The graphical display of recorded ground motions is referred to as "seismogram". Such a record contains information about the source, hypocentre and the released energy which are analysed by seismologists. Different kinds of sources, such as explosions, supersonic beams, mining collapses and landslides can be distinguished from tectonic earthquakes by specific waveforms they generate.
The first arrival of a seismic wave is termed "P-wave" (primary wave), which is the result of the compressional wave which propagates much faster than all other elastic waves at a speed of approximately 6000 m/s through the Earth. A much more stronger amplitude is usually generated by the second arrival - the shear wave (S-wave), which arrives later due to the lower velocity of propagation (approx. 3400 m/s). The different velocities are a result of the different responses of the material of the Earth's interior which depends on whether the material is deformed in direction or perpendicular to the path of the wave.
Wave paths of elastic waves emitted by strong earthquakes at great distances penetrate deep into the body of the Earth and may even intersect the Earth's core. Such waves have been coined "body waves".
From the arrival times of all these waves, the distance from the hypocenter (that is the actual source below the epicenter, where most effects are usually observed) can be determined. For local earthquakes, the time difference between the arrival of the S-wave and the P-wave (in seconds) can by multiplied by 8.2 to estimate the distance. Hence, a S-P-time-difference of 5 seconds indicates a hypocentral distance of 41 km.
Another onset in a seismogram results from the arrival of the surface wave. This wave takes even longer for it travels along the surface and does not take the faster short-cut through the Earth like body waves. The amplitude of surface waves depends on the focal depth of the source. Deeper earthquakes, which tend to occur along subduction zones like the "ring of fire" in the Pacific down to a depth of 700 km, generate much less surface waves than shallow earthquakes which are typical for e.g. the Alpine region in Austria.
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EMS-98 |
see Intensity Scale. |
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Earthquake
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Shaking of the ground. Often used as combined term for the whole earthquake event (epicenter, time, magnitude). Causes can be natural or induced. Most earthquakes result from sudden movements of the Earth´s crust. |
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Earthquake hazard
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Probability of occurrence of ground shaking due to a tectonic earthquake within a certain period of time. Gives no information on possible damage (see earthquake risk). |
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Earthquake risk
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Probability of damage ( = loss * earthquake hazard). |
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Epicenter
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Location of the most severe damage on the earth surface above the hypocenter. Derived from Greek "epi" meaning above. |
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Focal depth
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Depth of the hypocenter. |
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Hypocenter
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Location of the earthquake in the interior of the earth, below the epicenter, where the actual rupture process takes place. Derived from Greek "hypo" meaning below. |
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Intensity
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Damage through and perception of an earthquake on the earth surface. Measured on the basis of an intensity scale.
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Intensity Scale
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12-degree scale by which damage and perception of an earthquake is classified. The scale used in Europe today is the European Macroseismic Scale of 1998 (EMS-1998), based on the former Mercalli-Sieberg Scale. |
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Induced earthquake
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Man-made earthquake. Triggered or caused, amongst other reasons, by mining, water reservoirs building, nuclear tests, blasts, collapse of man-made structures and supersonic booms. |
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Isoseismal
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Line connecting points of equal intensity on a map. |
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Magnitude
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The logarithm of the energy released at the hypocenter as introduced by Richter, thus often referred to as Richter Scale. It should be noted that the energy release of an earthquake of magnitude 7 is 30 times larger than that of magnitude 6 and 1000 times larger than that of
magnitude 5.
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Ms
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Magnitude based on recorded surface waves generated by distant earthquakes. Deep earthquakes (e.g. 700 km) produce less surface waves than comparable shallow events thus cause no or little damage when compared with earthquakes at shallow depth (e.g. 10 km). |
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ML
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Magnitude determined from amplitude of shear wave (< 1000 km epicentral distance) |
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mb
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Magnitude based on the amplitude of the first arrival of body waves (P-waves) for distant earthquakes. The wave paths dive deep into the Earth's body. |
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Macroseismology
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Expert field for evaluation and interpretation of damage and perception reports. |
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Richter Scale
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see Magnitude |
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Seaquake
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Earthquake with its hypocenter beneath the ocean. |
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Seismogram
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Graphical display of recorded ground motions. |
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Seismograph
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Instrument for the enlarged presentation of the movement of the ground, measured by the seismometer. In earlier days when there was no digital data transfer, the seismograph was installed at the place of the seismometer or both instruments were combined to a unit. The term seismograph has changed since. While in former times, the vibration had to be enlarged by a mechanical system before being recorded, the measured signals are now automatically digitised and then sent to a computer for final analysis and storage. |
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Seismometer
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Instrument for measuring movements of the ground. The center piece of a seismic observation station. |
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Seismologist
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Expert in the field of seismology. |
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Seismology
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Science of earthquakes, derived from Greek "seismo" meaning tremor. |
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Tectonics
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Study of the Earth´s crustal structure and of the forces responsible for its deformation, derived from Greek "tektonikos" meaning builder. |
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Tsunami
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Great sea wave produced by a seaquake, a Japanese term. |
