Glossary of seismological terms

Accelerograph:

A type of seismograph used to measure ground acceleration as a function of time.

Active fault:

A fault along which slip has occurred in recent geological time, or where earthquake foci are located.

Active margin:

Continental margin characterized by earthquakes and volcanic activity (i.e. location of transform fault or subduction zone).

Aftershock:

An earthquake that occurs after a "mainshock" (or larger earthquake). Aftershocks occur in the same general region as the "mainshock" and result from readjustments of stress at places along the fault zone. For great earthquakes (M=8), aftershocks may occur over hundreds of kilometres. Depending on the size and depth of the earthquake, aftershocks may occur for many months after the mainshock; however, the rate of aftershock activity dies off quickly with time.

Amplitude (wave):

The maximum height of a wave crest or depth of a trough.

Array:

An ordered arrangement of seismometers or geophones, the data from which feeds into a central receiver.

Arrival:

The appearance of seismic energy on a seismic record.

Arrival time:

The time at which a particular wave phase arrives at a detector.

Aseismic:

Not associated with an earthquake, as in aseismic slip. Also used to indicate an area with no record of earthquakes: an aseismic zone.

Asperities (fault):

Roughness on the fault surface posing local resistance to slip.

Asthenosphere:

The layer below the lithosphere that is marked by low seismic-wave velocities and high seismic-wave attenuation.

Attenuation:

The reduction in amplitude of a wave with time or distance travelled.

Auxiliary fault-plane:

A plane orthogonal to the fault plane.

Barrier (fault):

An area of fault surface that is resistant to slip because of geometrical or structural changes.

Benioff zone:

A narrow zone, defined by earthquake foci, that is tens of kilometers thick dipping from the surface under the Earth's crust to depths of up to 700 kilometers. (Also Wadati-Benioff zone.)

Blind thrust:

A deep crustal thrust-fault with no or only indirect surface expression such as a fold structure.

Body wave:

A seismic wave that can travel through the interior of the earth. P-waves and S-waves are body waves.

Body-wave magnitude:

Magnitude of an earthquake as estimated from the amplitude of body waves.

Capable fault:

A fault along which it is mechanically feasible for sudden slip to occur.

Characteristic earthquake:

An earthquake with a size and generating mechanism typical for a particular fault source.

Coda:

The concluding train of seismic waves that follows the principal waves from an earthquake.

Consolidated:

Tightly packed. Composed of particles that are not easily separated.

Continental Drift:

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.

Continental shelf:

Part of the continental margin between the coast and the continental slope.

Core:

The innermost layers of the Earth. The inner core is solid and has a radius of about 1300 kilometres. (The radius of the Earth is about 6371 kilometres.) The outer core is fluid and is about 2300 kilometres thick. S-waves cannot travel through the outer core.

Crust:

See Earth's Crust.

Damping:

Loss of energy in wave motion due to transfer into heat by frictional forces.

Density:

The mass per unit volume of a substance, commonly expressed in grams per cubic centimetre.

Depth of an earthquake:

The value given is the depth below the surface of the mean spheroid.

Dilatancy (of rocks):

The increase in the volume of rocks mainly due to pervasive microcracking.

Dip:

The angle by which a rock layer or fault plane deviates from the horizontal. The angle is measured in a plane perpendicular to the strike.

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.

Dispersion (wave):

The spreading out of a wave train due to each wavelength travelling with its own velocity.

Duration (of strong shaking):

Time interval between the first and last peaks of strong ground motion above a specified amplitude.

Earthquake:

The sudden release of stored elastic energy caused by the sudden fracture and movement of rocks along a fault. Some of the energy released is in the form of seismic waves, that cause the ground to shake.

Earthquake occurrence (recurrence) interval:

The average interval of time between the occurrence of earthquakes in a particular region.

Earthquake swarm:

A series of minor earthquakes, none of which may be identified as the main shock, occurring in a limited area and time.

Earth's crust:

The layer of rock located immediately below the earth's surface. Beneath continents, it is typically about 35 km thick, and composed of granite. Under the ocean, the crust is about 5-10 kilometres thick and composed mainly of basalt.

Elastic wave:

A wave that is propagated by some kind of elastic deformation, that is, a change in shape that disappears when the stresses are removed. A seismic wave is a type of elastic wave.

Epicentre:

The point on the earth's surface directly above the focus (hypocentre) of an earthquake.

Fault:

A zone of fractures or breaks in rocks, where movements occur. Earthquakes often occur along faults because they are weak zones in the rock.

Fault plane:

The plane that most closely coincides with the rupture surface of a fault.

First arrival:

The first recorded signal attributed to seismic wave travel from a source.

Flower structure:

More-or-less symmetrical splays into sub-faults near the intersection of the main fault with the ground surface.

Focal zone:

See Rupture Zone.

Foreshock:

An earthquake that is smaller than, and precedes, a "mainshock". Foreshocks tend to occur in the same area as the mainshock. Foreshocks have not been observed before damaging earthquakes in British Columbia.

Frequency:

Number of oscillations per unit time; unit is Hertz (Hz),which equals 1 cycle per second.

Gouge:

In a fault zone, crushed, sheared, and powdered rock altered to clay.

Graben:

A crustal block of rock generally long and narrow, that has dropped down along boundary faults relative to the adjacent rocks.

Great Earthquake:

An earthquake having a magnitude of 8 or greater on the Richter scale.

Gutenberg discontinuity:

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

Hazard:

A situation that has the possibility of occurring.

Hertz:

The unit of frequency equal to 1 cycle per second, or 2 PI radians per second.

Holocene:

The current geological time period that started about 10,000 years ago.

Hypocentre:

The subsurface location (focus) at which the energy of an earthquake is released. Earthquakes generally occur at depths less than about 30 km, but may occur to a depth of 600 km or more in some areas.

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 about 1221 kilometers, discovered by Inge Lehmann in 1936.

Intensity:

The Modified Mercalli Scale is a numerical scale used to catagorize earthquakes based on descriptions of how the earthquake was felt. These effects may range from I (not felt except by a very few under especially favourable conditions) and XII (total damage).

Interplate earthquake:

Earthquake with its focus on a plate boundary. Offshore earthquakes of western Canada are of this type.

Intraplate earthquake:

Earthquake with its focus within a tectonic plate. Eastern Canadian earthquakes are of this type.

Island arc:

Chain of islands above a subduction zone (e.g., Japan, Aleutians).

Isoseismal Line:

A line connecting points on the Earth's surface at which earthquake intensity is the same. It is usually a closed curve around the epicentre.

Isostasy:

The way in which the lithosphere 'floats' on the asthenosphere.

Landslide:

An abrupt movement of geological materials downhill in response to gravity. Landslides can be triggered by an earthquake or other natural causes. Undersea landslides can cause tsunamis, such as the one triggered by the 1929 Grand Banks earthquake.

Latitude:

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.

Left-lateral fault: (sinistral)

A strike-slip fault on which the displacement of the far block is to the left when viewed from either side.

Liquefaction:

The process in which a granular 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 loses cohesion and starts flowing like a liquid.

Lithology:

Physical character of rocks.

Lithosphere:

The outer, rigid shell of the Earth above the asthenosphere. It contains the crust, continents, and plates.

L_g Wave:

A surface wave which travels through the continental crust. This wave type is the one which causes damage during large Eastern Canadian earthquakes.

Longitude:

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.

Love wave:

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.

Low-velocity zone:

Any layer in the Earth in which seismic wave velocities are lower than in the layers above and below.

Magnitude:

Magnitude is a measure of the amount of energy released during an earthquake. It may be expressed using the Richter scale.

See:

• What is the "magnitude" of an earthquake?
• What it is the difference between magnitudes M_L and m_N?
• Certain earthquakes have a negative magnitude, is this an error?
• Is there a maximum magnitude for an earthquake?
• At what magnitude do earthquakes begin to be felt? When does damage start do to be observed?
• Do several magnitude scales exist?

Mainshock:

The largest earthquake in a "cluster" of earthquakes. Mainshocks are sometimes preceded by "foreshocks", and generally followed by aftershocks.

Major Earthquake:

An earthquake having a magnitude of 7 to 7.99 on the Richter scale.

Mantle (of Earth):

The main bulk of the Earth, between the crust and the core, ranging from depths of about 40 to 3470 kilometres. It is composed of dense silicate rocks and divided into a number of concentric shells. Under Eastern Canada, it can be found at around 40 km depth.

Meizoseismal region:

The area of strong shaking and significant damage in an earthquake.

Microearthquake:

An earthquake having a magnitude of 2 or less on the Richter scale.

Microseism:

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.

Microzonation:

The division of a town or county into smaller areas according to their variation in seismic hazard.

Moderate earthquake:

An earthquake having a magnitude of 5 to 6 on the Richter scale.

Modified Mercalli Intensity Scale:

The Mercalli scale rates the intensity of shaking from an earthquake. The ratings vary from I (felt only under especially favourable circumstances) to XII (total destruction).

Modified Mercalli (MM) Intensity Scale

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. Under Eastern Canada, it can be found at around 40 km depth.

Moment (of earthquakes):

A measure of earthquake size related to the leverage of the forces (couples) across the area of the fault slip. The rigidity of the rock times the area of faulting times the amount of slip. Dimensions are dyne-cm (or Newton-metres).

Moment magnitude (M_W):

Magnitude of an earthquake estimated by using the seismic moment.

Normal fault:

A dip-slip fault in which the rock above the fault plane has moved downward with respect to the rock below.

Oblique faulting:

The slip on the fault has components both along the dip and along the strike of the fault.

Origin time:

The exact time at which an earthquake occurred.

Outer core:

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

P wave:

Also called 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. Their velocity in the crust varies between 5.0 and 7.0 km/s. 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.

Paleoseismology:

The study of ancient (prehistoric) earthquakes from their geological evidences.

Passive margin:

Continental margin formed during initial rifting apart of continents to form an ocean; frequently has thick sedimentary deposits.

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: (seismic)

The onset of a displacement or oscillation on a seismogram, indicating the arrival of a different type of seismic wave.

Plates and plate tectonics:

The crust and upper mantle of the earth are made up of about a dozen large plates and several smaller ones that are constantly moving. The movements are very slow - only a few centimetres per year. Where the plates rub against one another, strain builds up, especially at the edges. When the strength of the rock is exceeded, the earth's crust may break and suddenly shift by several metres, causing an earthquake.

Precursor:

A change in the geological or geophysical conditions that is a forerunner to earthquake generation on a fault. Precursors cannot reliably be recognized as such beforehand.

Prediction (of earthquakes):

The forecasting in time, place, and magnitude of an earthquake; the forecasting of strong ground motions. Currently, there is no reliable method of predicting earthquakes.

Probability:

The number of cases that meet a given description divided by the total number of (equally likely) cases possible.

Probability of exceedence of a given earthquake size:

The odds that the size of a future earthquake will exceed some specified value.

Quaternary:

The geological period covering about 2 million years ago to the present.

Rayleigh wave:

A type of surface wave having a retrograde elliptical motion at the Earth's surface. 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.

Recurrence Interval:

The approximate length of time between earthquakes in a specific seismically active area.

Reflect:

To bounce back from a surface that represents an acoustic impedance contrast.

Refract:

To bend or change direction due to an acoustic impedance contrast.

Reverse faulting:

The rock above the fault plane (the "hanging"wall) moves up and over the rock below ("foot" wall).

Rift:

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

Right-lateral fault: (dextral)

A strike-slip fault on which the displacement of the far block is to the right when viewed from either side. The Queen Charlotte fault is one of this type of fault.

Rigidity:

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

Risk (seismic):

The probabilistic risk is the odds of an earthquake occurring and causing damage within a given time interval and region.

Rossi-Forel Intensity Scale:

The Rossi-Forel scale is a measure of intensity of shaking from an earthquake. This scale was replaced by the Mercalli intensity scale.

See:

• Rossi-Forel Intensity scale of 1883

Runup height:

The elevation of the water level above the immediate tide level when a tsunami runs up onto the coastal land.

Rupture Zone:

The area of the Earth through which faulting occurred during an earthquake. For very small earthquakes, this zone could be a few millimetres long, but in the case of a great earthquake, the rupture zone may extend several hundred kilometres in length and tens of kilometres in width.

S wave:

Also called 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.

Sag (fault):

A narrow geological depression found in strike-slip fault zones. Those that contain water are called sag ponds.

Scarp (fault):

A cliff or steep slope formed by displacement of the ground surface.

Seiche:

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.

Seismic:

Of or having to do with earthquakes.

Seismic belt:

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.

Seismic discontinuity:

A surface or thin layer within the Earth across which P-wave and/or S-wave velocities change rapidly.

Seismic gap:

An area in an earthquake-prone region where there is a below-average release of seismic energy.

Seismic moment:

See Moment (of earthquakes).

Seismic Sea Wave:

A tsunami (see below) generated by an undersea earthquake.

Seismic Wave:

Seismic waves are vibrations generated by sudden movements of rock. After earthquakes occur, the seismic waves propagate from the hypocentre to the surface of the Earth. The speed at which the waves propagate is a function of the nature and type of rock traversed, but generally varies from 1 to 10 km/s. Some waves have a high enough frequency to be audible; others have a very low frequency corresponding to periods of several seconds or minutes.

Earthquakes generate two main types of waves: compressive (P) waves, and transverse (S) waves. The two types of waves travel through the interior of the Earth from the hypocentre, but only the compressive waves travel through the part of the Earth called the outer core, which is composed of molten matter.

Compressive waves travel faster; they are first to arrive at the surface. This is why they're called primary or P waves. Transverse waves do not travel as fast; they are therefore called secondary or S waves. Sometimes the first indication of an earthquake is a a sudden low sound, indicating the arrival of the P waves. Then, the S waves reach the surface and cause a more violent shaking.

Seismic Zone:

A region in which earthquakes are known to occur.

Seismicity:

The occurrence of earthquakes in space and time.

Seismogram:

Recording of ground motions made by a seismograph.

Seismograph:

A very sensitive instrument used to record and measure earthquakes. During an earthquake, vibrations initiated by fracturing of the earth's crust radiate outward from the point of fracture and are detected by seismographs. The visual record produced is called a "seismogram".

Seismologist:

A scientist who studies earthquakes, seismic sources, and wave propagation through the Earth.

See:

• What is a seismologist?
• What do scientists do after an earthquake?

Seismology:

The study of earthquakes, seismic sources, and wave propagation through the Earth.

Seismometer:

The sensor part of the seismograph, usually a suspended pendulum.

Seismometry:

The instrumental aspects of seismology.

Seismoscope:

A simple seismograph recording on a plate without time marks.

Seismotectonics:

The study of earthquakes and their relationships with faults.

Shadow zone:

The area on the Earth's surface protected from seismic wave arrivals.

Signal-to-noise ratio:

The comparison between the amplitude of the seismic signal and the amplitude of noise caused by seismic unrest and (or) the seismic instruments.

Slip (fault):

The relative motion of one face of a fault relative to the other.

Soil amplification:

Growth in the amplitude of earthquakes when seismic waves pass from rock into less rigid material such as soil.

Spread:

The layout of seismometer or geophone groups from which data from a single shot (the explosive charge) are recorded simultaneously.

Strain (elastic):

The geometrical deformation or change in shape of a body. The change in an angle, length, area, or volume divided by the original value.

Stress (drop):

The sudden reduction of stress across the fault plane during rupture.

Stress (elastic):

A measure of the forces acting on a body in units of force per unit area.

Strike of fault:

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

Strike-slip fault:

A fault whose relative displacement is purely horizontal.

Strong ground motion:

The shaking of the ground near an earthquake source made up of potentially damaging seismic waves of various types.

Subduction Zone:

A region where the earth's plates collide, with one plate sliding beneath the other. The world's largest earthquakes occur along this type of plate boundary. The Cascadia subduction zone, extending from northern California to the north end of Vancouver Island, is one such area. The subducting ocean plate is about 40 km beneath Victoria, BC, and about 70 km beneath Vancouver.

Surface Waves:

Waves that move over the surface of the Earth. Rayleigh and Love waves are surface waves.

Surface-wave magnitude: M_S

Magnitude of an earthquake estimated from measurements of the amplitude of surface waves.

Swarm: (or Earthquake swarm)

A series of minor earthquakes, none of which may be identified as the main shock, occurring in a limited area and time.

Tectonic earthquakes:

Earthquakes resulting from sudden release of energy stored by major deformation of the Earth.

Teleseism:

An earthquake that is distant (usually more than 20 degrees) from the recording station.

Thrust fault:

A reverse fault in which the upper rocks above the fault plane move up and over the lower rocks at an angle of 30 ° or less so that older strata are placed over younger.

Tidal wave:

See Tsunami.

Tomography:

Construction of the image of velocity variations inside the Earth from measurements of seismic waves at the surface.

Transform fault:

A strike-slip fault connecting the ends of an offset in a midoceanic ridge, an island arc, or an arc-ridge chain. Pairs of plates slide past each other along transform faults.

Travel time:

The time required for a wave train to travel from its source to the point of observation.

Travel-time curve:

A graph of travel time versus distance for the arrival of seismic waves from distant events. Each type of seismic wave has its own curve.

Trench:

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

Triple junction:

The point where three plates meet.

Tsunami:

(Japanese for "Harbour Wave"). A series of huge ocean waves caused by a rapid, large-scale disturbance of the sea water, such as a major earthquake beneath the seabed that causes large vertical movements. In deep water, Tsunami waves are less than a metre high, but they can travel at speeds exceeding 800 kilometres per hour and can easily cross an entire ocean basin. When they reach shallow water or narrow inlets the waves slow down and the height can build into a wall of water which causes devastation on the shore.

• More info on tsunamis

Unconsolidated:

Loosely arranged, not cemented together, so particles separate easily.

UTC:

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. In the most common usage, the terms GMT and UTC are identical.

Viscoelastic material:

A material which can behave as an elastic solid on a short time scale and as a viscous fluid on a long time scale.

Volcano:

An opening in the crust that has allowed magma to reach the surface.

Volcanism:

Geological process which involved the eruption of molten rock.

Wadati-Benioff zone:

See Benioff zone.

Wavefront:

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

Wavelength:

The distance between two successive crests or troughs of a wave.