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Glossary of Terms Used in Seismology (Adapted from USGS files)

Aftershock An earthquake which 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 a seismic wave on the seismic record.
Arrival time The time at which a particular wave phase arrives at a detector.
Aseismic area An area that is almost free of earthquakes.
Body wave A seismic wave that travels through the interior of the earth and is not related to a boundary surface.
Crust The outer layer of the Earth's surface.
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 Rock is an elastic material that when strained by normal external forces can return to its original state. When the strength of the rock is exceeded, the rock ruptures, generating elastic seismic or earthquake waves.
EMS The currently used 12-grade European macroseismic scale (EMS-92) is the updated version of the MSK-scale intensity scale.
Epicentre That point on the Earth's surface directly above the hypocentre of an earthquake.
Fault A weak area in the Earth's crust where two sides of a fracture or fracture zone move relative to each other.
First arrival The first recorded signal on a seismogram is the direction of the first P-wave, where upward ground motion is compressional and downward motion is dilatational.
Focus The point where earthquake rupture or fault movement originates.
Foreshock A small earthquake that may precede a larger earthquake or main shock and that originates at or near the focus of the larger event.
Frequency The frequency of a wave (Hz) is the number of wave cycles per second.
Hypocentre The calculated location of the focus of an earthquake.
Induced seismicity Non-natural events induced by man's activity. These include mining induced events, events caused by loading of dams or pumping of water in geothermal areas.
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 epicentre and the local geology at that point.
Isoseismal line A line enclosing points on the Earth's surface at which earthquake intensity is the same. It is usually elliptical in shape
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.
Magnitude A measure of the strength of an earthquake. There are several scales depending on which part of the seismogram is examined. These include Richter local magnitude (ML), Body wave magnitude (mb) and surface wave magnitude (Ms). Moment magnitude (Mw) is calculated from spectral analysis.
Mantle The layer that lies between the crust and the core of the earth.
Microseism A motion in the Earth that is unrelated to an earthquake. It is caused by a variety of natural and artificial agents, for example wave action, wind, traffic and industrial noise.
MSK MSK intensity is the intensity scale used in Europe before the introduction of the EMS scale. It is a 12-grade scale ranging from not felt to complete devastation.
P wave The first and faster of the body waves which moves by a series of compressions and dilatations, similar to a sound wave. They can travel through both solid and liquid.
Phase The onset of a displacement on a seismogram indicating the arrival of the different types of seismic wave.
Plate One of the segments which make up the Earth's crust. The plates are continuously moving relative to each other.
Plate boundary The place where two or more plates in the Earth's crust meet.
Prediction Predicting the time, place and magnitude of an earthquake.
Rayleigh wave A type of surface wave having a retrograde, elliptical motion at the free surface. It is named after Lord Rayleigh, the English physicist who predicted its existence.
Reflected wave A wave that has turned back from a boundary or discontinuity in the earth's crust.
Refraction The change in direction of a wave on reaching a boundary of different density and velocity.
Richter scale A popular name for the local magnitude scale (See Magnitude).
S wave The second arrival on a seismogram, the S wave, is slower than the P-wave. It is a shear wave and cannot travel through liquids.
Seismogram A record of an earthquake or ground vibration. The wave trace is made up of P-waves, S-waves and surface waves, the pattern of onsets of the first two arrivals help to determine the location. The seismogram can be either a paper record or a digital record that is analysed by computer.
Seismograph An instrument that registers the occurrence of an earthquake and the time it occurred as a written record.
Seismologist A scientist who studies earthquakes.
Seismometer An instrument that not only measures the time of the arrival of earthquake waves, but also allows the exact motion of the ground to be computed from the record.
Seismoscope An instrument that registers the occurrence of an earthquake, but not the time.
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.
Subduction zone An elongated region along which a crustal plate descends relative to another crustal block, for example, the descent of the Pacific plate beneath the Andean plate along the Andean trench.
Surface waves Seismic waves with motion restricted to near the ground surface (Love and Rayleigh)
Teleseism An earthquake that is distant from the recording station.
Travel time The time required for a wave train to travel from its source to a point of observation.
Tsunamis A huge sea wave caused by earthquakes. (Referred to by many as a tidal wave.)
Volcanic earthquake Earthquakes associated with volcanic activity.
Wavelength The distance between two successive crests or troughs of a wave.

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