Seismic Waves and the structure of the Earth

The Earthquake.

An Earthquake is a release of elastically stored energy from a focal point beneath the Earth's surface. They are triggered when a barrier containing this energy is removed. When plate movement in contrasting directions occurs, huge amounts of energy can be stored up where the two contacting plates meet. The release of such energy can cause huge destruction.

The energy produced by the earthquake is emitted radially from the point of release, the focus, in seismic waves. These travel to the Earths surface (the point directly above the focus on the earth's surface is called the epicentre).

To read into the structure of the Earth, man has very little evidence available to him. Lavas (extruded magma from the mantle) only gives clues of the earth's composition down to a maximum depth of around 50km. There are specimens of diamond originating from more than 150km down, with a few examples originating from up to 700km deep. However, this is still merely the outer layers: The earth has a radius of 6400km. We therefore rely instead on remote sensing techniques. Seismic waves provide very valuable information about the earth's interior.

The seismic (sometimes referred to as elastic) waves have a velocity, v = sqrt( elastic modulus/density ), where elastic modulus refers to the behaviour of a material under stress. A material with a high elastic modulus does not deform easily (ie. it is hard). There are four types of elastic waves.

The following types are body waves:

Love Waves

For the purposes of this study, we are only interested in surface waves.

P-Waves (primary waves) are a compressional wave, and travel by compressing and rarefacting the medium. This is very similar to the movement of a spring. Compressional waves can propagate through solids, liquids and gases because all three can sustain changes in density. They are called primary waves because they travel faster, they are the first waves to be recorded after the occurance of an earthquake.

S-Waves (secondary waves) are transverse or shear waves. This means that particles in the medium are pushed perpendicular to the direction of motion of the wave. S-waves only transmit through solids where particles have enough cohesion to be pulled (perpendicular tp the direction of travel) by one another. Click here for animation demonstrating the propagation of body waves.

The velocity of P-Waves therefore depends on the density and compressability of a medium (resistance to compression) wheras the velocity of S-Waves depends on the density and rigidity of a medium (resistance to distortion or shearing).

An approximation for the velocity of P-Waves is 6 - 14 km/s. The velocity of S- Waves, in contrast, is less than 6km/s. The arrival of these waves at various points on the Earth’s surface are detected by a seismogram.

This is a typical reading of a seismogram over the period of a disturbance. Note the time lag betwwen the respnse to P-Waves and the response to S-Waves.