MBMG - Glossary of Earthquake Terms         

Glossary of Earthquake Terms

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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. Sediments that accumulate and deform where oceanic and continental plates collide. These sediments are scraped off the top of the downgoing oceanic crustal plate and are added to the leading edge of the continental plate.

Active fault. A fault that is considered likely to undergo renewed movement within a period of concern to humans. Faults are commonly considered to be active if they have moved one or more times in the last 10,000 years, but they may also be considered active when assessing the hazard for some applications even if movement has occurred in the last 500,000 years.

Aftershocks. Secondary tremors that may follow the largest shock of an earthquake sequence. Such tremors can extend over a period of weeks, months, or years.

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. Zero-to-peak value of any wavelike disturbance.

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. Referring to a fault on which no earthquakes have been observed. Aseismic behavior may be due to lack of shear stress across the fault, a locked-fault condition with or without shear stress, or release of stress by fault creep.

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.

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.

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

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.

Basement. Igneous and metamorphic rocks that underlie the main sedimentary-rock sequences 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.

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.

Body wave. A seismic wave that propagates through the interior of the Earth, as opposed to surface waves that propagate near the Earth's surface. P and S waves are examples. Each type of wave has distinctive strain characteristics.

Brittle-ductile boundary. A depth in the crust across which the thermomechanical properties of the crust change from brittle above to ductile below. A large percentage of the earthquakes in the crust initiate at or above this depth on high-angle faults; below this depth, fault slip may be aseismic and may grade from high angle to low angle.

Bulk density. The mass of a material divided by its volume, including the volume of its pore spaces.

14C age date. An absolute age obtained for geologic materials containing bits or pieces of carbon using measurements of the proportion of radioactive carbon (14C) to daughter carbon (12C). These dates are independently calibrated with calendar dates.

Coherent slides. Landslides that consist of a few relatively intact blocks of rock or soil that move together. The basal failure surface of most of these slides is several meters or tens of meters below the land surface.

Cohesionless. Referring to the condition of a sediment whose shear strength depends only on friction because there is no bonding between the grains. This condition is typical of clay-free sandy deposits.

Colluvium. Loose soil or rock fragments on or at the base of gentle slopes or hillsides. Deposited by or moving under the influence of rainwash or downhill creep.

Compressional wave. See P wave.

Convolution. A mathematical operation that describes the action of a linear system on a signal, such as that of a filter on a seismic signal.

Corner frequency. The frequency at which the curve representing the Fourier amplitude spectrum of a recorded earthquake seismic signal abruptly changes slope. This frequency is a property of the source function related to fault size.

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 (see locked fault and interplate coupling).

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.

Damping. The reduction in amplitude of a seismic wave or oscillator due to friction and (or) the internal absorption of energy by matter. See Geometrical attenuation.

Deformational front. See Accretionary wedge.

Design earthquake. The postulated earthquake (commonly including a specification of the ground motion at a site) that is used for evaluating the earthquake resistance of a particular structure.

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.

Dip. Inclination of a planar geologic surface (for example, a fault or a bed) from the horizontal.

Dip slip. See Fault.

Directivity. An effect of a propagating fault rupture whereby earthquake ground motion in the direction of propagation is more severe than that in other directions from the earthquake source.

Displacement. The difference between the initial position of a reference point and any later position. (1) In seismology, displacement is the ground motion commonly inferred from a seismogram. For example, it may be calculated by integrating an accelerogram twice with respect to time and is expressed in units of length, such as centimeters. (2) In geology, displacement is the permanent offset of a geologic or man-made reference point along a fault or a landslide.

Disrupted slides and falls. Landslides that are broken during movement into chaotic masses of small blocks, rock fragments, or individual grains. The basal failure surface of most such slides is within a few meters of the land surface.

Earthquake. Ground shaking and radiated seismic energy caused most commonly by sudden slip on a fault, volcanic or magmatic activity, or other sudden stress changes in the Earth. An earthquake of magnitude 8 or larger is termed a great earthquake.

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 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.

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.

Epicenter. The point on the Earth's surface vertically above the point (focus or hypocenter) in the crust where a seismic rupture nucleates.

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.

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-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. Steplike linear landform coincident with a fault trace and caused by geologically recent slip on the fault.

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

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 ground motion as the P wave arrives at the seismometer. Upward ground motion indicates an expansion in the source region; downward motion indicates a contraction.

Focal depth. A term that refers to the depth of an earthquake focus.

Focal-mechanism solution. See Fault-plane solution.

Focus. See Hypocenter.

Focusing. See Amplification.

Forearc. The region between the trace of the subduction thrust fault and the volcanic chain (volcanic arc).

Foreshocks, mainshock, aftershocks. Foreshocks are relatively smaller earthquakes that precede the biggest earthquake in a series, which is termed the mainshock. Aftershocks are relatively smaller earthquakes that follow the mainshock.

Fourier amplitude spectrum.The relative amplitude at different component frequencies that are derived from a time history by Fourier analysis.

Fourier transform. The mathematical operation that resolves a time series (for example, a recording of ground motion) into a series of numbers that characterize the relative amplitude and phase components of the signal as a function of frequency.

Frequency. Number of cycles occurring in unit time.

Fundamental period. The longest period for which a structure shows a maximum response. The reciprocal of natural frequency.

G or g. See Acceleration.

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.

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

Geomorphology. The study of the character and origin of landforms.

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

Gravity. The attraction between two masses, such as the Earth and an object on its surface. Commonly referred to as the acceleration of gravity. Changes in the gravity field can be used to infer information about the structure of the Earth's lithosphere and upper mantle. 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.

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 failure. A general reference to landslides, liquefaction, and lateral spreads.

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.

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.

Hazard. See Earthquake hazard.

Hertz (Hz). A unit of frequency. Expressed in cycles per second.

Holocene. Refers to a period of time between the present and 10,000 years before present. Applied to rocks or faults, this term indicates the period of rock formation or the time of most recent fault slip. Faults of this age are commonly considered active, based on the observation of historical activity on faults of this age in other locales.

Hypocenter. The point within the Earth where an earthquake rupture initiates. Also commonly termed the focus.

Intensity. A subjective numerical index 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.

Intraplate and interplate. Intraplate pertains to processes within the Earth's crustal plates. Interplate pertains to processes between the plates.

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. See Locked fault.

Isoseismal. Referring to a line on a map bounding points of equal intensity for a particular earthquake.

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

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.

Landslide. The downslope movement of soil and (or) rock.

Lateral spreads and flows. Terms referring to landslides that commonly form on gentle slopes and that have rapid fluid-like flow movement.

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.

Lifelines. Structures that are important or critical for urban functionality. Examples are roadways, pipelines, powerlines, sewers, communications, and port facilities.

Liquefaction. Process by which water-saturated sediment temporarily loses strength and acts as a fluid. This effect can be caused by earthquake shaking.

Lithology. The description of rock composition and texture.

Lithosphere. The outer solid part of the Earth, including the crust and uppermost mantle. The lithosphere is about 100 km thick, although its thickness is age dependent.The lithosphere below the crust is brittle enough at some locations to produce earthquakes by faulting, such as within a subducted oceanic plate.

Locked fault. A fault that is not slipping because frictional resistance on the fault is greater than the shear stress across the fault. 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. See Interplate coupling.

Love wave. A type of seismic surface wave having a horizontal motion that is transverse to the direction of propagation.

Ma. An abbreviation for one million years ago (Megannum).

Magnetic polarity reversal. A change of the Earth's magnetic field to the opposite polarity that has occurred at irregular intervals during geologic time. Polarity reversals can be preserved in sequences of magnetized rocks and compared with standard polarity-change time scales to estimate geologic ages of the rocks. Rocks created along the spreading oceanic ridges commonly preserve this pattern of polarity reversals as they cool, and this pattern can be used to determine the rate of oceanridge 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 principle, 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.

Mantle. That part of the Earth's interior between the metallic core and the crust.

Microzonation. The geographic delineation at local or site scales of areas having different potentials for hazardous earthquake effects, such as groundshaking potential. Microzonation for any of the earthquake hazards can be produced.

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.

Moment magnitude. See Magnitude.

Natural frequency(ies). The discrete frequency(ies) at which a particular elastic system vibrates when it is set in motion by a single impulse and not influenced by other external forces or by damping. The reciprocal of fundamental period.

Newmark analysis. A numerical technique that models a potential landslide as a rigid block resting on a frictional slope, prescribing dynamic forces on the block from assumed ground shaking records in order to calculate the expected displacement of the block.

Normal stress. That stress component perpendicular to a given plane.

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.

Oceanic trench. A linear depression of the sea floor caused by and approximately coincident with a subduction thrust fault.

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.

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.

P wave. A seismic body wave that involves particle motion (alternating compression and extension) in the direction of propagation.

Paleoseismic. Referring to the prehistoric seismic record as inferred from young geologic sediments.

Peak acceleration. See Acceleration.

Pedogenic. Pertaining to processes that add, transfer, transform, or remove soil constituents.

Period. The time interval required for one full cycle of a wave.

Period band. That range of periods being considered in an analysis of ground motion.

Phase. (1) A stage in periodic motion, such as wave motion or the motion of an oscillator, measured with respect to a given initial point and expressed in angular measure. (2) A pulse of seismic energy arriving at a definite time. (3) Stages in the physical properties of rocks or minerals under differing conditions of pressure, temperature, and water availability.

Physiographic. Referring to the character and distribution of landforms.

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.

Poisson distribution. A probability distribution that characterizes discrete events occurring independently of one another in time.

Probabilistic earthquake hazard. See Earthquake hazard and Earthquake risk.

Pseudorelative acceleration spectrum. See Response spectrum.

PSRV. Pseudorelative velocity response spectrum. See Response spectrum.

Q. See Attenuation.

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

Radiometric. Pertaining to the measurement of geologic time by the analysis of certain radioisotopes in rocks and their known rates of decay.

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 wave. A seismic surface wave causing an elliptical motion of a particle at the free surface, with no transverse motion.

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.

Recurrence interval. The average time span between events (such as large earthquakes, ground shaking exceeding a particular value, or liquefaction) at a particular site. Also termed return period.

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.

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.

Resonance. An increase in the amplitude of vibration in an elastic body or system when the frequency of the shaking force is close to one or more of the natural frequencies of a shaking body.

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.

Return period. See Recurrence interval.

Rigidity. See Shear modulus.

Risk. See Earthquake risk.

Rheological properties. The properties of rocks that describe their ability to deform and flow as a function of temperature, pressure, and chemical conditions.

Root mean square. Square root of the mean square value of a random variable.

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 wave. A seismic body wave that involves a shearing motion in a direction perpendicular to the direction of propagation. When it is resolved into two orthogonal components in the plane perpendicular to the direction of propagation, SH denotes the horizontal component and SV denotes the orthogonal component.

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.

Sea-floor magnetic lineaments.See Magnetic polarity reversals.

Secular. Referring to long-term changes that take place slowly and imperceptibly. Commonly used to describe changes in elevation, tilt, and stress or strain rates that are related to long-term tectonic deformation.

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. Oscillation of the surface of an enclosed body of water owing to earthquake shaking.

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. See Earthquake hazard and Earthquake risk.

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

Seismicity. The geographic and historical distribution of earthquakes.

Seismic moment. A measure of the size of an earthquake based on the area of fault rupture, the average amount of slip, and the shear modulus of the rocks offset by faulting. Seismic moment can also be calculated from the amplitude spectra of seismic waves.

Seismic reflection or refraction line. A set of seismographs 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. See Probabilistic earthquake hazard.

Seismic wave. An elastic wave generated by an impulse such as an earthquake or an explosion. Seismic waves may propagate either along or near the Earth's surface (for example, Rayleigh and Love waves) or through the Earth's interior (P and S waves).

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. See Microzonation.

Seismogenic. Capable of generating earthquakes.

Seismogram. A record written by a seismograph in response to ground motions produced by an earthquake, explosion, or other ground-motion sources.

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.

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.

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 reinote forces transmitted through the surrounding rock.

Shear wave. See S wave.

Slab. In this document, slab refers to the oceanic crustal plate that underthrusts the continental plate and is consumed by the Earth's mantle. See Plate tectonics.

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

Slip model. A kinematic model that describes the amount, distribution, and timing of slip associated with 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.

Soil. (1) In engineering, all unconsolidated material above bedrock. (2) In soil science, naturally occurring layers of mineral and (or) organic constituents that differ from the underlying parent material in their physical, chemical, mineralogic, and morphologic character because of pedogenic processes.

Soil profile. The vertical arrangement of soil horizons down to the parent material or to bedrock. Commonly subdivided into A, B, and C horizons.

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.

Standard deviation. 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.

Standard penetration resistance. A measure of relative density expressed by the number of blows (blow count) needed to push a probe a standard distance into sediment. The standard penetration test determines the number of blows required to drive a standard sampling spoon 1 ft into the sediment by repeatedly dropping a 140-pound weight from a height of 30 in.

Station. A ground position at which a geophysical instrument is located for an observation.

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.

Stochastic. Applied to processes that have random characteristics.

Strain. Small changes in length and volume associated with deformation of the Earth by tectonic stresses or by the passage of seismic waves.

Strain rate. 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.

Stratigraphy. The study of the character, form, and sequence of layered rocks.

Stress. Force per unit area acting on a plane within a body. Six values are required to characterize completely the stress at a point: three normal components and three shear components.

Stress drop. The difference between the stress across a fault before and after an earthquake. 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).

Strike. Trend or bearing, relative to north, of the line defined by the intersection of a planar geologic surface (for example, a fault or a bed) and a horizontal surface.

Strike slip. See Fault.

Strong motion. Ground motion of sufficient amplitude and duration to be potentially damaging to a building's structural components or architectural features.

Subduction. A plate tectonics term for the process whereby the oceanic lithosphere collides with and descends beneath the continental lithosphere.

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.

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 wave. Seismic wave that propagates along the Earth's surface. Love and Rayleigh waves are the most common.

Surface-wave magnitude. See Magnitude.

Tectonic. Refers to rock-deforming processes and resulting structures that occur over regional sections of the lithosphere.

Teleseismic. Pertaining to earthquakes at distances greater than 1,000 km from the measurement site.

Tilt. An observed rate of change in the slope of the Earth's surface.

Thrust interface fault. See Benioff zone and subduction thrust fault.

Time history. The sequence of values of any time-varying quantity (such as a ground motion measurement) measured at a set of fixed times. Also termed time series.

Transform fault. A special variety of strike-slip fault that accommodates relative horizontal slip between other tectonic elements, such as oceanic crustal plates.

Traveltime curve. A graph of arrival times, commonly P or S waves, recorded at different points as a function of distance from the seismic source. Seismic velocities within the Earth can be computed from the slopes of the resulting curves.

Tsunami. An impulsively generated 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.

Tsunamigenic. Referring to those earthquake sources, commonly along major subduction-zone plate boundaries such as those bordering the Pacific Ocean, that can generate tsunamis.

Tsunami magnitude (Mt). A number used to compare sizes of tsunamis generated by different earthquakes and calculated from the logarithm of the maximum amplitude of the tsunami wave measured by a tide gauge distant from the tsunami source.

Turbidites. Sea-bottom deposits formed by massive slope failures where rivers have deposited large deltas. These slopes fail in response to earthquake shaking or excessive sedimentation load. The temporal correlation of turbidite occurrence for some deltas of the Pacific Northwest suggests that these deposits have been formed by earthquakes.

Unlocked fault. See Locked fault and Creep.

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.

Water table. The upper surface of a body of unconfined ground water at which the water pressure is equal to the atmospheric pressure.

Wavelength. The distance between successive points of equal amplitude and phase on a wave (for example, crest to crest or trough to trough).

YBP. An abbreviation for years before present.

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