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     EduGorilla 
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    Seismology is the study of earthquakes and seismic waves. The seismograph records the seismic waves generated by earthquakes, allowing the seismologist to determine where, and how deep, a particular earthquake is. Also, the seismic waves from earthquakes can be used to image the deep interior of the Earth, providing vital clues to the internal structure of our planet. The outer shell of the Earth, or lithosphere, is made up of a number of rigid segments called tectonic plates. These plates are continually moving at rates of a few centimetres per year (about as fast as your fingernails grow), driven by forces deep within the Earth.
    Below the lithospheric plates, lies the Earth’s asthenosphere. The asthenosphere behaves like a fluid over very long time scales, allowing it to convect. Convection acts like giant conveyor belts, moving the overlying plates around. At the boundaries between the plates, where they are moving together, apart or past each other, tremendous stresses build up, and are where most earthquakes occur. Movements within the
    Earth’s crust cause stress to build up at points of
    weakness, and rocks to deform. Stored energy builds
    up in the same way as energy builds up in the spring of
    a watch when it is wound. When the stress finally
    exceeds the strength of the rock, the rock fractures
    along a fault, often at a zone of existing weakness
    within the rock. The stored energy is suddenly released
    as an earthquake. Intense vibrations, or seismic waves,
    spread out from the initial point of rupture, the focus,
    like ripples on a pond.

    A seismogram is a record of the seismic waves from an
    earthquake. A seismograph or seismometer is the
    measuring instrument that creates the seismogram.
    Almost all seismometers are based on the principle of
    inertia: a suspended mass tends to remain still when
    the ground moves. The relative motion between the
    suspended mass and the ground will then be a
    measure of the ground’s motion. On a seismogram
    from an earthquake, the P-wave is the first signal to
    arrive, followed by the slower S-wave, then the surface
    waves. The arrival times of the P- and S-waves at
    different seismographs are used to determine the
    location of the earthquake. Given that we know the
    relative speed of P- and S-waves, the time difference
    between the arrivals of the P- and S-waves determines
    the distance the earthquake is from the seismograph. 

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