When an earthquake takes place, shockwaves of released energy that shake the earth and turn soft deposits like clay into jelly temporarily are known as seismic waves. Paul Favret points out that this term is derived from the Greek word ‘seismos’ meaning ‘earthquake’. Typically, seismic waves are generated by the movements of the tectonic plates of the earth, but might occur due to landslides, volcanoes and explosions as well.
Seismologists make use of seismographs for the purpose of recording the amount of time it takes seismic waves to travel through distinguished layers of the earth. These waves can be reflected and refracted as they travel through varying densities and stiffness. Owing to the distinguished behavior of waves in varied materials, seismologists are able to deduce the type of material these waves are travelling through. The result can provide a snapshot of the internal structure of the earth, and aid in locating and understanding the fault planes, as well as the strains and stresses acting on them.
The three basic types of seismic waves are:
- P-waves: Also known as pressure waves or primary waves, these waves travel at the greatest velocity through the earth. They take the form of sound waves when travelling through air. P-waves travel at the speed of sound (330 ms-1) through air. On the other hand, they might travel at 5000 ms-1 in granite. Due to their speed, they typically are the first waves to be recorded by a seismograph during an earthquake. P-waves differ from S-waves as they propagate through a material by alternately expanding and compressing the medium, where particle motion is parallel to the direction of wave propagation. This is more or less like a slinky that is partially stretched, laid flat and has its coils compressed at one end and then released.
- S-waves: Commonly referred to as shaking waves, shear waves or secondary waves, these transverse waves travel at a slower speed than the P-waves. Particle motion is perpendicular to the direction of wave propagation in this situation. One can again imagine a slinky partially stretched to understand it, except this time, a section has to be lifted and released, so that a transverse wave travels along the length of the slinky. S-waves cannot travel through water or air, but can be more destructive than P-waves owing to their larger amplitudes
- Surface waves: These waves are similar to water waves in their nature and travel under the surface of the earth. Surface waves are generated when the source of the earthquake is really close to the surface of the planet. Even though they do travel slower than S-waves, surface waves can be much larger in amplitude, and might be the most destructive type of seismic wave.
Paul Favret mentions that Surface waves can be of two types, Rayleigh waves and Love waves. The former travels as ripples similar to those on the surface of the water and are also called ground roll. Love waves, on the other hand, cause horizontal shearing of the ground.