After an earthquake, P waves are always detected at seismic stations well in advance of the S waves. From this we can make the assumption that P waves travel faster than S waves. The properties of the waves must therefore be different for the waves to be reaching one point at two different times.
The difference is in the wave type: P waves are longitudinal, their energy travels in a straight line.
S waves are transverse, their energy moves forwards and outwards.
See diagram 1-behaviour of P and S waves
Earthquakes are one of Mother Nature’s most deadly forces; they are extreme and can cause a lot of devastation and destruction in an ill-prepared area. The main hazard for humans is the immense ground shaking, this on its own is not too bad, the main injury’s come from today’s high-rise building’s toppling over and crushing people. As can be seen from the above diagram, an S wave will cause considerable damage to a building. If the area has been well planned out and built on tough bedrock then the structures stand a much greater chance of remaining standing. But if the area is built on poor material such as saturated mud- a poor consistency material, the building’s in the area will not remain standing after a quake. One such quake occurred in South America; a city had been built on the site of an old lake, which had dried up only about 10,000 years previous. The rock was therefore very weak and when a quake struck the area, mass destruction occurred.
When a quake hits an urban area, other risks stem from the quake itself. Fires start, but cannot be put out because the water mains are fractured. Landslides can also be a major hazard after a quake; they are unexpected and simply drown people in thick mud, similar to the effect of an avalanche. In some instances these may cause more destruction than the quake itself.
If a quake occurs at a plate margin under the ocean, the huge gap that is formed fills with water very rapidly, this causes huge waves to form, these can devastate coastal areas- they are known as tsunamis, meaning tidal wave.
There is little to tell geologists about the structure of the Earth, meteorites that come from the asteroid belts in space, formed at the same time as the Earth, during the ‘big bang’. They are useful because they tell us the exact age of Earth. Ocean trenches have also been extensively surveyed, primarily by the Americans. They can reach up to depths of 11-12 KM into the Earth’s crust. But when put into proportion of the crust being up to 40 KM deep in places, this seems a relatively small distance.
Geologists have calculated that the density of the Earth’s crust is 5.5g/cm³, but towards the centre of the Earth the density increases to 13.0g/cm³. Therefore we know that the density of the Earth increases towards the centre.
The Earth is divided into different segments, the outer-most being the crust has the lightest density, and the inner core being the most central part of the Earth has the heaviest density. This is because the inner core is being squashed from all sides, forcing a large amount of material into a small area, therefore giving it a greater density.
The area where there is a meeting of two zones, each with different densities, is called a discontinuity.
These different densities cause the waves to travel at different speed through the Earth. This tells us for sure that the Earth must be made of different materials.
Because of the changes in density throughout the Earth, P and S waves cannot reach everywhere on the Earth’s surface, these areas are called ‘shadow zones’.
There are two laws that exist, that are always correct over P waves:
P waves will speed up as they move into incompressible rocks.
P waves will slow down as they pass into rocks of a higher density.
Secondary waves behave differently. There are two laws that are always correct over S waves:
S waves will speed up as they move through rigid substances. They stop if they hit a surface that has no rigidity, such as water, or any other liquid.
S waves will slow down as they move through rocks of a higher density.
Because the waves have different properties, and move differently through different types of rock, this tells us that the waves will travel at different speeds trough the Earth.
As mentioned earlier, S waves travel well through solids. The low velocity zone of the Earth is 5% molten, therefore slowing the S wave down, even though the density has increased (which should speed the wave up).
The shadow zone of a P wave is between 103 and 143º from the earthquake’s focus, this is because the waves are refracted. Some P waves are recorded, but their energy is very small.
At locations over 103º no S waves are recorded, this indicates that the outer core could be liquid, as S waves cannot travel through liquids. An increase is recorded in P wave velocity at the boundary between the outer and inner core discontinuity.
See diagram 2-Shadow zones on the Earth’s surface (p263)
One problem that does exist is that as P waves travel from the crust into the mantle, they speed up. The density of the crust is 2.7-4.0g/cm³, and the density of the mantle is 2,3g/cm³. This is a problem because P wave velocity should decrease as the density decreases. The answer is that the mantle must consist of more incompressible material.
To conclude-What does the behaviour of P and S waves tell us about the interior of the Earth?
S waves- Behave identically to P waves in the crust and mantle. Both waves decrease in velocity at the boundary between the crust and low velocity zone, despite their being an increase in density in the rocks. We can also say that the outer core must be a liquid because the S waves cannot travel through it, indicating low rigidity levels.
P waves- A low velocity in the outer core indicates that the rocks are less incompressible, therefore are liquid. The increase in velocity at the outer core/inner core boundary indicates that the inner core is solid, and not a liquid.
