Seismic waves

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Earthquakes cause different types of waves.

When there’s an earthquake somewhere, it produces shock waves which travel out through the Earth. We record thee seismic waves all over the surface of the planet using seismographs.

Seismologists measure the time it takes for the shock waves to reach each seismographs.

They also note which parts of the world don’t receive the shock waves at all.

There are two types of seismic waves that travel through Earth – P-waves and S-waves.

P-waves are longitudinal – they travel through solids and liquids. They travel faster than S-waves.

S-waves are transverse – S- waves only travel through solids. They are slower than P-waves.

The seismograph results tells us what down there.

About halfway through the Earth, P-waves change direction abruptly.

This indicates that there’s a sudden change in properties – as you can go from the mantle to the core.

The fact that S-waves are not detected in the core’s shadow tells us that the outer core is liquid – S-waves only pass through solids.

P-waves seem to travel lightly faster through the mi9ddle of the core, which strongly suggests that there is a solid inner core.

Note that S-waves do travel through the mantle. Which shows that it’s solid. It only melts to form magma in small ‘hot spots’.

The waves curve with increasing depth.

The waves change speed as the properties of the mantle and the core change.

This change in speed causes the waves to change direction – which is refraction of course.

Mostly of the time the waves change speed gradually, resulting in a curved path.

But when the properties change suddenly, the wave speed changes abruptly, and the path has a kink.