Earthquakes
Introduction

An earthquake occurs when rocks in the Earth's crust suddenly release their built-up energy. This causes the ground to shake violently, and can cause damage which costs huge amounts of money to repair. The science of earthquakes is known as seismology.
Earthquakes
Earthquakes can shake the entire Earth. They are powerful enough to cause vast amounts of damage and their effects kill thousands of people every year. Earthquakes usually occur at the edges of plates which are moving towards each other or sliding past each other.
Figure 1.   Damage caused by an earthquake.
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Look at the diagram below. Move the mouse over the Earth to cause an
earthquake
An earthquake is when the earth releases built-up energy by shaking violently.
earthquake
.

Figure 2.   Seismic waves passing through the Earth.
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The earthquake produces three types of seismic wave. These waves are detected in seismographic stations all over the earth.

The first waves to reach the seismographic stations are primary or P-waves. These are longitudinal waves. They travel through the Earth in the same way as sound travels through air. Because P-waves can travel through the core, they can travel the whole way across the Earth.


Secondary or S-waves are transverse waves. They cannot travel through liquids.

Which parts of the Earth do S-waves pass through?
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Surface waves are the slowest type of wave caused by earthquakes. These are the waves that shake the ground and can cause considerable damage to buildings.

In which parts of the Earth do surface waves travel?
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Locating earthquakes
This is a typical seismogram. Click on the button to start.
Figure 3.   A typical seismogram.
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The first wave to arrive is the P-wave. This wave travels at 8 km/s. It is detected 60 seconds after the earthquake strikes.

How far away is the earthquake?
  •  km
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The S-wave arrives 100 seconds later.

How long after the earthquake strikes does the S-wave arrive at the seismogram station?
  •  s
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The S-wave travels the same distance as the P-wave from the source of the earthquake.

How fast does the S-wave travel?
  •  km/s
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If the earthquake happened further away, the time between the P- and the S-waves would be greater. By using information from different seismographic stations, scientists can find out exactly where the centre of the earthquake occurred.

The density of the Earth
The
speed
Speed is a measure of how fast something is moving. It is calculated by dividing the distance travelled by the time taken.
speed
of any wave depends on the
density
The density of an object is equal to its mass divided by its volume. The SI unit of density is kg/m³ although it is also common to use g/cm³.
density
of the material it is moving through. Light slows down in glass and sound speeds up in water. Seismic waves move faster in denser materials.

When a wave passes from one material to another, the direction of the wave changes.

What do we call this bending of waves?
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Look at this picture of a
seismic wave
A seismic wave is a wave or vibration that passes through the Earth. These can be caused be earthquakes.
seismic wave
moving through the Earth:

Figure 4.   Seismic waves travelling through the Earth.
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Press the button to see the seismic waves bending, or refracting, as they pass into layers with different densities.

Figure 5.   Seismic waves travelling through the Earth.
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Which layer is denser? (Remember, waves bend towards the
normal
The normal to a surface is an imaginary line at right angles to the surface.
normal
when entering a denser material.)
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Scientists know that seismic waves curve through the Earth, as shown in the diagram above. From this information they can tell that between 500 km and 2500 km below the surface, the density of the mantle increases from 3 g/cm3 to around 6 g/cm3.

The composition of the Earth
P-waves travel through both solids and liquids. S-waves travel through solids, but not through liquids. As the outer core of the Earth is hot molten metal, S-waves do not pass through it. Use the diagram in Fig.4 above to answer these questions.

At which seismographic stations will S-waves be detected?
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At which seismographic stations will P-waves be detected?
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On the other side of the earth to the earthquake, there are no S-waves. This region is called the S-wave shadow zone.

Scientists also use other methods in addition to seismic measurements to learn about the interior of the Earth. The material ejected by volcanoes can tell them about the composition of the upper mantle. Scientists have also measured the average density of the whole earth to be 5.5 g/cm3. The density of the crust is only 2.7 g/cm3, so we know there must be denser rocks within the Earth.

Summary


There are three types of vibration, or seismic wave, from earthquakes, S-waves, P-waves, and surface waves.

Primary P-waves are longitudinal waves and travel through solids and liquids.

Secondary S-waves are transverse waves and only travel through solids.

Surface waves are transverse waves that shake the surface of the earth and cause considerable damage.

The difference in time of arrival between S-waves and P-waves gives information on the distance of the earthquake from the seismograph station.

Seismic waves refract when passing from one layer to the next. This can give information on the composition of different layers of the Earth.

Exercises
1. Which type of seismic wave travels the fastest?
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2. Match the two seismic waves below to the correct wave type.
  • Primary P-waves
    Secondary S-waves
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3. Decide if the three seismic waves below travel through the various layers of the earth. Enter 'yes' if the wave travels through the given layer, and 'no' if it does not.

  • Seismic wave Crust Mantle Core
    Primary P-waves
    Secondary S-waves
    Surface waves

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Seismic wave Average speed (km/s)
Primary P-waves 8.0
Secondary S-waves 3.0
Surface waves 0.5

4. A seismographic station detects the P-wave of an earthquake 24 seconds after the event. Use the data in the table above to calculate how far the centre of the earthquake is from the station?
  • km
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5. How long after the P-wave arrives at the station will the S-wave be detected?
  • seconds
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6. How long does it take the surface wave to travel from the earthquake to the seismographic centre?
  • seconds
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7. A seismographic station detects a P-wave, but no S-wave. Where was the earthquake?
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