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What are P Waves and S Waves?

The two types of wave which travel through the planet from
an earthquake are called P waves and S waves.
S waves are transverse. P waves are longitudinal.
P waves travel faster than S waves.

In the picture below
P waves are shown in blue and S waves are shown in red.
The earthquake has occurred on the left side of the planet,
the waves are moving from left to right.

Earthquake Waves

How do P Waves and S Waves
give Information about the Structure of the Earth?

The transverse S waves cannot travel through the liquid outer core.
They can travel through the mantle because the mantle behaves
more like a solid than a liquid. The S waves curve as they move
through the mantle due to refraction as the density of the mantle
. There is a large part of the surface of the planet where
no S waves are detected. This shows that the outer core is liquid
because it blocks S waves. It also shows how big the outer core is.

The longitudinal P waves can travel through the whole planet.
They also curve with the changing density of both the mantle
and the core (except the wave passing through the centre,
which travels in a straight line, normal to the boundary).
The P waves change direction suddenly at the boundary
between the different layers of the Earth. This is due to
refraction caused by the different densities of the layers.
The P waves show how big the solid inner core is.

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