Glossary of Some Common Terms in Seismology
An earthquake which follows a larger earthquake or main shock and originates in or near the rupture zone of the larger earthquake. Generally, major earthquakes are followed by a larger number of aftershocks, decreasing in frequency with time.
The maximum height of a wave crest or depth of a trough.
An ordered arrangement of seismometers or geophones, the data from which feeds into a central receiver.
The appearance of seismic energy on a seismic record.
The time at which a particular wave phase arrives at a detector.
Not associated with an earthquake, as in aseismic slip. Also used to indicate an area with no record of earthquakes; an aseismic zone.
A seismic wave that can travel through the interior of the earth. P-waves and S-waves are body waves.
An angle with the vertex at the center of the Earth, with one ray passing through the hypocenter (and also the epicenter) and the other ray passing through the recording station.
Tightly packed. Composed of particles that are not easily separated.
The innermost layers of the Earth. The inner core is solid and has a radius of about 1300 kilometers. (The radius of the Earth is about 6371 kilometers.) The outer core is fluid and is about 2300 kilometers thick. S-waves cannot travel through the outer core.
The theory, first advanced by Alfred Wegener, that Earth's continents were originally one land mass. Pieces of the land mass split off and migrated to form the continents.
The thin outer layer of the Earth's surface, averaging about 10 kilometers thick under the oceans and up tp about 50 kilometers thick on the continents. This is the only layer of the Earth that humans have actually seen.
Shaking of the Earth caused by a sudden movement of rock beneath its surface.
A series of minor earthquakes, none of which may be identified as the main shock, occurring in a limited area and time.
A wave that is propagated by some kind of elastic deformation, that is, a change in shape that disappears when the forces are removed. A seismic wave is a type of elastic wave.
That point on the Earth's surface directly above the hypocenter of an earthquake.
A weak point in the Earth's crust and upper mantle where the rock layers have ruptured and slipped. Faults are caused by earthquakes, and earthquakes are likely to reoccur on pre-existing faults.
The first recorded signal attributed to seismic wave travel from a source.
That point within the Earth from which originates the first motion of an earthquake and its elastic waves.
See Rupture Zone.
A small tremor that commonly precedes a larger earthquake or main shock by seconds to weeks and that originates in or near the rupture zone of the larger earthquake.
An earthquake having a magnitude of 8 or greater on the Richter scale.
A risk. An object or situation that has the possibility of injury or damage.
The calculated location of the focus of an earthquake.
A measure of the effects of an earthquake at a particular place on humans, structures and (or) the land itself. The intensity at a point depends not only upon the strength of the earthquake (magnitude) but also upon the distance from the earthquake to the point and the local geology at that point.
A line connecting points on the Earth's surface at which earthquake intensity is the same. It is usually a closed curve around the epicenter.
An abrupt movement of soil and bedrock downhill in response to gravity. Landslides can be triggered by an earthquake or other natural causes. Undersea landslides can cause tsunamis.
The location of a point north or south of the equator. Latitude is shown on a map or globe as east-west lines parallel to the equator.
A surface seismic wave which is imperfectly trapped so that its energy leaks or escapes across a layer boundary causing some attenuation, or loss of energy.
The process in which a solid (soil) takes on the characteristics of a liquid as a result of an increase in pore pressure and a reduction in stress. In other words, solid ground turns to jelly.
A surface wave which travels through the continental crust.
The location of a point east or west of the prime meridian. Longitude is shown on a map or globe as north-south lines left and right of the prime meridian, which passes through Greenwich, England.
A major type of surface wave having a horizontal motion that is shear or transverse to the direction of propagation (travel). It is named after A.E.H. Love, the English mathematician who discovered it.
Any layer in the Earth in which seismic wave velocities are lower than in the layers above and below.
A measure of the strength of an earthquake or strain energy released by it, as determined by seismographic observations. This is a logarithmic value originally defined by Charles Richter (1935). An increase of one unit of magnitude (for example, from 4.6 to 5.6) represents a 10-fold increase in wave amplitude on a seismogram or approximately a 30-fold increase in the energy released. In other words, a magnitude 6.7 earthquake releases over 900 times (30 times 30) the energy of a 4.7 earthquake - or it takes about 900 magnitude 4.7 earthquakes to equal the energy released in a single 6.7 earthquake! There is no beginning nor end to this scale. However, rock mechanics seems to preclude earthquakes smaller than about -1 or larger than about 9.5. A magnitude -1.0 event release about 900 times less energy than a magnitude 1.0 quake. Except in special circumstances, earthquakes below magnitude 2.5 are not generally felt by humans.
An earthquake having a magnitude of 7 to 7.99 on the Richter scale.
The layer of rock that lies between the crust and the outer core of the Earth. It is approximately 2900 kilometers thick and is the largest of the Earth's major layers.
An earthquake having a magnitude of 2 or less on the Richter scale.
A more or less continuous motion in the Earth that is unrelated to an earthquake and that has a period of 1.0 to 9.0 seconds. It is caused by a variety of natural and artificial agents.
Modified Mercalli scale
Mercalli intensity scale modified for North American conditions. A scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, that is designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects.
Mohorovicic discontinuity (the Moho)
The boundary surface or sharp seismic-velocity discontinuity (pronounced Mo-ho-ro-vi-chich) that separates the Earth's crust from the underlying mantle. Named for Andrija Mohorovicic, the Croatian seismologist who first suggested its existence.
Primary, longitudinal, irrotational, push, pressure, dilatational, compressional, or push-pull wave. P waves are the fastest body waves and arrive at stations before the S waves, or secondary waves. The waves carry energy through the Earth as longitudinal waves, moving particles in the same line as the direction of the wave. P waves can travel through all layers of the Earth. P waves are generally felt by humans as a bang or thump.
The natural magnetic traces that reveal the intensity and direction of Earth's magnetic field in the geologic past. Also, the study of these magnetic traces.
The study of ancient (prehistoric) earthquakes.
The time between two successive wave crests.
The onset of a displacement or oscillation on a seismogram indicating the arrival of a different type of seismic wave.
One of the huge sections which make up the Earth's crust. The plates are continuously moving.
The place where two or more plates in the Earth's crust meet.
The theory that the Earth's crust and upper mantle (the lithosphere) is broken into a number of more or less rigid, but constantly moving, segments or plates.
A type of surface wave having a retrograde, elliptical motion at the Earth's surface, similar to the waves caused when a stone is dropped into a pond. These are the slowest, but often the largest and most destructive, of the wave types caused by an earthquake. They are usually felt as a rolling or rocking motion and in the case of major earthquakes, can be seen as they approach. Named after Lord Rayleigh, the English physicist who predicted its existence.
The approximate length of time between earthquakes in a specific seismically active area.
To bounce back from a surface.
To bend or change direction.
The system used to measure the strength of an earthquake. Developed by Charles Richter in 1935 as a means of categorizing local earthquakes. It is a collection of mathematical formulas; it is not a physical device.
The area of the Earth through which faulting occurred during an earthquake. For very small earthquakes, this zone could be the size of a pinhead, but in the case of a great earthquake, the rupture zone may extend several hundred kilometers in length and tens of kilometers in width.
Shear, secondary, rotational, tangential, equivoluminal, distortional, transverse, or shake wave. These waves carry energy through the Earth in very complex patterns of transverse (crosswise) waves. These waves move more slowly than P waves, but in an earthquake they are usually bigger. S waves cannot travel through the outer core because these waves cannot exist in fluids, such as air, water or molten rock.
A free or standing wave oscillation of the surface of water in an enclosed basin that is initiated by local atmospheric changes, tidal currents, or earthquakes. Similar to water sloshing in a bathtub.
An elongated earthquake zone, for example, circum-Pacific, Mediterranean, Rocky Mountain. About 60% of the world's earthquakes occur in the circum-Pacific seismic belt.
In building codes dealing with earthquake hazards, an arbitrarily-set acceleration value (in units of gravity) that a building must withstand.
Of or having to do with earthquakes.
Seismic Sea Wave
A tsunami generated by an undersea earthquake.
A region in which earthquakes are known to occur.
A written record of an earthquake, recorded by a seismograph.
An instrument that records the motions of the Earth, especially earthquakes.
A site at which one or more seismographs are set up and routinely monitored.
A scientist who studies earthquakes.
The instrumental aspects of seismology.
The comparison between the amplitude of the seismic signal and the amplitude of noise caused by seismic unrest and (or) the seismic instruments.
The layout of seismometer or geophone groups from which data from a single shot (the explosive charge) are recorded simultaneously.
An elongated region where two plates are being pulled away from each other. New crust is formed as molten rock is forced upward into the gap. Examples of spreading centers include the Mid-Atlantic Ridge and the East African Rift.
The process in which one lithospheric plate collides with and is forced down under another plate and drawn back into the Earth's mantle.
An elongated region along which a plate descends relative to another plate, for example, the descent of the Nazca plate beneath the South American plate along the Peru-Chile Trench.
Surface of the Earth
The value given is the depth below the surface of the mean spheroid. The mean spheroid is a uniform approximation to the true shape of the Earth. No adjustment is made to the depth due to any differences between the true Earth and the mean spheroid. For example, the minimum depth that will be given is 0 kilometers, even though a quake directly under Mount Everest (elevation 8848 meters) could legitimately have a depth of -6 kilometers and still be 2 kilometers underground. On the other hand, a depth of 10 kilometers would actually be more than 1 kilometer above the ocean floor of Challenger Deep (elevation -11,033 meters) in the Marianas Trench of the Pacific Ocean.
Waves that move over the surface of the Earth. Rayleigh waves and Love waves are surface waves.
An earthquake that is distant (usually more than 20 degrees) from the recording station.
A term that seismologists hate. The correct word for the big waves people often call "tidal waves" is tsunami. True "tidal waves" - or waves caused by the tides - are the ordinary waves people see on the ocean.
The time required for a wave train to travel from its source to a point of observation.
One or a series of huge sea waves caused by earthquakes or other large-scale disturbance of the ocean floor. (Referred to incorrectly by many as a tidal wave, but these waves have nothing to do with tides.) The word tsunami is Japanese, meaning "harbor wave."
Loosely arranged, not cemented together, so particles separate easily.
Coordinated Universal Time. The time scale based on the atomic second but corrected every now and again to keep it in approximate sync with the earth's rotation. The corrections show up as the leap seconds put into UTC - usually on New Year's Eve
The Glossar is taken from:
http://www.uwm.edu/Dept/Geosciences/Rec ... onary.html
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. You can e-mail suggestions to me: Brett Ketter
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" 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: 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.
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 t0 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" t0 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 (mb) to 20-s-period Rayleigh-wave energy (MS), 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.
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.
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 (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.
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 Pn.
heave: the horizontal component of separation or displacement on a fault.
Hertz: the unit of frequency equal to 1 cycle per second, or 2 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: 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: 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: 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: denotes a Love-wave (see also LQ).
Lg: 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.
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.
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.
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).
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).
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.
pPn: depth phase that leaves the focus upward as P, is reflected as Pn at the free surface and continues further as Pn.
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 103o (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.
Pg: 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.
Pn: 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'', PKPDF): 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 PKPBC, PKPAB): 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 ra