A type of seismograph used to measure ground acceleration as a function of time.
A fault along which a landslide has occurred in recent geological time, or where earthquakes are located.
Continental margin characterized by earthquakes and volcanic activity (ie, location of transform fault or subduction zone).
An earthquake that occurs after a "mainshock" (or major earthquake). Aftershocks occur in the same general region as the "major tremor" and are the result of voltage readjustments at locations along the fault zone. For large earthquakes (M=8), aftershocks can occur for hundreds of kilometers. Depending on the size and depth of the earthquake, aftershocks can occur for many months after the main earthquake; however, the aftershock activity rate quickly disappears over time.
The maximum height of a wave crest or the depth of a trough.
An ordered array of seismometers or geophones, whose data is fed to a central receiver.
The appearance of seismic energy in a seismic record.
The time when a given wave phase arrives at a detector.
Not associated with an earthquake, as in a seismic landslide. It is also used to indicate an area with no earthquake record: a seismic zone.
Roughness on the fault surface that presents local resistance to sliding.
The layer below the lithosphere that is marked by low seismic wave velocities and high seismic wave attenuation.
The reduction in amplitude of a wave with time or distance traveled.
Auxiliary failure plan:
A plane orthogonal to the fault plane.
A fault surface area that is resistant to slippage due to geometric or structural changes.
A narrow zone, defined by seismic pockets, tens of kilometers thick and that plunges from the surface under the Earth's crust to depths of up to 700 kilometers. (AlsoZona Wadati-Benioff.)
A deep crustal thrust fault without superficial or indirect expression, such as a folded structure.
Seismic wave that can propagate through the interior of the Earth. P waves and S waves are body waves.
Body Wave Magnitude:
Magnitude of an earthquake estimated from the amplitude of body waves.
A fault along which sudden slippage is mechanically feasible.
An earthquake with typical size and generation mechanism for a specific fault source.
The final train of seismic waves that follows the main waves of an earthquake.
crushed. Composed of particles that do not separate easily.
The theory, first put forward by Alfred Wegener, that the Earth's continents were originally one land mass. Pieces of the landmass broke apart and migrated to form the continents.
Part of the continental margin between the coast and the continental slope.
The innermost layers of the Earth. The inner core is solid and has a radius of about 1,300 kilometers. (Earth's radius is about 6,371 kilometers.) The outer core is fluid and about 2,300 kilometers thick. S waves cannot travel through the outer core.
Loss of energy in wave motion due to heat transfer by frictional forces.
The mass per unit volume of a substance, commonly expressed in grams per cubic centimeter.
Depth of an earthquake:
The value given is the depth below the surface of the mean spheroid.
Expansion (of rocks):
The increase in rock volume is mainly due to widespread microcracks.
The angle by which a rock layer or fault plane deviates from the horizontal. The angle is measured in a plane perpendicular to the course.
Dive Slip Fault:
A fault on which the relative displacement is along the dip of the fault plane; the displacement is normal or reverse.
The scattering of a wave train due to the fact that each wavelength travels at its own speed.
Duration (of strong tremors):
Time interval between the first and last peak of strong ground movement above a specified amplitude.
The sudden release of stored elastic energy caused by the sudden fracture and movement of rocks along a fault. Part of the energy released is in the form of seismic waves, which make the ground shake.
Interval of occurrence (recurrence) of earthquakes:
The average time interval between the occurrence of earthquakes in a given region.
A series of smaller earthquakes, none of which can be identified as the main earthquake, occurring in a limited area and time.
The layer of rock located just below the Earth's surface. Below the continents, it is typically about 35 km thick and is composed of granite. Beneath the ocean, the crust is about 5 to 10 kilometers thick and is mostly composed of basalt.
Wave that propagates 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.
The point on the Earth's surface directly above the focus (hypocenter) of an earthquake.
A zone of fractures or breaks in rocks where movement occurs. Earthquakes usually occur along fault lines because they are weak zones in the rock.
The plane that comes closest to the failure surface of a fault.
The first recorded signal attributed to the propagation of seismic waves from a source.
More or less symmetrical implantations on sub-faults close to the intersection of the main fault and the ground surface.
An earthquake that is minor and precedes a "mainshock". Pre-shocks tend to occur in the same area as the main shock. No previous tremors were observed before the damaging earthquakes in British Columbia.
Number of oscillations per unit of time; The unit is Hertz (Hz), which is equal to 1 cycle per second.
In one fault zone, crushed, cut and pulverized rock was transformed into clay.
A block of crustal rock, usually long and narrow, that has descended along faults bordering adjacent rocks.
Earthquake of magnitude 8 or greater on the Richter scale.
Discontinuity in seismic velocity that marks the boundary between the core and the mantle; named after the seismologist Beno Gutenberg.
A situation that has the possibility of occurring.
The unit of frequency equals 1 cycle per second, or 2 PI radians per second.
The current geological period that began about 10,000 years ago.
The location below the surface (focus) where energy from an earthquake is released. Earthquakes usually occur at depths less than about 30 km, but can occur at depths of 600 km or more in some areas.
An index of the resistance of an elastic body, such as a rock, to change in volume.
Solid central region of Earth's core, probably mostly iron; radius of about 1221 kilometers, discovered by Inge Lehmann in 1936.
LosModified Mercalli scaleis a numerical scale used to classify earthquakes based on descriptions of how the earthquake felt. These effects can range from I (not felt, except for some under especially favorable conditions) to XII (full damage).
Earthquake between plates:
Earthquake with focus on the boundary of a plate. Offshore earthquakes in western Canada are of this type.
Earthquake with focus inside a tectonic plate. Earthquakes in eastern Canada are of this type.
Chain of islands above a subduction zone (eg Japan, Aleutians).
A line connecting points on the Earth's surface where the earthquake intensity is the same. It is usually a tight curve around the epicenter.
The way the lithosphere 'floats' in the asthenosphere.
An abrupt downward movement of geological materials in response to gravity. Landslides can be triggered by an earthquake or other natural causes. Undersea landslides can cause tsunamis, such as the one caused by the Grand Banks earthquake in 1929.
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 side fault: (sinistral)
A bearing fault in which the offset of the far block is to the left when viewed from either side.
The process in which a granular solid (soil) acquires the characteristics of a liquid as a result of an increase in pore pressure and a reduction in stress. In other words, solid earth loses cohesion and begins to flow like a liquid.
Physical character of the rocks.
The rigid outer layer of Earth above the asthenosphere. Contains the crust, continents and plates.
Surface wave propagating through the continental crust. This type of wave is what causes damage during large earthquakes in eastern Canada.
The location of a point east or west of the prime meridian. Longitude is shown on a map or globe as north-south lines to the left and right of the prime meridian, which runs through Greenwich, England.
A major type of surface wave that has a horizontal motion that is shearing or transverse to the direction of propagation (path). It bears the name of A.E.H. Love, the English mathematician who discovered it.
Low speed zone:
Any layer of the Earth in which seismic wave velocities are less than those in the layers above and below.
Magnitude is a measure of the amount of energy released during an earthquake. It can be expressed using the Richter scale.
- What is the "magnitude" of an earthquake?
- What is the difference between ML and mN magnitudes?
- Certain earthquakes have 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 begin to be observed?
- Are there multiple scales of magnitude?
The largest earthquake in a "group" of earthquakes. Major earthquakes are sometimes preceded by "presismo" and, in general, followed by aftershocks.
An earthquake that has a magnitude of 7 to 7.99 on the Richter scale.
Cloak (from Earth):
Most of Earth, between the crust and core, ranges from depths of about 40 to 3,470 kilometers. It consists of dense siliceous rocks and is divided into several concentric layers. In eastern Canada, it can be found about 40 km deep.
The area of strong tremor and significant damage in an earthquake.
An earthquake that has a magnitude of 2 or less on the Richter scale.
A more or less continuous motion on Earth that is not related 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.
The division of a city or county into smaller areas according to its variation in seismic risk.
An earthquake that has a magnitude of 5 to 6 on the Richter scale.
Modified Mercalli intensity scale:
The Mercalli scale rates the tremor intensity of an earthquake. Ratings range from I (perceived only in especially favorable circumstances) to XII (total destruction).
Modified Mercalli Intensity Scale (MM)
Mohorovicic discontinuity (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 after Andrija Mohorovicic, the Croatian seismologist who first suggested its existence. In eastern Canada, it can be found about 40 km deep.
Moment (two earthquakes):
A measure of earthquake size related to the leverage of forces (torques) in the fault slip area. Rock stiffness times fault area times amount of landslide. Dimensions are dyne-cm (or Newton-meters).
Magnitude do momento (MC):
Magnitude of an earthquake estimated using seismic moment.
A dip-slip fault in which the rock above the fault plane has moved downward relative to the rock below.
Fault slip has components both along the dip and along the fault direction.
The exact time when an earthquake occurred.
Liquid outer layer of Earth's core, probably iron with some oxygen; inner radius, 1221 kilometers, outer radius, 3480 kilometers.
Also called primary, longitudinal, irrotational, impulse, pressure, expansion, compression or push-pull waves. P waves are the fastest body waves and arrive at stations before S waves or secondary waves. Its velocity in the crust varies between 5.0 and 7.0 km/s. Waves carry energy through the Earth as longitudinal waves, moving particles in line with the direction of the wave. P waves can travel through all layers of the Earth. Humans usually experience P waves as an explosion or a thump.
The study of ancient (prehistoric) earthquakes based on their geological evidence.
Continental margin formed during the initial separation of continents to form an ocean; often has thick sedimentary deposits.
The time interval between successive crests in a sine wave train; the period is the inverse of the frequency of a cyclic event.
The beginning of a change or wobble in a seismogram, indicating the arrival of a different type of seismic wave.
Plates and tectonic plates:
The Earth's crust and upper mantle are made up of a dozen large plates and several smaller plates that are in constant motion. Movements are very slow, just a few centimeters per year. Where the boards rub against each other, stress increases, especially at the edges. When the strength of the rock is overcome, the Earth's crust can suddenly rupture and shift several meters, causing an earthquake.
A change in geological or geophysical conditions that is a precursor to the generation of earthquakes on a fault. Precursors cannot be reliably recognized as such in advance.
Forecast (of earthquakes):
The prediction in time, place and magnitude of an earthquake; anticipation of strong ground movements. Currently, there is no reliable method for predicting earthquakes.
The number of cases meeting a given description divided by the total number of possible (equally probable) cases.
Probability of exceeding a given earthquake size:
The probabilities that the size of a future earthquake will exceed a specified value.
The geological period spanning from about 2 million years ago to the present.
A type of surface wave that has a retrograde elliptical motion on the Earth's surface. These are the slowest but usually the biggest and most destructive types of waves caused by an earthquake. They are usually felt as a rocking or rocking motion and, in the case of large earthquakes, can be seen as they approach. Named after Lord Rayleigh, the English physicist who predicted its existence.
The approximate time between earthquakes in a specific seismically active area.
Bouncing off a surface representing an acoustic impedance contrast.
Bending or changing direction due to an acoustic impedance contrast.
The rock above the fault plane (the "hanging" wall) moves up and over the rock below (the "footing" wall).
Region where the crust has separated, usually marked by a rift valley (eg, East African Rift, Rhine Graben).
Right Side Fault: (dextral)
A rolling fault in which the displacement of the far block is to the right when viewed from either side. The Queen Charlotte glitch is one such glitch.
An index of the resistance of an elastic body to shear. The relationship between shear stress and the amount of angular rotation it produces in a rock sample.
Probabilistic risk is the probability that an earthquake will occur and cause damage within a given time range and region.
Rossi-Forel intensity scale:
The Rossi-Forel scale is a measure of the tremor intensity of an earthquake. This scale was replaced by the Mercalli intensity scale.
- Rossi-Forel intensity scale from 1883
The rise in water level above the immediate tide level when a tsunami hits the coast.
The area of the Earth through which faults occurred during an earthquake. For very small earthquakes, this zone can be a few millimeters long, but in the case of a large earthquake, the rupture zone can extend to several hundred kilometers in length and tens of kilometers in width.
Also called shear, secondary, rotational, tangential, equiluminous, distorted, transverse, or jolt waves. These waves carry energy through the Earth in very complex transverse (cross) wave patterns. These waves move more slowly than P waves, but in an earthquake they are usually larger. S waves cannot travel through the outer core because these waves cannot exist in fluids such as air, water or molten rock.
A narrow geological depression found in strike fault zones. Those that contain water are called underwater ponds.
A steep cliff or slope formed by displacement of the ground surface.
A free or standing wave oscillation of the water surface in a closed basin, initiated by local atmospheric changes, tidal currents or earthquakes. Similar to splashing water in a bathtub.
Of or pertaining to earthquakes.
An elongated earthquake zone, eg circum-Pacific, Mediterranean, Rocky Mountains. About 60% of the world's earthquakes occur in the earthquake belt around the Pacific.
A surface or thin layer within the Earth through which P-wave and/or S-wave velocities change rapidly.
An area in an earthquake-prone region where there is a below-average release of seismic energy.
verMoment (of earthquakes).
Sea seismic wave:
A tsunami (see below) generated by an underwater earthquake.
Seismic waves are vibrations generated by sudden movements of rocks. After earthquakes occur, seismic waves propagate from the hypocenter to the Earth's surface. The speed with which the waves propagate is a function of the nature and type of rock crossed, but generally varies from 1 to 10 km/s. Some waves are high enough in 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: compression waves (P) and transverse waves (S). Both types of waves travel through the Earth's interior from the hypocenter, but only compression waves travel through the part of the Earth called the outer core, which is made of molten matter.
Compression waves travel faster; They are the first to reach the surface. That's why they're called primary waves or P waves. Transverse waves don't travel as fast; therefore, they are called secondary waves or S waves. Sometimes the first indication of an earthquake is a sudden low sound, signaling the arrival of P waves. The S waves then strike the surface, causing a more violent jolt.
A region where earthquakes are known to occur.
The occurrence of earthquakes in space and time.
Recording of earth movements made by a seismograph.
A very sensitive instrument used to record and measure earthquakes. During an earthquake, vibrations initiated by fracture of the Earth's crust radiate outward from the fracture point and are detected by seismographs. The visual record produced is called a "seismogram".
A scientist who studies earthquakes, seismic sources, and the propagation of waves through the Earth.
- What is a seismologist?
- What do scientists do after an earthquake?
The study of earthquakes, seismic sources, and wave propagation through the Earth.
The sensor part of the seismograph, usually a suspended pendulum.
The instrumental aspects of seismology.
A simple seismographic recording on a plate without time stamps.
The study of earthquakes and their relationships with faults.
The area on the Earth's surface protected from incoming seismic waves.
The signal to noise ratio:
The comparison between the amplitude of the seismic signal and the amplitude of the noise caused by the seismic disturbances and (or) by the seismic instruments.
The relative movement of one fault face relative to the other.
Increased amplitude of earthquakes when seismic waves pass from rock to less rigid material such as soil.
The arrangement of groups of seismometers or geophones from which data from a single shot (the explosive charge) is recorded simultaneously.
The geometric deformation or change of shape of a body. The change in an angle, length, area, or volume divided by the original value.
The sudden reduction of stress in the fault plane during failure.
A measure of the forces acting on a body in units of force per unit area.
The line of intersection between the fault plane and the Earth's surface. Its orientation is expressed as the west or east angle of true north.
A fault whose relative displacement is purely horizontal.
Strong ground movement:
Ground shaking near a seismic source comprises potentially harmful seismic waves of various types.
A region where Earth's plates collide, with one plate sliding under the other. The world's largest earthquakes occur along this type of plate boundary. The Cascadia subduction zone, which stretches from northern California to the northern tip of Vancouver Island, is one such area. The subducting oceanic plate lies about 40 km below Victoria, BC, and about 70 km below Vancouver.
Waves moving on the Earth's surface. Rayleigh and Love waves are surface waves.
Surface wave magnitude: MS
Magnitude of an earthquake estimated from surface wave amplitude measurements.
Swarm: (or earthquake swarm)
A series of smaller earthquakes, none of which can be identified as the main earthquake, occurring in a limited area and time.
Earthquakes resulting from the sudden release of stored energy by a large deformation of the Earth.
An earthquake far away (usually more than 20 degrees) from the recording station.
A reverse fault in which the upper rocks on the fault plane move up and over the lower rocks at an angle of 30° or less, so that older strata are placed on top of younger ones.
Construction of the image of velocity variations inside the Earth from measurements of seismic waves on the surface.
An attack fault connecting the ends of an offset in a mid-ocean ridge, island arc, or chain of arcs and ridges. Pairs of plates slide past each other along transform faults.
Time of travel:
The time required for a wave train to travel from its origin to the observation point.
Travel time curve:
A plot of travel time versus distance for incoming seismic waves from distant events. Each type of seismic wave has its own curve.
A long, narrow arched depression in the seafloor that results from the curvature of the lithospheric plate as it descends into the mantle in a subduction zone.
The point where three plates meet.
(Japanese for "Harbour Wave"). A series of huge ocean waves caused by rapid, large-scale disturbance of sea water, such as a large earthquake below the seafloor causing large vertical movements. In deep water, tsunami waves are less than a meter high, but they can travel at speeds of over 800 kilometers per hour and can easily traverse an entire ocean basin. When they reach shallow water or narrow coves, the waves subside and the height can form a wall of water that wreaks havoc on the coast.
- More information about tsunamis
Loosely arranged, not cemented together, so the particles separate easily.
Coordinated Universal Time🇧🇷 The time scale is based on the atomic second, but is corrected from time to time to keep it approximately synchronized with the Earth's rotation. Fixes appear as leap seconds set to UTC, usually on New Year's Eve. In most common usage, the terms GMT and UTC are identical.
A material that can behave as an elastic solid on a short time scale and as a viscous fluid on a long time scale.
An opening in the crust that allowed magma to reach the surface.
Geological process involving the eruption of molten rock.
verZone de Benioff.
An imaginary surface or line joining points where waves from a source are in phase (for example, all at maximum or all at minimum).
The distance between two successive crests or troughs of a wave.
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