given station. Is used to determine the longitudinal and transverse directions for an incoming ray at a prescribed station. The longitudinal component lies along the great circle, and the transverse component is perpendicular to the great circle.

**backstop:** Continental rocks in the backarc that are landward from the trace of the subduction thrust fault and that are strong enough to support stress accumulation. These rocks are both igneous and dewatered, lithified, consolidated sediments that probably were part of the accretionary wedge. The softer accretionary-wedge rocks are strongly deformed as they accumulate against the backstop. The exact position and dip direction of the backstop is not well determined, and more than one backstop may exist.

**band-pass filter:** filters signal to retain only those frequencies within the required range, e.g. 5-40 Hz.

**barrier (fault):** an area of fault surface resistant to slip because of geometrical or structural changes.

**basement:** Igneous and metamorphic rocks that underlie the main sedimentary- rock sequences of a region and extend downward to the base of the crust.

**basin depression:** depression in which sediments collect.

**bedrock:** Relatively hard, solid rock that commonly underlies softer rock, sediment, or soil.

**Benioff zone:** A dipping planar 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 (sometimes referred to as the thrust interface fault because it is the interface between the continental plate and the oceanic plate) or by slip on faults within the downgoing plate as a result of bending and extension as the plate is pulled into the mantle. Slip may also initiate between adjacent segments of downgoing plates. The Benioff zone in the Pacific Northwest is not as well developed as it is in other subduction zones. The earthquakes in this region do not appear to be produced by slip along the thrust fault. Also known as the **Wadati-Benioff zone**.

**Bessel Function (mathematics):** a type of function used in describing Earth's normal modes in spherical coordinates. They have the form of a decaying sinusoid.

**blind thrust:** a thrust-fault deep in the crust with no or only indirect surface expression such as a fold structure.

**body wave:** a seismic wave that travels through the interior of an elastic material.

**body-wave magnitude:** magnitude of an earthquake as estimated from the amplitude of the body waves.

**bore:** a single water wave with an almost vertical front.

**boxcar (function):** a mathematical function that represents filter in a time series of equal value (time or frequency) that goes to zero at a specified amount on each end of the boxcar. In convolution, the amplitude of the input signal is multiplied by the filter (boxcar) resulting in time shift of the signal.

**brittle-ductile transition:** a zone within the Earth's crust that separates superjacent brittle rocks from subjacent ductile rocks. Commonly identified as the zone defining the deepest earthquakes in the crust.

**bulk modulus:** see **incompressibility**

## C Return to Top

**c:** symbol used to indicate the reflection at the core-mantle boundary for waves incident from the mantle.

**CTBT:** Comprehensive Test Ban Treaty. A treaty to put an end to clandestine nuclear tests.

**capable fault:** a fault along which it is mechanically feasible for sudden slip to occur.

**caustic (waves):** focusing of a wave where the energy is turning above and reflecting off a discontinuity, respectively - constructively interfering. The amplitude may be large, but not infinite. There are two branches, the AB branch and the BC branch. Another example of a caustic is the cusp associated with the termination of a shadow zone.

**characteristic earthquake:** an earthquake with a size and generating mechanism typical for a particular fault source.

**checkerboard test (tomography):** one type of resolution test that can be performed by varying the velocity as a sinusoidal function in the x and y directions with a wavelength larger than the distance between two adjacent nodes. The resolution of at a particular node is directly correlatable with the number of rays crossing the node.

**coda:** the concluding train of seismic waves that follows the principal part of an earthquake.

**coefficient of friction:** a property of rocks that describes the amount of stress that can be accumulated before two bodies of rock will slide past each other. It is related to the roughness or protrusions that the rocks have (called asperities).

**compressional (P) axis:** in a double-couple system, the dipole that is directed toward the source and lies in the quadrants of dilatation. The ground is being pushed toward the source, giving dilatational first P-waves at the receiver.

**Conrad discontinuity:** a midcrustal velocity discontinuity found in some regions (denoted by **P*** on seismograms).

**continental shelf:** part of the continental margin between the coast and the continental slope; slopes about 0.1 degrees.

**converted waves:** conversion of P to S waves occurs at a discontinuity for non-normal incidence.

**convolution:** the mathematical link between the input seismic signal, the transfer function, and the output signal. Usually performed in the frequency domain.

**core (of Earth):** the central part of the Earth below a depth of 2900 kilometers. It is though to be composed mainly of iron and silicates and to be molten on the outside with a solid central part.

**corner frequency:** the frequency at which the asymptotes to the low-frequency and high-frequency portion of the spectrum intersect. For the convolution of two boxcars, there will be two corner frequencies defining the corresponding trapezoid. This frequency is a property of the source function related to fault size.

**correlation (mathematics):** takes two functions and compares them by shifting one function by different times and evaluating the integral of the product as a function of of those different times. Correlation can be used to estimate the difference in arrival times when seismic energy travel along different paths from the source to receiver. Knowing the signals should be the same when they left the source and followed different paths the waveforms should still be similar.

**covariant matrix (modeling):** indicates the uncertainty in the model due to both the nature on the inverse problem and the errors in the observations.

**creep**: Slow, more or less continuous movement occurring on faults due to ongoing tectonic deformation. Also applied to slow movement of landslide masses down a slope because of gravitational forces. Faults that undergo significant and (or) ongoing creep are likely to be aseismic or capable of only small or moderate earthquakes. This fault condition is commonly referred to as unlocked.

**critical facilities:** Structures whose ongoing performance during an emergency is required or whose failure could threaten many lives. May include (1) structures such as nuclear power reactors or large dams whose failure might be catastrophic; (2) major communication, utility, and transportation systems; (3) involuntary- or high-occupancy buildings such as schools or prisons; and (4) emergency facilities such as hospitals, police and fire stations, and disaster-response centers.

**crust:** The outermost major layer of the Earth, ranging from about 10 to 65 km in thickness worldwide. The continental crust is about 40 km thick in the Pacific Northwest. The thickness of the oceanic crust in this region varies between about 10 and 15 km.The crust is characterized by P-wave velocities less than about 8 km/s. The uppermost 15-35 km of crust is brittle enough to produce earthquakes. The seismic crust is separated from the lower crust by the brittle-ductile boundary.

## D Return to Top

**damping:** (1) loss of energy in a wave motion to transfer into heat by frictional forces.(2) in seismic wave modeling, a factor that limits the amount of model variation between successive iterations. Damping controls how fast the model converges (the number of iterations needed to make the change between observed and predicted models negligible). A low damping value makes the model converge fast and can introduce unnecessary complexity such as low-velocity zones. A high damping factor changes the model slowly and produces a smoother, simpler model.

**deconvolution:** the inverse of convolution. Used in extracting information (e.g. source-time function, instrument response) from the raw seismic data to retain the source signal or ground displacement.

**delta function:** a mathematical generalized function that can be regarded as the limit of any several sequences of continuous function. It also has the "sifting " property where the delta function "picks out" the value of a function at t_{0} if it is multiplied by the function and integrated over all time. A final definition is the derivative of of a step function since it is zero except "near" t_{0} when it goes to infinity. Also known as a impulse.

**density:** the mass per unit volume of a substance, commonly expressed in grams per cubic centimeter.

**depth phases:** upgoing rays that travel from the source to the free surface, reflect, and travel on to the receiver.

**deterministic hazard assessment:** an assessment that specifies single-valued parameters such as maximum earthquake magnitude or peak ground acceleration, without consideration of likelihood.

**deterministic methods:** Refers to methods of calculating ground motions for hypothetical earthquakes based on earthquake-source models and wave- propagation methods that exclude random effects.

**dilatancy (of rocks):** the increase in the volume of rocks mainly due to pervasive microcracking.

**dilatation (waves):** initial motion toward (pulling) the source. The opposite of compression.

**dip:** the angle by which a rock layer or fault plane deviates from the horizontal. The angle is measured in a plane.

**dip-slip fault:** a fault in which the relative displacement is along the direction of dip of the fault plane; the offset is either normal or reverse.

**directivity:** the phenomena where azimuth dependence is due to fault propagation effects (i.e. stations lying in the directions of rupture propagation may get higher amplitudes than those lying in an area where the direction of rupture in going away from it).

**discriminant (explosion detection):** a repeatable characteristic that can be used to distinguish a natural earthquake from an explosion. The most common discriminant is the ratio of short-period (1-s) P-wave energy (m_{b}) to 20-s-period Rayleigh-wave energy (M_{S}), which is higher for explosions than for earthquakes larger than magnitude 3.5.

**dispersion (wave):** the spreading out of a wave train due to each wave length traveling at its own velocity.

**displacements (seismograms):** a measure of the actual ground movement at the receiver, usually in microns. This is obtained by deconvolving a seismogram, removing the instrument response and rotating the seismogram into radial and transverse components.

**divergence (vector operator):** measures the flux of a vector field through a unit of volume.

**double couple:** a model to describe the equivalent body forces for a dislocation source. This model is used to balance the moment of the force system so that no net moment is added to the medium and so that the force couple will be directed along the auxiliary plane. It can be represented by a pair of orthogonal dipoles without shear, called primary axes (see **compressional axis (P) and tensional axis (T)**).

**ductile (rocks):** said of a rock that is able to sustain, under a given set of conditions, 5-10% deformation before fracture or faulting.

**duration (of strong shaking):** the duration is the time interval between the first and last peaks of strong ground motion above a specified amplitude.

## E Return to Top

**earthquake:** the vibrations of the Earth caused by the passage of seismic waves radiating from some source of elastic energy.

**earthquake cycle:** for a particular fault, fault segment, or region, a period of time that encompasses an episode of strain accumulation and subsequent seismic relief.

**earthquake hazard:** Any physical phenomenon associated with an earthquake that may produce adverse effects on human activities. This includes surface faulting, ground shaking, landslides, liquefaction, tectonic deformation, tsunami, and seiche and their effects on land use, manmade structures, and socioeconomic systems. A commonly used restricted definition of earthquake hazard is the probability of occurrence of a specified level of ground shaking in a specified period of time.

**earthquake occurrence (recurrence) interval:** the average interval time between the occurrence of earthquakes in a particular region.

**earthquake risk:** The expected (or probable) life loss, injury, or building damage that will happen, given the probability that some earthquake hazard occurs. Earthquake risk and earthquake hazard are occasionally used interchangeably.

**earthquake segment:** that part of a fault zone or fault zones that has ruptured during individual earthquakes.

**eigenfrequencies:** discrete frequencies of motion that satisfy the boundary conditions of a mode.

**eigenfunction:** the corresponding displacement patterns of the eigenfrequencies (also called normal modes). When the solution is zero with the given boundary conditions, that is the fundamental mode. Higher values equal higher modes.

**eigenvalue:** in tensor mathematics, it represents the values of the matrix in a principal coordinate system.

**eigenvector:** in tensor mathematics, they give the principal coordinate axis "directions".

**elastic dislocation theory:** In seismology, a theoretical description of how an elastic Earth responds to fault slip, as represented by a distribution of displacement discontinuities.

**elastic rebound theory:**the theory of earthquake generation proposing that faults remain locked while strain energy slowly accumulates in the surrounding rock and then suddenly slip, releasing this energy in the form of heat and seismic waves.

**epicenter:** the point on the Earth's surface directly above the focus (or hypocenter) of an earthquake.

**equations of motion:** the fundamental equations of seismology that relates the forces in the medium to measurable displacements. The equations include parameters such as density, body force per unit volume, stress and strain tensors, and time.

**Euler pole (of rotation):** a point that describes how two plates are rotating relative to each other.

**evanescent wave:** a P-wave that is "trapped" propagating along the free surface. This type of wave decays exponentially with depth.

## F Return to Top

**Fmax:** The frequency above which little seismic energy is observed at most strong-motion stations. This frequency cutoff may be produced by attenuation of shaking by unconsolidated sediments underlying the recording site or may be a property of the source function.

**far-field term (displacements):** decays slowly, dominating displacement at large distances and disproportional to the time derivative of the reduced displacement potential, that is if there is a step in the effective pressure at the source there is an impulsive far-field ground motion.

**fault:** A fracture along which there has been significant displacement of the two sides relative to each other parallel to the fracture. Strike-slip faults are vertical (or nearly vertical) fractures along which rock masses have mostly shifted 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 along which rock masses have mostly shifted vertically. If the rock mass above an inclined fault is depressed by slip, the fault is termed normal, whereas if the rock above the fault is elevated by slip, the fault is termed reverse (or thrust). Oblique-slip faults have significant components of both slip styles.

**fault-angle depression:** an arrow, downdropped crustal block bounced by a fault on only one side.

**fault-bend fold:** a fold within the hanging wall block of a fault, formed by bending of the hanging block as it slips over a nonplanar fault surface.

**fault length:** (1) the total length of a fault or fault zone. (2) the rupture length along a fault or fault zone associated with a specific earthquake, representing either the observed surface rupture length or the rupture length at depth, usually determined from the aftershock distribution of the earthquake.

**fault-propagation fold:** a fold formed in front of a fault surface as the fault surface lengthens over a period of geologic time. Commonly associated with the upward termination of a thrust fault.

**fault-plane solution:** An analysis using stereographic projection or its mathematical equivalent to determine the attitude of the causative fault and its direction of slip from the radiation pattern of seismic waves using earthquake records at many stations. The analysis most commonly uses the direction of first motion of P waves and yields two possible orientations for the fault rupture and the direction of seismogenic slip. From these data, inferences can be made concerning the principal axes of stress in the region of the earthquake. The principal stress axes determined in this method are the compressional axis (also called the P-axis, the axis of greatest compression, or s1), the tension axis (also known as the T-axis, axis of least compression, or s3), and the intermediate stress axis ( s2)

**fault scarp:** a slope formed by the offset of the surface of the Earth by a fault.

**fault slip rate:** the rate of slip on a fault averaged over a time period involving several large earthquakes.

**filter:** In seismology, a physical system or a mathematical operation that changes the waveform or amplitude of a signal.

**filtering:** Attenuation of certain frequency components of a seismic signal and the amplification of others. For a recorded signal, the process can be accomplished electronically or numerically in a digital computer. Filtering also occurs naturally as seismic energy passes through the Earth.

**first motion:** on a seismogram, the direction of motion at the beginning of the arrival of a P wave. Conventionally, upward motion indicates a compression of the ground; downward motion, a dilatation.

**floating earthquake:** a conceptual earthquake that is considered equally likely to occur at any point within a specified region.

**flower structure:** more-or-less symmetrical splays into sub-faults near the intersection of the main fault with the ground surface.

**focal depth (of earthquakes):** the depth of the focus below the surface of the Earth.

**focal mechanism:** a lower hemisphere representation of a fault. It shows the strike, dip and rake of a fault as wells as the tension and compression poles along with the fault plane and its corresponding auxiliary fault plane.

**focus:** the place at which rupture commences resulting in an earthquake.

**footwall:** the underlying side of a nonvertical fault surface.

**forearc basin:** sedimentary basin on the trench side of the volcanic arc of a subduction zone.

**foreland basin:** sedimentary basin on the trench side of a continent-continent collision zone.

**foreshocks:** smaller earthquakes preceding the largest earthquake of a series concentrated in a restricted crustal volume.

**forward modeling problem (for seismology):** describing the seismic waves generated by a know source in a known medium. This is the opposite of the inverse modeling problem.

**Fourier spectra:** the corresponding amplitude and phase signals resulting from the Fourier transform of a time series.

**Fourier Transform:** a mathematical tool that converts seismic signals from the time domain to the frequency domain.

**free oscillation:** vibration of the whole Earth after a major earthquake.

**frequency:** number of oscillations per unit time; unit is Hertz (Hz), which equals one cycle per second.

**frequency response function:** obtained from the Fourier transform of the output signal divided by the Fourier transform of the input signal. The values of the frequency response function are the **eigenvalues** of the system. It is also the Fourier transform of the **impulse response function**.

**fundamental period:** The longest period for which a structure shows a maximum response. The reciprocal of natural frequency.

**fundamental mode (frequencies):** the primary frequency produced where there are no places of zero motions (see **Higher modes**).

## G Return to Top

**G (or Gn):** another name for long-period mantle Love waves.

**G1, G2:** LQ-type mantle wave that travels the direct and anticenter routes. Waves that have, in addition, traveled once or several times around the Earth are denoted G3, G4, G5, G6, etc.

**Gaussian distribution:**a statistical relationship where data falls within a specified "bell curve".

**Gaussian filter:** a "bell" shaped filter used to isolate a specific part of a seismic signal (e.g. fundamental mode Rayleigh wave for group velocity estimation).

**gaussian noise spectrum:** The spectrum of a time history whose sample values are generated by random selection from a statistical population that has a specified mean and standard deviation. The values (ordinates) have a bell-shaped distribution about the mean. In earthquake studies, this type of spectrum is commonly multiplied by a theoretical earthquake source spectrum to obtain predicted ground-motion spectra for hypothetical earthquakes.

**geodetic:** Referring to the determination of the size and shape of the Earth and the precise location of points on its surface.

**geodimeter:** a surveying instrument to measure the distance between two points on the Earth's surface.

**geometrical attenuation:** That component of attenuation of seismic-wave amplitudes due to the radial spreading of seismic energy with distance from a given source.

**geophone:** an instrument used to record ground in reflection/refraction exploration geophysics and ocean acoustic surveys.

**geotechnical:** Referring to the use of scientific methods and engineering principles to acquire, interpret, and apply knowledge of earth materials for solving engineering problems.

**gouge:** crushed, sheared, and powdered rock altered to clay. Usually found within the fault zone itself.

**gradient (vector operator):** the partial derivative of the vectors in three directions. The gradient vector point in the direction of steepest slope.

**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. Interpretations of changes in the gravity field are generally applied to gravity values corrected for extraneous effects. The corrected values are referred to by various terms, such as free-air gravity, Bouguer gravity, and isostatic gravity, depending on the number of corrections.

**gravity wave:** a type of wave where gravity is the main restoring force in the system. Tsunami's are the most prevalent type of gravity wave.

**great circle path:** an arc on the Earth's surface that represents the path that seismic energy has traveled to get from the source to the receiver.

**green's function:** A mathematical representation that, in reference to earthquake shaking, is used to represent the ground motion caused by instantaneous slip on a small part of a fault. Green's functions can be summed over a large fault surface to compute the ground shaking for a large earthquake rupturing a fault of finite size. The fractional fault-slip events that are summed can be records from small earthquakes on the fault or they can be theoretically computed small-earthquake records.

**ground motion (shaking):** General term referring to the qualitative or quantitative aspects of 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.

**ground roll:** a term used in exploration seismology refer to surface waves generated from explosions. They are characterized by low velocity, low frequency, and high amplitude, and are observed in regions where near-surface layering consists of poorly consolidated, low velocity sediments overlying more competent bed with higher velocity.

**group velocity:** wave disturbances with a wide spectrum of periods interfere with each other producing constructive and destructive patterns. The constructive patters travel along the surface as wave packets with well defined group velocities that depend on the medium parameters and the variation in phase velocity with frequency. Group velocities can be used to estimate the shear-velocity crustal structure with depth.

**Gutenberg discontinuity:** discontinuity in seismic velocity that marks the boundary between the core and the mantle; named after seismologist Beno Gutenberg.

**Gutenberg-Richter recurrence relationship:** the observed relationship that, for large areas and long time periods, number of earthquakes of different magnitudes occur systematically, with the relationship *M* = *a - bN*, where *M* is magnitude, *N* is the number of events per unit area per unit time, and *a* and *b* are constants representing, respectively, the overall level of seismicity and the ratio of small to large events.

## H Return to Top

**halfspace**: A mathematical model bounded by a planar surface but otherwise infinite. Properties within the model are commonly assumed to be homogeneous and isotropic, unlike the Earth itself, which is heterogeneous and anisotropic.

**hanging wall:** the overlying side of a nonvertical fault.

**head waves:** head waves are observed in a half-space that is in welded contact with another half-space with higher velocity when the seismic source is located in the lower velocity medium. The ray path of the head wave is along the interface. One example is P_{n}.

**heave:** the horizontal component of separation or displacement on a fault.

**Hertz:** the unit of frequency equal to 1 cycle per second, or 2p radian per second.

**heterogeneous (material):** material that has a varied composition.

**high-pass filter:** filters a signal to remove frequencies below a given frequency, passes only the high frequencies.

**higher-modes (overtones):** secondary frequencies generated that have one or more nodes of zero motion (see also **Fundamental mode**).

**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, based on the observation of historical activity on faults of this age in other locales.

**homogeneous (material):** a material that has the same composition throughout.

**Hooke's Law:** is an empirical relationship expressing a linear relationship between stress and strain.

**hot spot:** a volcanic center, 100 to 200 kilometers across and persistent for at least a few tens of millions of years, thought to be the surface expression of a persistent rising plume of mantle material. Hot spots are not linked with volcanic arcs, but some are associated with oceanic ridges. Hawaii is a classic example of a hot spot.

**hypocenter:** (see **focus**).

## I Return to Top

**i:** symbol used to indicate reflection at the boundary between the outer and inner core.

**I:** symbol used to indicate the part of the ray path that lies within the Earth's inner core (e.g. PKIKP).

**impulse response function:** a mathematical representation of an impulsive signal. By taking the Fourier transform of the impulse response of the system, you obtain the **frequency response function**.

**incompressibility:** an index of the resistance of an elastic body, such as a rock, to volume change.

**inner core:** central solid region of the Earth's core, probably mostly iron; radius is about 1221 kilometers, discovered by Inge Lehmann in 1936.

**instrument response:** a set of parameters built into the physics of the recording instrument. These are well known and can be removed from a seismogram by deconvolution of the transfer function and the seismic signal to recover the source derived signals.

**intensity (of earthquakes):** a measure of ground shaking obtained from the damage done to structures built by humans, changes in the Earth's surface, and felt reports.

**interplate earthquake:** earthquake with its focus on a plate boundary.

**interplate coupling:** The qualitative ability of a subduction thrust fault to lock and accumulate stress. Strong interplate coupling implies that the fault is locked and capable of accumulating stress, whereas weak coupling implies that the fault is unlocked or only capable of accumulating low stress. A fault with weak interplate coupling could be aseismic or could slip by creep.

**intraplate earthquake:** earthquake with its focus within a plate.

**inverse Fourier transform:** a mathematical tool that converts a seismic signal from the frequency domain to the time domain.

**inverse modeling problem (for seismology):** deriving the source and medium parameters from the seismic signals. This is the opposite of the **forward modeling** problem.

**island arc:** chain of islands above a subduction zone (e.g. Japan, Aleutians).

**isotropy:** a material where its mechanical properties are the same in all directions.

**isoseismal:** Referring to a line on a map bounding points of equal intensity for a particular earthquake.

**isostasy:** the way in which the lithosphere "floats" on the asthenosphere. There are two models generally accepted, the Airy model which is load "floating" in the asthenosphere with a root sinking down with higher density and the Pratt model that has the depth at the base of the layers being constant along with the densities.

## J Return to Top

**J:** the symbol used to indicate that a part of the ray path lies inside the Earth's inner core as S-waves.

**J-B table (Jeffreys-Bullen):** a table used to predict the arrival times of P waves to any point on the Earth's surface with very good accuracy.

## K Return to Top

**K:** symbol used to indicate that part of the ray path of a P-wave that lies within the Earth's outer core.

**kernel (excitation):** the complete seismogram Green's function for each of the moment tensor elements. They are obtained by summing the normal modes of the Earth. Used in calculating centroid moment tensors.

**kinematic:** Referring to the general movement patterns and directions of the Earth's rocks that produce rock deformation.

## L Return to Top

**L:** denotes a Love-wave (see also **LQ**).

**L _{g}:** Short-period (1-6 sec) large amplitude arrivals with predominantly transverse motion. Denotes a phase that is the result of the combination of higher-mode Rayleigh waves as well as some high frequency Love-wave overtone energy. They propagate along the surface with velocities close to the average shear velocity in the upper part of the continental crust. The waves are only observed the wave path is entirely continental.

**LQ:** another denotation of a surface Love wave (Q stands for Querwellen, a German word used to describe Love waves; see also **L**).

**LR:** another denotation of a surface Rayleigh wave (see also **R**).

**Laplacian (vector operator):** is the divergence of the gradient (see **divergence** and **gradient**).

**Late Quaternary:** Referring to an 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.

**lava:** magma or molten rock that has reached the surface.

**leaky barrier:** an earthquake segment boundary that is, from time to time, broken across by an earthquake.

**least-squares fit:** An approximation of a set of data with a curve such that the sum of the squares of the differences between the observed points and the assumed curve is a minimum.

**least-squares inverse:** a solution to matrix inversion problem where the squared error is minimized.

**left-lateral fault:** a strike-slip fault on which the displacement of the far block is to the left when viewed from the other side.

**Legendre functions:** a type of function used in describing Earth's normal modes in spherical coordinates. They have the form of a decaying sinusoid.

**lifelines:** Structures that are important or critical for urban functionality. Examples are roadways, pipelines, power lines, sewers, communications, and port facilities.

**linear filter system:** a means of describing seismic waves (ground displacement) in terms of the source function, propagating function and the instrument recording function.

**liquefaction (of soil):** process of soil and sand behaving like a dense fluid rather than a wet solid mass during an earthquake.

**lithology:** physical character of rocks.

**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.The lithosphere below the crust is brittle enough at some locations to produce earthquakes by faulting, such as within a subducted oceanic plate.

**local distance (from earthquake):** a source-receiver distance of less than 100 km.

**locked fault:** A fault that is not slipping because frictional resistance on the fault is greater than the shear stress across the fault. Such faults may store strain for extended periods that is eventually released in an earthquake when frictional resistance is overcome. A locked fault condition contrasts with fault-creep conditions and an unlocked fault.

**longitudinal waves:** displacement associated with far-field P-waves in a homogeneous isotropic solid is parallel to the direction of propagation (e.g. P-waves).

**Love waves:** seismic surface waves with only horizontal shear motion transverse to the direction of propagation.

**low-pass filter:** filters a signal to remove frequencies above a given frequency, passes only the low frequencies.

**lurching of the ground:** disruption of soil by lateral spreading under gravity.

## M Return to Top

**Ma:** million years, mega-annum (million years ago).

**magma:** molten rock material that forms igneous rocks upon cooling.

**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 spreading oceanic 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(sometimes for earthquake waves of a particular frequency), corrected for attenuation to a standardized distance. 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. In principal, all magnitude scales could be cross calibrated to yield the same value for any given earthquake, but this expectation has proven to be only approximately true, thus the need to specify the magnitude type as well as its value.

**mainshock:** the largest earthquake within a closely-spaced and temporally-clustered series of earthquakes. Typically followed by aftershocks, smaller earthquakes which become less frequent with increased time since the mainshock.

**mantle (of Earth):** the main bulk of the Earth, between the crust and core, ranging in depths of about 40 to 3470 kilometers. It is composed of dense silicate rocks and divided into a number of concentric shells.

**mantle wave:** a very long-period (> 60 sec) surface wave (Rayleigh) with corresponding wavelengths of several hundred to about 1200 kilometers and are generated by the largest earthquakes. They can make a complete circuit around the globe on a great circle path.

**mare:** a dark, low-lying lunar plain, filled to an undetermined depth with volcanic rocks. (Plural: maria).

**maximum credible earthquake:** the maximum earthquake that is capable of occurring in a given area or on a given fault during the current tectonic regime.

**mean (statistics):** the sum of the values divided by the number of values

**median (statistics):** the midpoint of the values after they have been arranged from the smallest to the largest (or the largest to the smallest). There will be as many values above the median as below the median.

**meizoseismal region:** the area of strong shaking and significant damage in an earthquake.

**microseism:** weak, almost continuous background seismic waves or Earth "noise" that can be detected only by seismographs often caused by surf, ocean waves, wind, or human activity.

**microzonation:** the division of a town or county into smaller areas according to the variation in seismic hazard.

**Moho:** A discontinuity in seismic velocity that marks the boundary between the Earth's crust and mantle. 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.

**Mohr circle:** a diagram that plots the normal stress versus the shears stress. They can show how the stress at a point is related to the surface stresses on planes through the point.

**moment (of earthquakes):** (see **seismic moment**).

**moment tensor, seismic:** (see **seismic moment tensor**).

**moveout:** the ratio of travel time to the distance traveled.

**multipathing:** a phenomena where the geologic structure and/or velocity gradient "bend" the rays from the predicted path causing later arrival times.

## N Return to Top

**near-field term (displacements):** involves displacements directly proportional to the reduced displacement potential and decay rapidly. If there is any step in the effective pressure a permanent deformation of the surrounding elastic medium will occur.

**normalizing (data):**

**normal fault:** a dip-slip fault in which the rock above the fault plane has moved downward to the rock below.

**normal modes:** (see **free oscillations**).

**Nyquist frequency:** the highest angular frequency found in the spectra of digital data used to avoid aliasing (see **aliasing**).

## O Return to Top

**oceanic spreading ridge:** A fracture zone along the ocean bottom that accommodates upwelling of mantle material to the surface, thus creating new crust. This fracture is topographically marked by a line of ridges that form as molten rock reaches the ocean bottom and solidifies.

**oscillator:** A mass that moves with oscillating motion under the influence of external forces and one or more forces that restore the mass to its stable at-rest position. In earthquake engineering, an oscillator is an idealized damped mass-spring system used as a model of the response of a structure to earthquake ground motion. A seismograph is also an oscillator of this type.

**oblique faulting:** the slip on the fault has components both along the dip and along the strike of the fault.

**operation basis earthquake:** a US Nuclear Regulatory Commission term that specifies the maximum ground motion for which a reactor is expected to continue operating during and after the earthquake.

**origin time:** the time of initiation of the seismic waves at an earthquake source (usually given in Universal Time Coordinate, UTC).

**orthogonal function:** a set of functions where the integral of the product of two different functions over a specified interval is always zero. Examples include Bessel functions and Legendre functions.

**outer arc ridge:** A zone landward from the trace of the subduction thrust fault of elevated sea floor probably related to the compression of the rocks in the accretionary wedge. Also referred to as the outer arc high.

**outer core:** outer liquid shell of the Earth's core, probably iron with some oxygen; inner radius, 1221 kilometers, outer radius, 3480 kilometers.

**overtone:** (see **free oscillations**).

## P Return to Top

**pP:** depth phase that leaves the focus upward as P (p leg) is reflected as P at the free surface and continues further as P.

**pPP:** depth phase that leaves the focus upward as P (p leg) is reflected as PP at the free surface and continues further as PP.

**pPS:** depth phase that leaves the focus upward as P (p leg) is converted to PS at the free surface and continues further as PS.

**pP _{n}:** depth phase that leaves the focus upward as P, is reflected as Pn at the free surface and continues further as P

_{n.}

**pS:** depth phase that leaves the focus upward as P (p leg) is converted to S at the free surface and continues further as S (arrives after pP).

**P*:** a seismic P-wave that travels along the Conrad discontinuity.

**P':** another symbol for PKP

**PcP:** a seismic wave that travels to the core and is reflected of the core boundary (see **c**).

**P-diff (or Pc):** a P-wave that grazes the Earth's core and emerges at an epicentral distance of about 103^{o} (see **Shadow zone**).

**PdP:** P wave reflected at the underside of a discontinuity ad a depth *d* in the upper part of the Earth. *d* is given in kilometers, e.g. P400P.

**P _{g}:** a P-wave that traveled within the granitic layer of the crust (direct arrivals) at a distance of less than 100 km.

**PmP:** a P-wave that was reflected of the Moho and converts to a P-wave.

**PmS:** a P-wave that was reflected of the Moho and converts to a S-wave.

**P _{n}:** a P-wave that traveled at or just below the Moho at a distance greater than 100 km; also called a head wave.

**PKIKP, (or P'', PKP _{DF}):** P wave traversing the outer and inner core.

**PKiKP:** a P-wave that travels through the mantle, outer core (K), reflected at the inner core (i) and back out to the surface.

**PKIIKP:** P wave reflected from the inside of the inner-core boundary.

**PKKP:** P wave reflected from the inside of the core-mantle boundary.

**PmKP (m=3,4,...):** P wave reflected m-1 times from the inside of core-mantle boundary.

**PKP:** P wave traversing the outer core.

**PKP1, PKP2 (or PKP _{BC}, PKP_{AB}):** Different branches of PKP that are refracted ONLY within the outer core.

**PKPPKP (or P'P'):** PKP reflected from the free surface, passing twice through the core.

**PKS:** P wave converted into S on refraction when leaving the core.

**PP or PPP:** P wave reflected once or twice at the Earth's surface.

**PPS, PSP, PSS:** P wave twice reflected/converted at the Earth's surface.

**P wave:** the primary or fastest wave traveling away from a seismic event through the rock and consisting of a train of compressions and dilatations of the material. P-waves can travel through the entire earth (including the liquid outer core). Their motion is analogous to sound waves.

**paleoseismology:** that part of earthquake studies that deals with evidence for earthquakes before instrumental recording of seismic waves or damage from felt reports.

**passive margin:** continental margin formed during initial rifting apart from continents to for an ocean; frequently has thick sedimentary deposits.

**pedogenic**: Pertaining to processes that add, transfer, transform, or remove soil constituents.

**period (wave):** the time interval between successive crests in a sinusoidal wave train; the period is the inverse of the frequency of a cyclic event.

**phase velocity:** the velocity of each harmonic component of the surface wave period that depends on the medium parameters such as layer thickness and P and/or S wave velocities. To get good estimates of phase velocity, the moment tensor and depth of the event must be accurate.

**plane wave:** a family of rays that are parallel straight lines.

**plate (tectonic):** a large, relatively rigid segment of the Earth's lithosphere that moves in relation to other plates over the deeper interior. Plates meet in convergence zones and separate at divergence zones.

**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.

**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.

**point force:** in solving for displacement, it is a force applied a to a point in a elastic homogeneous medium.

**point source:** an approximation used in describing the waves radiated from a seismic source.

**poisson distribution:**A probability distribution that characterizes discrete events occurring independently of one another in time.

**Poisson's ratio:** gives the absolute value of ratio given by the extension normal to applied compressive stress, divided by the extension parallel to the applied compression.

**Poisson solid:** when the rigidity and the other Lame (unnamed) constant are equal, Poisson's ratio is 0.25 and is used as the approximation in most seismology problems.

**poles and zeros:** the real and complex solutions to the instruments transfer function. Used in removing the instrument response from the signal.

**precursor:** a change in the geological conditions that is a forerunner to earthquake generation on a fault.

**prediction (of earthquakes):** the forecasting in time, place, and magnitude of an earthquake; the forecasting of strong ground motion.

**preshock:** an earthquake that occurs before the mainshock, but is not an unambiguous foreshock, either because it originated months or years before the mainshock or because it originated far from the epicenter of the mainshock.

**primary surface rupture:** surface rupture that is directly connected to subsurface displacement on a seismic fault.

**probability:** the number of cases that actually occur divided by the total number of cases possible.

**probability of exceedence of a given earthquake:** the odds that the size of a future earthquake will exceed some specified value.

## Q Return to Top

**Q:** the letter used to denote the attenuation (energy loss through nonelastic processes) factor for seismic waves (See **attenuation**). Large values for Q imply small attenuation (e.g. mantle material). Low values of Q imply strong attenuation (e.g. shale and sandstone).

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

## R Return to Top

**R:** denotes a mantle Rayleigh wave (see also **LR**).

**R1, R2:** LR-type mantle wave that travels the direct and anticenter routes. Waves that have, in addition, traveled once or several times around the Earth are denoted R3, R4, R5, R6, etc.

**R _{g}:** denotes a short period (<3 sec) fundamental mode Rayleigh wave measured at local or regional distances and is very dependent on the focal depth.

**radial (seismogram):** seismic waves arriving at a station propagating at some angle to the vertical along the great-circle path connecting the source and receiver.

**radiation pattern (of waves):** a geometric description of the amplitude and sense of initial motion distributed over the P and S wavefronts in the vicinity of the source.

**random vibration theory:** A theoretical probabilistic formulation that links band-limited Gaussian noise spectra, representing the spectra of earthquake ground motions, with corresponding time history peak values.

**Rayleigh waves:** seismic surface waves with ground motion only in the vertical plane containing the direction of propagation of the wave. The particle motion is retrograde elliptical at the surface grading into forward elliptical as the wave gets deeper. Rayleigh waves are usually the largest amplitude waves seen on a raw seismogram and are found typically found n the vertical and Radial records.

**rays (ray theory):** the normals to the wavefront and point in the direction of propagation.

**ray theory:** a standard set of approximations used to interpret seismic body-waves based on plane waves and/or spherical waves traveling through a medium.

**ray-tracing method:** A computational method of computing ground-shaking estimates that assumes that the ground motion is composed of multiple arrivals that leave the source and are reflected or refracted at velocity boundaries according to Snell's Law. The amplitudes of reflected and refracted waves at each boundary are recalculated according to the Law of Conservation of Energy.

**receiver function:** a time series calculated from teleseismic P wavetrains that can be interpreted in terms of reflections and transmissions of mode-converted waves at discrete boundaries beneath the recording station. They can be used to approximate the crustal structure in the region around the recording station.

**recurrence interval:** the average time interval between earthquakes in a seismic region.

**reflection:** The energy or wave from a seismic source that has been returned (reflected) from an interface between materials of different elastic properties within the Earth.

**reflector:** An interface between materials of different elastic properties that reflects seismic waves.

**refraction:** (1) The deflection 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.

**regional distance:** a source-receiver distance greater than 100 km but less than 1000 km.

**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. (**See Least-squares fit**).

**relaxation theory:** Concept wherein radiated seismic waves of an earthquake result when stored strain within the Earth is released at the time of slip along a fault; adjacent fault blocks reach new states of equilibrium.

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

**resolution matrix (resolving kernel):** indicate how well the value in the corresponding layer could be determined independently of that of the other layers, if the data had no errors. They illustrate "vertical smearing:" they are largest at the depth for which they are computed, but have nonzero amplitudes at other depths. The best resolution occurs when the kernel is sharply peaked at the desired depth.

**resonance:** the largest vibration of a mechanical system (such a soil layer) due to the enhancement of the energy at a a frequency special to that system. Liquefaction can be the result of an earthquake vibrating the surrounding material at its resonating frequency causing the soil to act as a fluid.

**response**: The motion in a system resulting from shaking under specified conditions.

**response spectrum:** A curve showing the mathematically computed maximum response of a set of simple damped harmonic oscillators of different natural frequency(ies) to a particular record of earthquake ground acceleration. Response spectra, commonly plotted on tripartite logarithmic graph paper, show the oscillator's maximum acceleration, velocity, and displacement as a function of oscillator frequency for various levels of oscillator damping. A computational approximation to the response spectrum is referred to as the pseudorelative velocity response spectrum (PSRV). These curves are used by engineers to estimate the maximum response of simple structures to complex ground motions. For example, the 5-percent spectral acceleration at 1 second is the maximum acceleration of the top of a structure with 5 percent damping whose natural period of vibration is 1 second when subjected to a given input ground-acceleration record.

**reverse faulting:** the rock above the fault plane (the "hanging" wall) moves up and over the rock below ("foot" wall).

**ridge (midoceanic):** a major linear elevated landform of the ocean floor, many hundreds of kilometers in extent. It resembles a mountain range with a central rift valley.

**rift:** region where the crust has split apart, usually marked by a rift valley (e.g. East African Rift, Rhine Graben).

**right-lateral fault:**a strike-slip fault on which the displacement of the far block is to the right when viewed from the other side.

**rigidity:** an index of the resistance of an elastic body to shear. The ratio of the shearing stress to the amount of angular rotation it produces in a rock sample.

**risk (seismic):** the relative risk is the comparative earthquake hazard from one site to another. The probabilistic risk is the odds of earthquake occurrence within a given time interval and region.

**root mean square:** Square root of the mean square value of a random variable.

**rotate (seismograms):** changing the orientation or angle of a seismogram to the true radial or transverse directions.

**runup height:** the elevation of the water level above the immediate tide level when a tsunami runs up onto the coastal land.

**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 propagates during an earthquake.

## S Return to Top

**sP:** depth phase that leaves the focus upward as S (s leg) is converted to a P at the free surface and continues further a P.

**sPP:** depth phase that leaves the focus upward as S (s leg) is converted to PP at the free surface and continues further as PP.

**sPS:** depth phase that leaves the focus upward as S (s leg) is converted to PS at the free surface and continues further as PS.

**sPn:** depth phase that leaves the focus upward as S, is reflected and converted into P at the free surface and continues further as Pn.

**sS:** depth phase that leaves the focus upward as S (s leg) is reflected as S at the free surface and continues further a S.

**S*:** a seismic S-wave that travels along the Conrad discontinuity.

**ScP:** S wave converted into P upon reflection at the core mantle boundary.

**ScS:** S wave reflected at the core-mantle boundary.

**S-diff (or Sc):** S wave diffracted around the core-mantle boundary.

**S _{g}:** denotes a S-wave that traveled within the granitic layer of the crust (direct arrivals) at a distance of less than 100 km.

**SmKS (m=3,4,...):** S wave converted into P on refraction at the outer core, reflected m-1 times from inside the core-mantle boundary and finally converted back into S when entering the mantle.

**SmP:** an S-wave that was reflected of the Moho and converts to a P-wave.

**SmS:** an S-wave that was reflected of the Moho.

**SKP:** S wave converted into P on refraction into the outer core.

**SKS:** S wave traversing the outer core a P and converted back into S when entering the mantle.

**SH:** a component of an S-wave having displacements parallel to the surface. Part of the S wave that helps form Love waves.

**SV:** a component of an S-wave having displacements perpendicular to the surface.

**S wave:** the secondary seismic wave, traveling more slowly than the P wave and consisting of elastic vibrations transverse to the direction of travel. It cannot propagate in a liquid (e.g. outer core).

**safe shutdown earthquake:** a U.S. Nuclear Regulatory Commission term that specifies the maximum ground motion for which a reactor is expected to shut down safely without significant risk to the public.

**sag (fault):** a narrow geological depression found in strike-slip fault zones. Those that contain water are called sag ponds.

**sand boil:** Sand and water ejected to the ground surface as a result of liquefaction at shallow depth; the conical sediment deposit that remains as evidence of liquefaction.

**scarp (fault):** a cliff or steep slope formed by displacement of the ground surface.

**sea-floor spreading:** the process by which adjacent plates along midoceanic ridges move apart to make room for new sea-floor crust. This process may continue at 0.5 to 10 centimeters per year through many geological periods.

**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.

**segmentation:** The braking of faults along their lengths by other faults that cross them or their limitations in length by other factors such as topography or bends in the strikes 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:** oscillations (standing waves) of the water in a bay or lake.

**seismic discontinuity:** a surface or thin layer within layer the Earth across which P-wave and/or S-wave velocities changes rapidly.

**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, this latter case may apply to the Cascadia thrust fault.

**seismic hazard:** the physical effects such as ground shaking, faulting, landsliding, and liquefaction that underlie the earthquake's potential danger.

**seismic impedance:** Seismic P-wave velocity multiplied by the bulk density of a medium.

**seismic moment:** a measure of earthquake size related to the leverage of the forces (couples) across the area of the fault slip. The rigidity of the rock times the the area of faulting times the amount of slip. Dimensions are dyne-cm (or Newton-meters).

**seismic moment tensor:** a mathematical representation of a full set of solutions to produce a wide variety of effective source deformations and time histories.

**seismic reflection or refraction line**: A set of seismographs commonly distributed 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 risk**: The probability of social or economic consequences of an earthquake.

**seismic wave:** an elastic wave in the Earth usually generated by an earthquake source or explosion.

**seismic zonation:** Geographic delineation of areas having different potentials for hazardous effects from future earthquakes. Seismic zonation can be done at any scale - national, regional, local, or site.

**seismicity:** the occurrence of earthquakes in space and time.

**seismogenic:** produced by earthquakes.

**seismogram:** the paper record produced by a seismograph that contains the "wiggles".

**seismology:** the study of earthquake, seismic sources, and wave propagation through the Earth.

**seismometer or seismograph:** A seismometer is a damped oscillating mass, such as a damped mass-spring system, used to detect seismic-wave energy. The motion of 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.

**seismoscope:** a simple seismograph recording on a plate with time marks.

**seismotectonic province:** a region within which the active geologic and seismic processes are considered to be relatively uniform.

**seismotectonics:** a subfield of active tectonics concentrating on the seismicity, both instrumental and historical, and dealing with geological and other geophysical aspects of seismicity.

**separation:** The distance between any two parts of a reference plane (for example, a sedimentary bed or a geomorphic surface) offset by a fault, measured in any plane. Separation is the apparent amount of fault displacement and is nearly always less than the actual slip.

**shadow zone:** (1) the area on the Earth's surface protected from seismic wave shaking by some blocking object in the Earth. (2) The area of the Earth where the first arrival of energy is in the form of a P-wave that "glanced" off of the outer core boundary (called P-diff). This region is usually found between 103-144 degrees from the earthquake source.

**shear modulus:** The ratio of shear stress to shear strain of a material during simple shear.

**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 remote forces transmitted through the surrounding rock.

**shear wave:** (See **S wave**).

**shield volcano:** a volcano in the shape of a flattened dome, broad and low, built by flows of very fluid basaltic lava or by silicic ash flows.

**singular value decomposition (SVD):** a matrix transformation technique used to find the inverse of a square matrix.

**single station method:** technique to find the group and/or phase velocities from a single dispersed waveform. The origin time must be accurately known.

**slab**: Refers to the oceanic crustal plate that underthrusts the continental plate and is consumed by the Earth's mantle.

**slab pull:** The force of gravity causing the slab to sink into the mantle. The downdip component of this force leads to downdip extensional stress in the slab and may produce earthquakes within the subducted slab. Slab pull may also contribute to stress on the subduction thrust fault if the fault is locked.

**slip (fault):** the relative motion of one face of a fault relative to the other.

**slip model:** A kinematic model that describes the amount, distribution, and timing of slip associated with a real or postulated earthquake.

**slip rate:** The average rate of displacement at a point along a fault 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 separation of geologic markers.

**slip vector:** the magnitude and orientation of dislocation of formally adjacent features on opposite sides of a fault.

**slowness (tomography):** the reciprocal of velocity. Most seismic tomography methods involve subdividing the medium into blocks and solving for slowness perturbations that cause the predicted times to match observed times better than an initial model.

**soil amplification:** growth in the amplitude of earthquakes when seismic waves pass from rock into less rigid material such as soil.

**source:** (1) The geologic structure that generates a particular earthquake. (2) The explosion used to generate acoustic or seismic waves.

**source function:** The ground motion generated at the fault during rupture, usually as predicted by a theoretical model and represented by a time history or spectrum. The terms Brune spectrum, Aki spectrum, and Haskel model refer to varying representations of the source function, each based on different assumptions, as devised by the scientist for which the model is named.

**spectral acceleration:** Commonly refers to either the Fourier amplitude spectrum of ground acceleration or the PSRV.

**spectral amplification:** A measure of the relative shaking response of different geologic materials. The ratio of the Fourier amplitude spectrum of a seismogram recorded on one material to that computed from a seismogram recorded on another material for the same earthquake or explosion.

**spectral ratio:** See Spectral amplification.

**spectrum:** In seismology, a curve showing amplitude and phase as a function of frequency or period.

**spherical wave:** a wave with rays that radiate out from the source like spokes.

**spike test (tomography):** one type of resolution test that can be performed by looking at the impulse response to a spike. The spike is simulated by a localized velocity perturbation consisting of adjacent nodes at a particular depth in a homogeneous 3-D model, where except for the higher velocity values at the chosen nodes all other nodes are assigned the same background value. This test gives an indication of whether or not the network geometry and event distribution are adequate to resolve the spike.

**Snell's Law:** a geometric property of rays that describes how a ray will travel from one medium into another depending on the angle of incidence and the medium velocity. It can help predict at which angle the ray will be deflected in the new medium (velocity) and where the ray will "land" within that layer.

**source-time function:** contain all of the information available on the details of the earthquake rupture (e.g. duration of rupture, directivity effects, moment release).

**source depth:** (see **focus**).

**standard deviation (statistics):** the square root of the arithmetic mean of the squared deviations from the mean (the square root of the variance; see **variance**).

**station:** A ground position at which a geophysical instrument is located for an observation.

**stochastic**: Applied to processes that have random characteristics.

**strain (elastic):** the geometrical deformation or change in shape of a body. The change in an angle, length, area, or volume divided by the original value.

**strain rate:** Strain measurements are computed from observed changes in length on the Earth's surface, commonly along multiple paths. Because the changes in length are observed over varying time periods and path lengths, they are expressed as the change in length divided by the measurement distance divided by the measurement time period. This number, which is expressed as the change in length per unit length per unit time, is termed the strain rate. These measurements are used to infer the directions of principal strain and stress rates near the Earth's surface.

**step function:** a mathematical function where its value is zero before time t_{0} and one afterwards. Its derivative is the delta function.

**stress (elastic):** a measure of the forces acting on a body in units of force per unit area.

**stress drop:** The difference between the stress across a fault before and after an earthquake. A parameter in many models of the earthquake source that has a bearing on the level of high-frequency shaking that the fault radiates. Commonly stated in units termed bars or megapascals (1 bar equals 1 kg/cm2, and 1 megapascal equals 10 bars).

**stick slip**: The rapid displacement that occurs between two sides of a fault when the shear stress on the fault exceeds the frictional stress. Stick-slip displacement on a fault radiates energy in the form of seismic waves, creating an earthquake.

**strike of fault:** the line of intersection between the fault plane and the surface of the Earth. Its orientation is expressed as the angle west or east of true north.

**strike-slip fault:** a fault whose relative displacement is purely horizontal (e.g. San Andreas).

**Stonely waves:** interface waves with the largest amplitudes confined to the neighborhood of a plane interface of two elastic media.

**strong ground-motion:** the shaking of the ground near an earthquake source made up of large amplitude seismic waves of various types.

**subduction thrust fault:** The fault that accommodates the differential motion between the downgoing oceanic crustal plate and the continental plate as subduction occurs. This fault is the contact between the top of the oceanic plate and the bottom of the newly formed continental accretionary wedge. Also alternately referred to as the plate-boundary thrust fault, the thrust interface fault, or the megathrust fault.

**subduction zone:** a dipping ocean plate into the Earth away from an ocean trench. It is usually the locus intermediate and earthquakes defining the Wadati-Benioff zone (e.g. Aleutian, Chile, Izu-Bonin).

**surface faulting:** Displacement that reaches the Earth's surface during slip along a fault. Commonly accompanies moderate and large earthquakes having focal depths less than 20 km. Surface faulting also may accompany aseismic tectonic creep or natural or man-induced subsidence.

**surface reflections:** a seismic wave that initially travels downward but then is reflected back to the surface, often repeating the cycle. Some examples of surface reflections are PP, PS and PPP.

**surface-wave magnitude:** magnitude of an earthquake estimated from measurements of the amplitude of surface waves.

**surface waves (of earthquakes):** seismic waves that follow the Earth's surface only, with a speed less than that of S-waves. There are two types of surface waves - Rayleigh and Love waves.

**swarm (of earthquakes):** a series of earthquakes in the same locality, no one earthquake being of outstanding size.

**synthetic seismogram:** a mathematical construction of a seismogram based on realistic model of the Earth.

## T Return to Top

**T:** compressional wave propagating through the ocean (tertiary wave).

**TPg (TSg, TRg):** wave that travels the ocean and land portion of the total transmission path as T and Pg (Sg, Rg), respectively.

**takeoff angle:** the angle that a ray makes with a downward vertical. Used in determining depth phases.

**taper:** a function whose corners are less sharp resulting in less distortion of the function. Taper is used to help minimize the decay of the signal into the noise that result from having a finite record length. A typical taper used is the cosine taper in surface wave processing.

**Taylor series:** a mathematical tool to expand a function into a series of partial derivatives in that approximates the highest order values.

**tectonic earthquakes:** earthquakes resulting from the sudden release of energy stored by major deformation of the Earth.

**tectonics:** large-scale deformation of the outer part of the Earth resulting from forces in the Earth.

**teleseism:** an earthquake that occurs at a large distance (<1000 km) from the recording station (30-180 degrees). The waves recorded sample the lower mantle and core or reverberate in the upper mantle.

**teleseismic distance:** a source-receiver distance greater than 1000 km.

**tensional (T) axis:** in a double-couple system, the dipole that is directed outer from the source and found in the compressional quadrant of a focal mechanism. The ground is being pulled away from the source, giving compressional first P-waves at the receiver.

**tensor (mathematics):** a mathematical tool to describe a scalar (magnitude but no direction; zero-order tensor), a vector (magnitude and directionality; first-order tensor), and a second-order tensor which describes the interaction between vectors and directional operators.

**tesseral harmonic:** higher order surface spherical harmonics combining latitudinal and longitudinal nodal patterns.

**threshold (nuclear verification):** the smallest size of explosion that can be detected as an explosion.

**throw:** the vertical component of displacement on a fault.

**thrust fault:** a reverse fault in which the upper rocks above the fault plane mover and over the lower rocks at an angle of 30 degrees or less so that older strata are placed over younger.

**time-predictable model:** with reference to earthquakes, a recurrence model in which the time interval between two successive large earthquakes is proportional to the amount of seismic displacement of the preceding earthquake.

**tomographic:** construction of the image of an internal object or structure (in 3-D) from measurements of seismic waves at the surface.

**torodial mode:** spherical motions of the entire Earth that involves purely horizontal twisting of the Earth.

**torsional mode:** a spherical mode where it depends only on the shear-velocity structure and thus are confined to motions the solid shell of the mantle.

**transfer function:** predescribed rules that distort a seismogram in terms of the series of filters that make up the signal. For example, every recording instrument has a known transfer function that can be removed by mathematical means. The Earth also has its own transfer function. Can also be explained as the Laplace transform of the output signal divided by the Laplace transform of the input symbol.

**transcurrent fault:** a nearly vertical strike-slip fault that cuts not only supercrustal sedimentary rocks, but also igneous and metamorphic rocks (same as strike-slip).

**transform fault:** a strike-slip fault connecting the ends of an offset in a midoceanic ridge, an island arc, or an arc-ridge chain. Pairs of plates slides past along transform faults.

**transverse (seismogram):** seismic waves that arrive entirely horizontal to the great-circle path direction.

**travel-time curve:** a graph of travel time verses distance for the arrival of seismic waves from distant events. Each type of seismic wave has its own curve.

**trench:** long, narrow arcuate depression in the seabed which results from the bending of the lithospheric plate as it descends into the mantle at a subduction zone.

**triggered earthquake:** an earthquake that occurs soon after or during a mainshock, but well out of the region of the aftershocks.

**triggered slip:** aseismic fault slip that occurs during or soon after seismogenic rupture of a nearby fault.

**triple junction:** point where three plates meet.

**tsunami:** a long-period ocean wave usually caused by sea-floor movements in an earthquake.

**tsunami earthquake:** an earthquake with a relatively low magnitude that generates a large tsunami.

**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.

**tsunamiogenic earthquake:** an earthquake that generates a tsunami.

**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.

**two-station method:** another approach to measuring group and phase velocities using two stations lying on the same great-circle path.

## V Return to Top

**variance (statistics):** the arithmetic mean of the squared deviations from the mean.

**vector:** quantity having both magnitude and direction.

**velocity:** In reference to earthquake shaking, velocity is the time rate of change of ground displacement of a reference point during the passage of earthquake seismic waves commonly expressed in centimeters per second.

**velocity structure**: A generalized regional model of the Earth's crust that represents crustal structure using layers having different assumed seismic velocities.

**viscoelastic material:** a material which can behave as an elastic solid on a short-time scale and as a viscous fluid on a long-time scale.

**volcano:** an opening in the crust that has allowed magma to reach the surface. There are three major types of volcanoes, a shield volcano that has a low dipping angle because of the molten material being more fluid-like (e.g. Kilauea, Hawaii) and a stratvolcano that has steeper sides because the material is more dense and a cinder cone volcano that is produced by pyroclastic debris ejected into the air.

**volcanic earthquakes:** earthquakes associated with volcanic activity.

**volcanic rocks:** igneous rock which involves the eruption of molten rock.

**volcanic tremor:** the more-or-less continuous vibration of the ground near an active volcano.

**volcanism:** geological process which involved the eruption of molten rock.

## W Return to Top

**Wadati-Benioff zone:** See **Benioff zone.**

**waveform modeling:** comparing synthetic and observed seismograms to better understand Earth's internal structure and fault rupture processes.

**wavefront:** imaginary surface or line that joins points at which the waves from a source are in phase (e.g., all at a maximum or all at minimum).

**wavelength:** the distance between two successive crest or troughs of a seismic signal.

## Y Return to Top

**yield:** the amount of energy released (in kilotons of TNT) from an explosion.

**Young's modulus (E):** the ratio of tensional stress to extensional strain along the axis where the tension was applied.

## Z Return to Top

**Z-transform:** A generalized version of a Fourier Transform for discrete-time signals and systems. In a continuous-time system this would be a Laplace transform.

**Zonal Harmonics:**Used to describe Earth's normal modes using Legendre polynomials. The Zonal harmonics represents the lowest order of the polynomial where the number of great circles through the pole with zero displacement (m = 0) and the angular order number which give the number of nodal lines on the surface with zero displacement is 1 or 2 (l = 1 or 2). Higher order Harmonics would be Sectoral (l = m) and Tesseral (0 < abs(m) < l).

**Sources: Aki and Richards (1980), Bolt (1995) and Lay and Wallace (1995), Yeats et. al. (1997) plus other various web sources.**