Earthquake Glossary

Aftershock

An earthquake which follows a larger earthquake or main shock.

An aftershock 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.

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 timeThe 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.

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

Central Angle An angle with the vertex at the center of the Earth. A Central Angle has one ray passing through the hypocenter (and also the epicenter) and the other ray passing through the recording station. Consolidated Tightly packed. Composed of particles that are not easily separated.

CoreThe 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.

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.

Crust The thin outer layer of the Earth's surface, averaging about 10 kilometers thick under the oceans and up to about 50 kilometers thick on the continents. This is the only layer of the Earth that humans have actually seen.

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 in a limited area and time.

Elastic wave 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.

EpicenterThat point on the Earth's surface directly above the hypocenter of an earthquake.

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

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

Focal ZoneSee Rupture Zone.

Foreshock 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.

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

Hazard A risk. An object or situation that has the possibility of injury or damage.

Hypocenter The calculated location of the focus of an earthquake.

Intensity

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.

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.

Landslide Abrupt movement of soil/bedrock downhill in response to gravity. Landslides can be triggered by an earthquake or other

natural causes. Undersea landslides cause tsunamis.

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.

Leaking Mode 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.

Liquefaction

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.

You will hear this term used repeatedly.

Large Wave A surface wave which travels through the continental crust.

LongitudeThe 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 Major type of surface wave. Has a horizontal motion that is shear or transverse to the direction of propagation (travel). Named after AEH Love, the English mathematician who discovered it.

Low-Velocity Zone Any layer in the Earth where seismic wave velocities are lower than in the layers above and below.

Magnitude

An earthquake’s strength or the strain energy released by it, as determined by seismographic observations. A logarithmic value originally defined by Charles Richter (1935).

A one unit increase of magnitude (e.g., from 4.6 to 5.6) represents a 10-fold increase in wave amplitude on a seismogram or a 30-fold increase in the energy released.

For example, a 6.7 earthquake releases over 900 times (30 x 30) the energy of a 4.7 earthquake. In other words, 900 4.7 earthquakes equal the energy of one 6.7 earthquake!

Magnitude (continued)

There is no beginning or end to this scale.

However, rock mechanics seems to preclude earthquakes smaller than about (-1) or larger than about 9.5.

A (-1) event release about 900 times less energy than a 1.0 quake. Except in special circumstances, earthquakes below magnitude 2.5 are not generally not felt by humans.

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

MantleThe 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.

Microearthquake An earthquake having a magnitude of 2.0 or less on the Richter scale.

Microseism A continuous motion in the Earth that is unrelated to an earthquake and has a period of 1-9 seconds. It is caused by a variety of natural and artificial agents.

Modified Mercalli Scale

Mercalli intensity scale modified for North American conditions.

Composed of twelve increasing levels of intensity that range from imperceptible shaking to catastrophic destruction. Designated by Roman numerals.

Has no mathematical basis. 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.

P Wave Primary, longitudinal, irrotational, push, pressure, dilatational, compressional, or push-pull wave.

P waves are the fastest body waves. They can travel through all

layers of the Earth and are felt by humans as a bang or thump.

P waves carry energy through the Earth as longitudinal waves, moving particles in the same line as the direction of the wave. They arrive before the S waves, or secondary waves.

PaleomagnetismThe natural magnetic traces that reveal the intensity and direction of Earth's magnetic field in the geologic past. Another word for the study of these magnetic traces. Paleoseismology

The study of ancient (prehistoric) earthquakes.

Period The time between two successive wave crests. Phase The onset of a displacement or oscillation on a seismogram indicating the arrival of a different type of seismic wave.

Plate One of the huge sections that make up the Earth's crust. Plates are constantly moving.

Plate BoundaryWhere two or more plates in the Earth's crust meet.

Plate TectonicsThe theory that the Earth's crust and upper mantle (the lithosphere) are broken into a number of rigid, but constantly moving, segments or plates.

Rayleigh WaveA 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.

Rayleigh waves are the slowest but often the largest and most destructive wave types caused by an earthquake. They are usually felt as a rolling or rocking motion. In the case of

major earthquakes, they can be seen as they approach.

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

Reflect To bounce back from a surface.

Refract To bend or change direction.

Richter Scale

The system used to measure the strength of an earthquake. Developed by Charles Richter in 1935 as a means of categorizing local earthquakes. A collection of mathematical formulas - not a physical device.

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. In the case of a great earthquake, the rupture zone may extend several hundred kilometers in length and tens of

kilometers in width.

S WaveShear, secondary, rotational, tangential, equivoluminal, distortional, transverse, or shake wave.

These waves carry energy through the Earth in very complex patterns of transverse waves, or 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.

Seiche Free or standing wave oscillation on the surface of water in an enclosed basin. Initiated by local atmospheric changes, tidal currents, or earthquakes. Similar to water sloshing in a bathtub.

Seismic BeltElongated earthquake zone. For example, Circum-Pacific, Mediterranean, Rocky Mts. About 60% 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 Of or having to do with earthquakes.

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.

Seismometry The instrumental aspects of seismology.

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.

Spreading Center An elongated region where two plates are being pulled away from each other.

New crust is formed as molten rock, then forced upward into the gap. Examples of spreading centers include the Mid-Atlantic Ridge and the East African Rift.

Subduction When one lithospheric plate collides with and is forced down under another plate, then drawn back into the Earth's mantle.

Subduction ZoneAn elongated region where a plate descends relative to another plate, for example, the descent of the Nazca plate beneath the So. Amer. plate along the Peru-Chile Trench.

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

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

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

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

UTCCoordinated Universal Time.

The time scale based on the atomic second but corrected - usually on New Year's Eve - to keep it in approximate sync with the earth's rotation.

The corrections show up as leap seconds put into UTC. In the most common usage, the terms GMT and UTC are identical. For more information on time, see Time Information Links.

Glossary Provided Courtesy of United States Geological Survey

National Earthquake Information Center – Public Information