P&S Waves

Seismic waves are caused by the energy released when rocks fracture under deforming forces. There are four types of seismic waves, which fall into two categories: body and surface waves. 

Body waves 

P-waves are the fastest seismic waves and can cross through any material—solid, liquid, or gas. They are sound waves (or acoustic waves), also called compressional waves, as rocks compress and extend repeatedly before returning to their original shape after the wave passes. 

S-waves are slower than P-waves but still faster than surface waves. They cannot travel through liquids or gases; therefore they cannot travel through the earth’s outer core, which is made of molten metal. 

Surface waves 

Rayleigh and Love waves travel parallel to the ground. 

They are less rapid than body waves but lose their energy more slowly. Rayleigh waves have a vertically elliptical motion and are typically described as a rocking movement, similar to the waves you feel on a boat. Love waves result from the interaction of S-waves, moving the ground sideways in a horizontal plane and causing no vertical motion.

Sketch of the ray path (long arrows), particle motion (short arrows), and wave front (rectangular plane) associated to P- and S-body waves (arrows in blue), and surface waves (arrows in red). The particle motion of S-waves can be on the horizontalplane (SH) or on the vertical plane (SV). S-particle motion will be a combination of the two.

Sketch of the ray path, particle motion, and wave front associated to P- and S-body waves (arrows in blue) and surface waves (arrows in red). Surface waves propagate parallel to the surface, while body waves propagate along curved rays through the interior of a heterogeneous earth, illustrated by red and blue colours. Dashes show the location of two discontinuities in the earth’s mantle.

Did you know that in 1939, Jeffreys established travel-time tables for the most important seismic phases, which say how much time each seismic wave takes to travel a given distance, and today they are still precise to 0.2%? The figure shows a recent and more comprehensive version of the Jeffreys travel-time curves. The variety of seismic waves is a consequence of the body waves inside the earth undergoing reflection and refraction at the major interfaces.

We observe a variety of seismic waves in our planet because body waves travelling inside the Earth undergo reflection and refraction at the major interfaces. The type of wave and the layer where each ray travels is identified by capital letters, P, S, K, I, J. Small cap letters show reflections, c and i.

A seismogram is the recording of the ground motion caused by an earthquake. When we are very close to the earthquake, the seismometer (accelerometer) records the waves generated progressively from the whole fault. It is usually impossible to identify clearly where the S-waves begin. Surface waves are not yet visible.

For smaller events at close range to the source, P- and S-waves can be clearly identified on the seismogram. Surface waves may, or may not, be present.

At large distances from large earthquakes the seismograms are complex, and many seismic waves can be identified due to the heterogeneous structure of the Earth,which leads to reflections and/or refractions. For shallow earthquakes, at large distances from earthquakes, the largest amplitude (and longest period) waves are the surface waves.

For very large earthquakes, the surface waves arriving at one station travel around the earth along opposite directions and circle the globe several times. At large distances from the earthquake, surface waves have the greatest amplitude.