When an earthquake occurs, the shockwaves of released energy that shake the Earth and temporarily turn soft deposits, such as clay, into jelly (liquefaction) are called seismic waves, from the Greek ‘seismos’ meaning ‘earthquake’. Seismic waves are usually generated by movements of the Earth’s tectonic plates but may also be caused by explosions, volcanoes and landslides.
Seismologists use seismographs to record the amount of time it takes seismic waves to travel through different layers of the Earth. As the waves travel through different densities and stiffness, the waves can be refracted and reflected. Because of the different behaviour of waves in different materials, seismologists can deduce the type of material the waves are travelling through.
The results can provide a snapshot of the Earth’s internal structure and help us to locate and understand fault planes and the stresses and strains acting on them.
This wave behaviour can also be used on a smaller scale by recording waves generated by explosions or ground vibrators in the search for oil and gas.
Types of seismic waves
There are three basic types of seismic waves – P-waves, S-waves and surface waves. P-waves and S-waves are sometimes collectively called body waves.
P-waves, also known as primary waves or pressure waves, travel at the greatest velocity through the Earth. When they travel through air, they take the form of sound waves – they travel at the speed of sound (330 ms-1) through air but may travel at 5000 ms-1 in granite. Because of their speed, they are the first waves to be recorded by a seismograph during an earthquake.
They differ from S-waves in that they propagate through a material by alternately compressing and expanding the medium, where particle motion is parallel to the direction of wave propagation – this is rather like a slinky that is partially stretched and laid flat and its coils are compressed at one end and then released.
S-waves, also known as secondary waves, shear waves or shaking waves, are transverse waves that travel slower than P-waves. In this case, particle motion is perpendicular to the direction of wave propagation. Again, imagine a slinky partially stretched, except this time, lift a section and then release it, a transverse wave will travel along the length of the slinky.
S-waves cannot travel through air or water but are more destructive than P-waves because of their larger amplitudes
Surface waves are similar in nature to water waves and travel just under the Earth’s surface. They are typically generated when the source of the earthquake is close to the Earth’s surface. Although surface waves travel more slowly than S-waves, they can be much larger in amplitude and can be the most destructive type of seismic wave. There are two basic kinds of surface waves:
- Rayleigh waves, also called ground roll, travel as ripples similar to those on the surface of water. People have claimed to have observed Rayleigh waves during an earthquake in open spaces, such as parking lots where the cars move up and down with the waves.
- Love waves cause horizontal shearing of the ground. They usually travel slightly faster than Rayleigh waves
What can seismic waves tell us?
Studies of the different types of seismic waves can tell us much about the nature of the Earth’s structure.
For example, seismologists can use the direction and the difference in the arrival times between P-waves and S-waves to determine the distance to the source of an earthquake. If the seismographs are too far away from the event to record S-waves, several recordings of P-waves can be crunched in a computer program to give an approximate location of the source.