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1 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. 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. Shock waves travel out from the “epicentre”. They are also called Seismic Waves. This is the Earthquake. 3 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 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. 4 P waves It is useful to think of these as “push” waves. They are longitudinal waves. Compression Rarefaction It seems to push here. 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. 5 S Waves It is useful to think of these as “shake” waves. They are transverse waves. It seems to “shake” here 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. 6 Summary P waves •It is useful to think of these as push waves. •They are longitudinal waves S Waves •It is useful to think of these as shake waves. •They are transverse waves •The motion of the particles in a •The motion of the particles in a longitudinal wave, is parallel to the transverse wave, is perpendicular direction of the motion of the wave. to the direction of the motion of the wave. P waves can travel through solids S waves can travel through solids and liquids. only. 7 S waves Epicentre The waves spread out through the Earth and are received at the surface. We assume that the shadow region forms behind something that the S waves cannot travel through. Cross-section of the Earth Liquid Core There is a huge region where no S waves are received at all. A shadow region perhaps? S waves can’t travel through a liquid 8 P waves We also suspect a solid inner core. Again there is a shadow region. But the longitudinal P waves also pass through the core. 9 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 change occurs as we pass to the MANTLE, a very hot region. Temperatures reach 1,200oC. 10 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 5,000oC. The SOLID INNER CORE is very dense - perhaps 3 times that of the crust. 11 THE MANTLE THE INNER CORE THE OUTER CORE THE CRUST 12 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. If the speed changes gradually, the path is gently curved. Fast Slow Fast Slow 13 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. The Seismic Waves behave as if they have speeded up as they travel deeper into the Earth. 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. Earth 14 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 15 More about Seismic S wave Shake wave Transverse Waves The shake waves cause the buildings to move from left P wave to right - in the Push wave horizontal Longitudinal 16 More about Seismic S wave Shake wave Transverse Waves The push waves cause the buildings to move up and P wave down - in the Push wave vertical Longitudinal 17 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? S waves displacement P waves time L waves 18 This seismometer detects vertical vibrations It detects P waves The drum rotates. When there is an earthquake, the heavy mass tries to keep still whilst everything around it vibrates up and down. spring pivot But to us it seems as if the mass moves and then the pen leaves a trace on the drum. Earth Earth 19 This seismometer detects vertical vibrations It detects P waves The drum rotates. When there is an earthquake, the heavy mass tries to keep still whilst everything around it vibrates up and down. spring pivot But to us it seems as if the mass moves and then the pen leaves a trace on the drum. Earth 20 This seismometer detects horizontal vibrations It detects S waves The drum and moving heavy mass is mounted so that it can move horizontally 21 22