A map of the Earth’s tectonic plates
When these plates rub against, or collide with each other, they often cause an earthquake. This releases energy as sound waves. These waves then travel through the Earth in two main forms, Primary (P) and Secondary (S). Primary waves are longitudinal waves, meaning that the wave action is parallel 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 liquid inside the earth.
Primary waves act in a different manner. They are detected on the opposite side of the Earth as the epicentre. A shadow zone where no Primary waves are observed is found from 103° to 142° from the epicentre. However, Primary waves are observed again after this. This shows that there is something refracting the Primary waves towards the centre of the earth. This evidence shows that inside the liquid core there is a solid part with a different density to the liquid part (if the densities were the same then no refraction would take place). There is a discontinuity in the speed of Primary waves at a point called the Moho. The change in velocity of Primary waves at the Moho (from about 6 km s-1 to 8 km s-1) is due to one of two things:
- A change in phase of the Earth (i.e. solid to liquid)
- A change in density (i.e. more dense to less dense)
The diagram below shows how the speeds of Primary waves and Secondary waves change as they get deeper into the Earth.
It can be seen that there are two large changes in the velocities of both Primary and Secondary waves. There is a large drop at about 2800 km, which is the boundary between the mantle and the outer core. The density of the Earth suddenly gets much greater, hence slowing the waves down. The velocity of the Secondary waves falls to zero here because they cannot travel through liquid. There is another large change, an increase this time, at about 5200 km. This is the boundary between the outer core and the inner core. The speed of Secondary waves rises quite dramatically from zero, while the speed of the Primary waves rises only a small amount. The rise is due to a change in phase of the Earth – sound waves travel faster in a solid than a liquid.
In conclusion, natural phenomena are very useful in determining the inner structure of the Earth. Using them has enabled us to build up a complete picture of the Earth, which looks like this.
