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2 Earthquakes occur due to movements that take place within the Earth. Large pressures build up between huge plates of land both above and below the sea. The release of these pressures is accompanied by huge releases of energy as one land mass slides against another.

3 Shock waves travel out from the “epicentre”. They are also called Seismic Waves. This is the Earthquake. Earthquakes occur due to movements that take place within the Earth. Large pressures build up between huge plates of land both above and below the sea. The release of these pressures is accompanied by huge releases of energy as one land mass slides against another. Have you answered the question on the sheet yet?

4 Shock Waves There are three types of shock waves called L, P and S. L waves only travel close to the surface so they do not tell us much about the structure of the Earth. They travel through a thin surface called the crust. P waves travel more slowly than L waves and penetrate below the crust. S waves travel slower then P waves and they too penetrate below the crust. We shall only concern ourselves with P and S waves,

5 P waves It is useful to think of these as “push” waves. They are longitudinal waves.Compression Rarefaction The motion of the particles in a longitudinal wave is parallel to the direction of the motion of the wave. It is important to remember that a P wave (which is longitudinal) can travel through solids and liquids. It seems to push here. Have you answered the question on the sheet yet?

6 S Waves It is useful to think of these as “shake” waves. They are transverse waves. The motion of the particles in a transverse wave, is perpendicular to the direction of the motion of the wave. It is important to remember that an S wave (which is transverse) can travel through solids only. It seems to “shake” here Have you answered the question on the sheet yet?

7 Summary P waves S Waves It is useful to think of these as push waves. It is useful to think of these as shake waves. The motion of the particles in a longitudinal wave, is parallel to the direction of the motion of the wave. The motion of the particles in a transverse wave, is perpendicular to the direction of the motion of the wave. S waves can travel through solids only. P waves can travel through solids and liquids. They are longitudinal wavesThey are transverse waves Have you answered the question on the sheet yet?

8 S waves Epicentre The waves spread out through the Earth and are received at the surface. There is a huge region where no S waves are received at all. A shadow region perhaps? We assume that the shadow region forms behind something that the S waves cannot travel through. S waves can’t travel through a liquid Liquid Core Cross-section of the Earth Have you answered the question on the sheet yet?

9 P waves Again there is a shadow region. But the longitudinal P waves also pass through the core. We also suspect a solid inner core. Have you answered the question on the sheet yet?

10 The Structure of the Earth There is a thin CRUST between 5 and 50 km thick. There is a sharp change in density below this (the Mohorovic discontinuity). The change occurs as we pass to the MANTLE, a very hot region. Temperatures reach 1,200 o C.

11 The Structure of the Earth There is a thin CRUST. The MANTLE is a very hot region. The OUTER CORE is LIQUID with temperatures ranging from 2,000 to 5,000 o C. The SOLID INNER CORE is very dense - perhaps 3 times that of the crust.

12 THE CRUST THE MANTLETHE INNER CORE THE OUTER CORE Have you answered the question on the sheet yet?

13 More about Seismic Waves It is clear that the waves change direction below the surface of the Earth. They do this due to a change in speed and the process is called refraction. SlowFast If the speed changes gradually, the path is gently curved. Slow Fast

14 Earth More about Seismic Waves The speed change is caused by a change in density of the material. The more dense the material, the faster it travels. Note that light slows down in more dense materials which is exactly the opposite to Seismic Waves and sound waves. You should be able to convince yourself from the path the seismic waves take, that the Earth must be getting more dense the deeper you go down. The Seismic Waves behave as if they have speeded up as they travel deeper into the Earth. Have you answered the question on the sheet yet?

15 More about Seismic Waves You will have noticed that when the Seismic Waves reach the surface again, they come up from almost directly below. This has important consequences for the way in which earthquakes shake buildings. S wave Shake wave Transverse P wave Push wave Longitudinal

16 More about Seismic Waves S wave Shake wave Transverse P wave Push wave Longitudinal The shake waves cause the buildings to move from left to right - in the horizontal

17 More about Seismic Waves S wave Shake wave Transverse P wave Push wave Longitudinal The push waves cause the buildings to move up and down - in the vertical

18 Click here to see the seismic waves again Next Slide Have you answered the question on the sheet yet?

19 Examiner’s corner If you were an examiner, you might think that earthquakes would be an interesting topic to set questions on. The only difficulty would be that there is a limited amount of material to set questions on. The examiner can only really ask the following questions: What is the structure of the earth? How do we know this? What are the different sorts of waves? What are the properties of the waves? The examiner’s job is to build interesting questions around these questions. If you have done your job thoroughly, you will be able to say “Ah, so that’s how he’s asking me if a P wave is longitudinal”. Have you answered the question on the sheet yet?

20 And Finally P waves are really called PRIMARY waves S waves are really called SECONDARY waves And L waves travel the slowest Can you guess which is which on the seismograph below? P waves S waves L waves Extension time displacement Have you answered the question on the sheet yet?

21 The drum rotates. When there is an earthquake, the heavy mass tries to keep still whilst everything around it vibrates up and down. But to us it seems as if the mass moves and then the pen leaves a trace on the drum. spring pivot Earth This seismometer detects vertical vibrations It detects P waves

22 The drum rotates. When there is an earthquake, the heavy mass tries to keep still whilst everything around it vibrates up and down. But to us it seems as if the mass moves and then the pen leaves a trace on the drum. spring pivot Earth This seismometer detects vertical vibrations It detects P waves Have you answered the question on the sheet yet?

23 This seismometer detects horizontal vibrations It detects S waves The drum and moving heavy mass is mounted so that it can move horizontally Have you answered the question on the sheet yet?

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