Seismology is a study of the motions of the Earth's crust.
Earthquakes are the most well known of these motions, but there are
places where the crust is in motion all the time. Plate tectonics
involves seismology, because the motion of plates relative to each
other is what causes many earthquakes. A seismograph is a device
for measuring such motions, which consists of a weight suspended
from a point, and a rotating drum. The weight has a marking pen, or
stylus, attached to it, which draws a line an the drum. As the drum
turns, it raises or lowers, so that the trace does not overlap.
When an earthquake hits, the suspended weight does not move very
much, because of its inertia, so the trace on the drum, which is
moving up and down from the waves of the earthquake, shows peaks
and valleys. Interpreting these traces, to determine the amplitude
of the earthquake, is what the science of seismology is
specifically. Seismographs are so sensitive that some can measure
earthquakes on the other side of the globe, explosions, and
Seismographs are one of the primary tools of telling what is
happening in the ground below us, and in trying to predict when an
earthquake will occur. They are also used in monitoring volcanoes,
which create motions detected by the seismographs such as harmonic
tremors, which is when magma is moving under the ground.
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