... to the motion. Secondary waves are transverse waves, meaning that the wave action is at 90? to the direction of motion. Primary waves travel faster than Secondary waves. They will also travel through liquid, whereas Secondary waves will not. We can look at how these waves travel (or do not travel) through the Earth and hence get some idea of the structure. It is the Secondary waves that yield the most information about the Earth's structure. When there is an earthquake, Secondary waves are only detected at about a quarter of the way around the earth from the epicentre (the point on the surface above the hypocentre which is the source of the earthquake). After this there is a shadow zone. This means that there must be something impeding the travel of the Secondary waves. Knowing that Secondary waves cannot travel through liquids, it can be deduced that there must be ...

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