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Geology & Earth Sciences

By Tas -

Earthquake Basics - Seismic Waves

Seismic waves provide indications of deep planetary composition and structure through measurements of their speed of travel, recorded by seismographs located around the planet. There are two main types of seismic wave - body and surface waves. Of these, body waves are the most useful in determining the planets composition as they pass through it.

Body waves come in two forms - Primary Waves (P waves) and Secondary waves (S waves), and their interaction with the planet is our best indicator of core composition.

P waves are the fastest wave but the material they pass through affects their speed. By studying travel time graphs and comparing the speed of the wave to its speed through known materials, the composition of the rock can be determined. Below depths of a few hundred km, P waves speed up and some of these waves are refracted to the surface - at distances over 200km they arrive before the "normal" speed waves indicating the change in composition. The P wave refraction caused by the Earth's core means there is a "shadow zone" (area where no P waves are detected) between 105° and 143° from the epicentre.

S waves are slower than P waves and cannot pass through liquids. Like P waves they are refracted by the material they pass through, although as they are blocked by the Earth's liquid core there is an S wave shadow zone from 105° onwards.

Other lines of enquiry support the seismographic evidence. Deep drilling brings samples of rock, which can then be compared to the wave speed to confirm. Planetary density can also be used to identify the composition - density of known materials is compared to the planets and the result gives an indication of the main components.

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