Earthquake waves are sound waves, and sound
waves travel at different speeds depending on what
they are traveling through. Different layers of
the Earth have different densities, and this
causes the paths of earthquake waves to bend as
they cross these boundaries just as light waves
bend when traveling through a prism. As a result,
by noticing where earthquake waves bend and where
they don't, and how long they take to reach you
depending on where you are on the Earth relative
to the earthquake, you can get an idea of what the
layers of the Earth are beneath the surface.
Earthquakes produce two kinds of waves, S-waves
and P-waves. S-waves only move through solids,
while P-waves can move through solids and liquids.
Therefore, by looking at the region where S-waves
can reach, we can see how large the liquid magma
core of the earth is.
Also, by looking at the speeds of the waves, we
can learn about density of the material, as it
affects wave speed. (Think like how underwater,
sound moves more slowly.) Here
please click to see is a good diagram of S- and
P-waves and how they move around the liquid core.
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