Earthquake Terminology, Earthquake Epicenter, Intensity Magnitude

Select the first letter of the word from the list below to jump to an appropriate section of the glossary:
A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

A

Aftershock:

An earthquake that follows a larger earthquake or main shock and originates at or near the focus of the larger earthquake. Generally, major earthquakes are followed by a larger number of aftershocks, decreasing in frequency with time.

Amplitude:

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:

Unassociated with an earthquake.

Asthenosphere:

The layer of mantle underlying the lithosphere which is close to its melting point and therefore much less rigid than the lithosphere.

B

Body wave:

A seismic wave that travels through the interior of the earth and is not related to a boundary surface.

C

Continental Crust:

Outermost solid layer of the earth that forms the continents and is composed of igneous, metamorphic, and sedimentary rocks. Overall, the continental crust is broadly granitic in composition. Contrast with oceanic crust.

Continental Drift:

The theory, first advanced by Alfred Wegener, that the earth's continents were originally one land mass called Pangaea. About 200 million years ago Pangaea split off and the pieces migrated (drifted) to form the present-day continents. The predecessor of plate tectonics.

Convergent Plate Boundary:

See subduction, and subduction zone.

Crust:

The outer layer of the earth's surface.

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D

Dilatancy:

An increase in the bulk volume of rock during deformation. Possibly related to the migration of water into microfractures or pores.

Divergent Plate Boundary:

The boundary between two crustal plates that are pulling apart (e.g. sea floor spreading).

E

Earthquake:

Shaking of the earth caused by a sudden movement of rock beneath its surface.

Earthquake swarm:

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

Elastic wave:

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

Epicenter:

That point on the earth's surface directly above the hypocenter of an earthquake.

F

Fault:

A weak point in the earth's crust where the rock layers have ruptured and slipped.

First arrival:

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

Focal zone:

The rupture zone of an earthquake. In the case of a great earthquake, the focal zone may extend several hundred kilometers in length.

Focus:

That point within the earth from which originates the first motion of an earthquake and its elastic waves.

Foreshock:

A small tremor that commonly precedes a larger earthquake or main shock by seconds to weeks and that originates at or near the focus of the larger earthquake.

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H

Harmonic Tremor:

A continuous release of seismic energy typically associated with the underground movement of magma, often preceding volcanic eruptions. It contrasts distinctly with the sudden release and rapid decrease of seismic energy associated with the more common type of earthquake caused by slippage along a fault.

Hypocenter:

The calculated location of the focus of an earthquake.

I

Igneous:

As in igneous rock. A rock formed when magma, or molten rock, cools and solidifies. If it cools slowly, the rock will have a coarse crystalline texture. If it cools quickly, it will have a fine crystalline texture. If it cools very quickly ("quenched"), it forms a glass, which has no crystalline structure. The three main types of rocks are sedimentary, igneous and metamorphic.

Intensity:

A measure of the effects of an earthquake at a particular place on humans and/or structures. 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 epicenter and the local geology at that point.

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 epicenter.

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K

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L

Leaking mode:

A surface seismic wave which is imperfectly trapped so that its energy leaks or escapes across a layer boundary causing some attenuation.

Lg Wave:

A surface wave that travels through the continental crust.

Liquefaction:

The process in which a solid (such as 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.

Lithosphere:

The rigid crust and uppermost mantle of the earth. Thickness is on the order of 62 miles (100 kilometers). Stronger than the underlying asthenosphere.

Love wave:

A major type of surface wave having a horizontal motion that is shear or transverse to the direction of propagation. 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. More commonly the "slow" layer just beneath the lithosphere.

M

Magma:

Molten rock beneath the surface of the earth. Molten rock erupted at the surface is termed "lava."

Magnitude:

A quantitative measure of the strength of an earthquake. Magnitude is calculated from ground motion as measured by seismograph and incorporates the distance of the seismograph from the earthquake epicenter so that, theoretically, the magnitude calculated for an earthquake would be the same from any seismograph station recording that earthquake. This is a logarithmic value originally defined by Wadati (1931) and 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 seem to preclude earthquakes smaller than about -1 or larger than about 9.5. A magnitude -1.0 event releases about 900 times less energy than a magnitude 1.0 quake. Except in special circumstances, earthquakes below magnitude 2.5 are not generally not felt by humans. See also Richter scale.

Major earthquake:

An earthquake having a magnitude of 7 or greater on the Richter scale.

Mantle:

The layer of rock that lies between the outer crust and the core of the earth. It is approximately 1,802 miles (2,900 kilometers) thick and is the largest of the earth's major layers.

Metamorphic:

As in metamorphic rock. A rock formed from any other type of rock by elevated temperatures and pressures, but which has not undergone complete melting. Two common examples of metamorphic rocks are slate (usually formed from shale), and marble (formed from limestone). The three main types of rocks are sedimentary, igneous and metamorphic.

Micro earthquake:

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.

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, designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects. Contrast with Richter scale, a type of magnitude scale.

Mohorovicic discontinuity:

The boundary surface or sharp seismic-velocity discontinuity that separates the earth's crust from the underlying mantle.

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N

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O

Oceanic crust:

The outermost solid layer of Earth that underlies the oceans. Composed of the igneous rocks basalt and gabbro, and therefore basaltic in composition. Contrast with continental crust.

P

P (Primary) wave:

Also called compressional or longitudinal waves, P waves are the fastest seismic waves produced by an earthquake. They oscillate the ground back and forth along the direction of wave travel, in much the same way as sound waves (which are also compressional), move the air back and forth as the waves travel from the sound source to a sound receiver.

Pangaea:

The supercontinent composed of all the present-day continents, which existed about 200 million years ago. Continental drift refers to the breakup of Pangaea into the present configuration of continents.

Phase:

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

Plate:

Pieces of crust and brittle uppermost mantle, perhaps 100 kilometers thick and hundreds or thousands of kilometers wide, that cover the earth's surface. The plates move very slowly over, or possibly with, a viscous layer in the mantle at rates of a few centimeters per year.

Plate boundary:

The place where two or more plates in the earth's crust meet.

Plate tectonics:

A widely accepted theory that relates most of the geologic features near the earth's surface to the movement and interaction of relatively thin rock plates. The theory predicts that most earthquakes occur when plates move past each other.

Q

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R

Rayleigh wave:

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.

Recurrence interval:

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

Richter magnitude scale:

The system used to measure the strength or magnitude of an earthquake. The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a collection of mathematical formulas to compare the size of earthquakes. A similar scale was developed in 1931 by Wadati, so it is more appropriate to call such scales "Wadati-Richter" scales. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included for the variation in the distance between the various seismographs and the epicenters of the earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value.

Rift system:

The oceanic ridges formed where tectonic plates are separating and new crust is being created; also refers to the on-land counterparts such as the East African Rift.

Ring of Fire:

A 40,000 kilometer (24,855 mile) band of seismicity including mountain-building, earthquakes, and volcanoes, stretching up the west coasts of South and Central America and from the North American continent to the Aleutians, Japan, China, the Philippines, Indonesia, and Australasia.

Rupture zone:

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.

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S

S (secondary or shear) wave:

A seismic body wave that involves particle motion from side to side, perpendicular to the direction of wave propagation. S-waves are slower than P-waves and cannot travel through a liquid such as water or molten rock.

Seafloor Spreading:

The mechanism by which new oceanic crust is created at oceanic ridges and slowly spreads away as the plates separate.

Sedimentary:

As in sedimentary rock. A rock made up of sediments, or rock fragments, laid down in water or deposited by wind or ice. The fragments can be microscopic, like the clays in a shale, or large, like the boulders in a coarse conglomerate. Sandstone, shale, and limestone are common sedimentary rocks. About 70% of the earth's crust is covered with sedimentary rocks (covering igneous or metamorphic rocks). The three main types of rocks are sedimentary, igneous and metamorphic.

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 75% of the world's earthquakes occur in the circum-Pacific seismic belt.

Seismic constant:

In building codes dealing with earthquake hazards, an arbitrarily-set acceleration value (in units of gravity) that a building must withstand.

Seismicity:

Earthquake activity.

Seismic sea wave:

A tsunami generated by an undersea earthquake.

Seismic zone:

A region in which earthquakes are known to occur.

Seismogram:

A written record of an earthquake, recorded by a seismograph.

Seismograph:

An instrument that records the motions of the earth, especially earthquakes.

Seismograph station:

A site at which one or more seismographs are set up and routinely monitored.

Seismologist:

A scientist who studies earthquakes.

Seismology:

The study of earthquakes and earthquake waves.

Seismometer:

The part of a seismograph which actually senses ground motion, ground velocity or ground acceleration.

Strike-slip fault:

A nearly vertical fault with side-slipping displacement.

Subduction:

The process in which one lithospheric plate collides with and is forced down under another plate and drawn back into the earth's mantle.

Subduction zone:

The zone of convergence of two tectonic plates, one of which is subducted beneath the other. 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 waves:

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

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T

Tectonic:

Pertaining to the forces involved in the deformation of the earth's crust, or the structures or features produced by such deformation.

Transform Fault:

A plate boundary where one plate slides past another; essentially a large strike-slip fault.

Tremor:

Low amplitude, continuous earthquake activity commonly associated with magma movement.

Tsunami:

One or a series of great sea waves produced by a submarine earthquake, volcanic eruption, or large landslide. (Referred to incorrectly by many as a tidal wave, but these waves have nothing to do with tides). The word tsunami is Japanese for "harbor wave."

U

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V

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W

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X

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Y

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Z

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Earthquake