It is currently impossible for us to explore literally what lies below the Earth's crust. Seismology provides a means of determining what might be present.

Seismology is the study of earthquakes and the different kinds of seismic waves produced by them.

Seismic waves can be divided into:

  • Body (or volume) waves travelling through the Earth:
    • P-waves (push-pull waves)
    • S-waves (transverse waves)
  • Surface waves travelling over the Earth's surface:
    • L-waves (Love waves)
    • R-waves (Rayleigh waves)

As these waves pass through the Earth, they behave differently according to the materials they are passing through. This is termed seismic refraction.

When seismic waves suddenly shift in direction and speed, we can therefore determine the materials they are passing through and the depths at which these layers are located. So, by studying the behaviour of seismic waves we can gain an understanding of the Earth's internal structure.


P-waves (primary waves) are the fastest type of seismic wave so is the first-arriving energy on a seismogram. These waves propagate through solid rock (such as granite mountains) and liquid material (such as volcanic magma or the oceans).

A p-wave is a 'push-pull' wave in the same direction as the wave is propagating. As p-waves move through rock, the rock is repeatedly compressed (pushed) and then dilated (pulled). Material returns to its original shape after the wave passes.

The effect of a p-wave is similar to a sonic boom rattling and shaking windows.

representation of a P wave


S-waves (secondary or shear waves) are slower than p-waves and so arrive after them. They propagate through solid but not liquid.

As the waves move through rock, the rock is shifted up and down and from side to side, shaking the ground horizonatally and vertically. This wave is the most damaging for structures.

They travel more slowly than P-waves so arrive after them. S-waves are a 'transverse wave'.

The motion shown above is vertical (the most common) but it can be in any direction. Material returns to its original shape after the wave passes.

The speed of P- and S- waves depends upon the density and properties of the rocks and soil that they pass through.

representation of an S wave

Love wave

Surface waves (Love and Rayleigh) are restricted to near the ground surface. This kind of wave travels more slowly than body waves and these are the waves that are most noticeable for anyone experiencing an earthquake.

Love waves are fast surface waves that are generally parallel to Earth's surface. They move the ground from side to side in a way that is particularly damaging to the foundations of structures. In contrast to S-waves, because of their vertical component, they can affect bodies of water. Material returns to its original shape after the wave passes.

Love waves are largest on Earth's surface and decrease with depth. The lower the frequency of a Love wave, the greater the depth penetration.

Love wave

Rayleigh wave

The second type of surface waves are Rayleigh waves (also known as ground roll).

These travel like ocean waves over Earth's surface, moving both vertically and horizontally. They cause most of the shaking at the ground surface during an earthquake.

These waves also decrease with depth in Earth. Material returns to its original shape after the wave passes.

S wave
representation of a P wave ×
representation of an S wave ×
representation of a Love wave ×
representation of a Rayleight wave ×









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