Seismic Waves and the Structure of the Earth

Our knowledge of the structure of the Earth comes mainly from studying how the shockwaves from (seismic waves) travel through it. These waves are detected using seismographs.

The Earth is nearly spherical and has a layered structure comprising:

• a thin ;
• a , extending almost halfway to the Earth’s centre which has all the properties of a solid except that it can flow very slowly;
• a , with just over half of the Earth’s radius, made of nickel and iron the outer part of which is liquid and the inner part of which is solid.

The overall density of the Earth is much than the mean densities of the rocks which form the crust. This indicates that the interior of the Earth is made of material different from, and denser than, that of the crust.

Earthquakes produce surface waves that can cause earthquake damage and two types of waves that can travel through the Earth:

• Longitudinal faster travelling Primary, P () waves, which move through liquids as well as solids;
• Transverse slower travelling Secondary, S waves, which travel only through solids.

The speed of both types of wave with depth through the mantle. The waves travel in paths as their speed changes gradually through a material. When the state of the transmitting changes abruptly, e.g. when moving from solid to liquid, the wave direction also changes abruptly. It is by observing the path of these waves that scientists have been able to work out details of the Earth’s layered structure.

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