- 05/17/2019 at 5:35 am #874908EduGorillaKeymasterSelect Question Language :
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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