Seismic waves: P and S waves

​​In a nutshell

Earthquakes produce seismic waves which travel through the Earth and can be detected on the surface. The two different types of seismic waves are P waves and S waves. P waves can travel through the Earth's liquid core but S waves cannot.

Seismic waves

An earthquake produces vibrations when it occurs. These vibrations form seismic waves which travel through the inside of the Earth. 

Seismologists use seismometers to detect seismic waves on the Earth's surface. They can work out the time it takes for the waves to reach the seismometer. This tells them about properties of the materials that make up the inside of the Earth.

Seismic waves can either be absorbed or refracted when they reach a boundary. Boundaries occur between different layers of materials, as they differ in densities. If the waves are refracted, they will change speed which results in a gradually curving path. A sudden change in the density of layers will result in a large change in speed and the path will have a kink.

P waves and S waves

The two types of seismic waves are P waves and S waves.

P waves

P waves are longitudinal seismic waves which can travel through solids and liquids. This means they can travel through the Earth's liquid core. They travel faster than S waves.

1. Seismic event (e.g. earthquake) 2. P wave shadow zone 3. Mantle 4. Liquid outer core 5. Solid inner core

S waves

S waves are transverse seismic waves which cannot travel through liquids (only through solids). This means they can't travel through the Earth's liquid core. They travel slower than P waves.

As S waves can't travel through Earth's core, there is a part of the Earth's crust it cannot travel to. This is known as the S wave shadow zone.

1. Seismic event (e.g. earthquake) 2. S wave shadow zone 3. Mantle 4. Liquid outer core 5. Solid inner core

When the theory was first introduced, there was assumed to be a P wave shadow zone in a certain region. However, unexpectedly a few weaker P waves were detected in this zone. This could only occur if there was a solid inner core to reflect the waves. Therefore, the P wave shadow zone was moved to a different region to account for this.