What are earthquakes?
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09-10-2010, 04:17 PM



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What are earthquakes?

What are earthquakes?
Earthquakes are powerful manifestations of sudden releases of strain energy accumulated within the crust and propagated as seismic wave
Seismograph
Seismograph is the instrument for recording motions of the earth’s surface caused by seismic waves, as a function of time. The simplest earthquake recording system consists of a sensor and an analog or digital recorder. The record is known as a seismogram. Location and magnitude of an earthquake are calculated from seismograms.
Seismic waves
In earthquake studies, we generally deal with P, S and surface waves. P wave is the primary or the fastest wave travelling away from an earthquake source, consisting of a train of compressions and dilatations parallel to the direction of travel of the wave. S wave is the secondary wave, travelling more slowly than the P wave and consisting of elastic vibrations transverse to the direction of travel. Earthquakes also generate surface waves that follow the Earth’s surface only, with a speed less than the S waves. Propagation of surface waves also causes considerable damage during an earthquake. The difference in arrival time between the P wave and the S wave can suggest the approximate distance from the epicentre. From records of earthquakes at several places, the epicentre can be determined by reading the time differences.
Intensity
Intensity is description of the effects of an earthquake at a particular place, based on observations of damage, using a descriptive scale like the Modified Mercalli Scale. A map showing intensities at individual locations may be contoured based on isoseismals, which are lines of equal intensity. An isoseismal map provides a representation of broad variations of shaking over the region surrounding the earthquake.
Magnitude
Magnitude is a measure of the size of the earthquake, calculated from the amplitude of the seismic waves and is closely related to the energy released by the earthquake. If the magnitude increases by 1, then the energy is about 30 times larger; if it increases by 2, then the energy is about 900 times. Richter magnitude, surface-wave and body-wave magnitudes are commonly used to indicate this measure. Duration or coda- magnitude based on the duration of the seismic signal is also in use..
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