[Introduction]
This is what happened in a Japanese television news room during an earthquake. Of the innumerable earthquakes on our planet the smallest pass unnoticed, while the largest are felt over hundreds of kilometres. Since the dawn of civilisation earthquakes have caused damage and destruction as they shake the Earth. The largest earthquakes can destroy buildings and other structures and can cut power supplies, telephone and computer links. We will investigate the seismic waves produced during earthquakes and how they can be used to find out about the interior of the Earth.

[Seismic waves - P waves]
The faster of the body waves is called the primary or p wave, as a p wave travels through the Earth it alternately compresses and stretches the rock. Because p waves compress rock they are also referred to as compressional waves. P wave motion is the same type of motion as a sound wave in the air which compresses the air as it travels.

[Seismic waves - S waves]
The slower type of body wave is called a secondary or S wave. An s wave is a transverse wave which means the rock moves at right-angles to the waves direction of travel. The sideways motion can be at any angle between vertical and horizontal. S waves cannot travel through a liquid, because liquids are not rigid. So s waves do not travel through liquid parts of the Earth such as oceans.

[Seismic waves - Love waves]
Surface waves travel only near the surface of the Earth. There are two types of surface waves named after two British physicists about a century ago. Love waves move the Earth from side to side horizontally, like s waves they do not travel through the liquid parts of the Earth. Love waves behave slightly differently from s-waves because the Earth's surface can move more freely than underground rocks. The largest motion occurs at the surface and reduces with depth inside the Earth. The horizontal motion produced by Love waves is particularly damaging to buildings and other structures.

[Seismic waves - Rayleigh waves]
The slower type of surface wave is called a Rayleigh wave. The ground surface is moving up and down. This motion decreases rapidly with depth. These waves are very similar to waves at the surface of water. Although Rayleigh waves are the slowest type of seismic waves they can persist for great distances, sometimes circling the Earth.

[Travel time graphs - Introducing travel time graphs]
This is a cross-section through the Earth with an earthquake focus on the left. The earthquake can cause damage to nearby buildings and other structures. Seismic waves spread out from the focus and are recorded by seismometers at different distances from the earthquake.

The results are plotted on a travel time graph. With time plotted vertically and the distance between the epicentre and the seismometers measured around the surface of the Earth plotted horizontally.

For short distances a travel time graph will be a straight line.

[Travel time graphs - Interpreting travel time graphs]
This is a p wave travel time graph for three different crustal rocks, use it to decide which of the rocks has the highest p-wave speed and which has the lowest.

[Travel time graphs - A deeper layer]
A simple straight line travel time graph is observed upto distances of about two hundred kilometres. At greater distances another p wave arrives at the seismometer at earlier times.

What has caused this? Let's consider a simple model which has a deeper layer with a higher p wave speed below the first layer. As well as waves that have travelled in a direct line from the earthquake, p waves are also recorded that have travelled down to the deeper layer, along the interface within the deeper layer at a higher speed and back to the surface. At distances greater than about 200 kilometres the p wavesfrom the deeper layer are arrive at a seismometer earlier than the p waves that have travelled directly through the upper layer. This earlier p wave must have travelled faster than the direct p wave to arrive at the seismometer at the earlier time. This happens because the p wave speed is higher in the deeper layer, so although this p wave has travelled further it travels faster, so it gets to the seismometer first. Waves do not just travel in the direction we have shown here, but in all directions from the earthquake.

[Refraction]
The change in direction of the seismic wave at the boundary between the upper/lower speed layer and the deeper/higher speed layer is called refraction.

Most refracted waves travel through the boundary into the lower layer. The wave that changes direction sufficiently to travel along the boundary, travels along just one of the possible parts of refracted waves.

[A deeper Layer]
A change in the depth of the Moho will alter the position of the second line on the travel time graph. This means that we can use the travel time graph to determine the depth of the boundary, and you will do this in the next step of this activity.

[Mohorovicic]
Mohorovicic calculated the Moho depth of 54 kilometres under his part of Yugoslavia from these data and your answer should have been about the same. It is also possible to use the travel time graph to calculate the average p wave speeds of the crust and top of the mantle beneath Yugoslavia. The inverse gradient of the line up to about 200 kilometres gives the crustal p wave speed, and the inverse gradient of the line over 200 kilometres gives the mantle p wave speed.

[Down to the centre - Waves in the mantle]
Seismograms from seismometers at increasing distances from an earthquake epicentre have recorded p waves that have travelled deeper into the mantle.

S-waves travel on similar paths but travel more slowly.

This is the travel time graph for p waves and the slower s waves up to an epicentral angle of 105°.

[Down to the centre - Below the mantle]
What happens at greater epicentre seismometer distances which record earthquake waves that have penetrated more deeply into the Earth. This is the travel time graph for epicentral angles up to 180°, right through the centre of the Earth.

We can reproduce this travel time graph using a model in which the waves travel in more complex paths. What happens to the p waves that travel closer to the centre of the Earth?

[Down to the centre - Seismic discontinuity]
The s waves on a travel time graph can tell us more about the core. Watch what happens to s waves that travel towards the core. There is an s wave shadow zone from 105° around the rest of the Earth. This can only happen if s waves cannot travel through the core.

[Down to the centre - Earth structure and composition]
So earthquake waves can give us important information about the interior of the Earth, and much more comfortably than in Jules Verne's book 'Journey to the centre of the Earth'.