contemporary science issues lesson 14: journey to the centre of the earth © 2006 gatsby technical...
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Contemporary science issues
Lesson 14: Journey to the centre of the Earth
© 2006 Gatsby Technical Education Projects
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SEISMIC WAVESSEISMIC WAVES
Can they tell us anything Can they tell us anything about the Earth’s structure?about the Earth’s structure?
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For each earthquake fact..For each earthquake fact..
• Decide whether or not it can help us to build up a model of the Earth’s structure
• If you think it can, describe what the fact tells us
• You may want to look at some facts again as you gain more information
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Waves can change direction Waves can change direction when they change speedwhen they change speed
Example – light can change direction when it enters glass from air and slows down
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Earthquake waves are like Earthquake waves are like sound wavessound waves
They travel faster through denser materials
Examples: speed of sound in air 330m/sspeed in water 1500m/sspeed in rock > 5000m/s
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Scientists know how fast earthquake waves travel but the time they take to reach their destination is longer than it should be if they followed a straight line
through the Earth
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There are 2 types of There are 2 types of earthquake wavesearthquake waves
• P-waves (primary or pressure waves)
• S-waves (secondary or shear waves)
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• P-waves can travel through solids, liquids and gases
• S-waves can only travel through solids
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• P-waves travel faster than S-waves so reach their destination sooner, but..
• S-waves do more damage to buildings
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P-waves travel as a longitudinal wave; the particles vibrate in the same line as the
waves travel
Movement of particleswavelength
Direction of travel of wave
Particles close togetherParticles
far apart
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S-waves travel as a transverse S-waves travel as a transverse wavewave
The particles vibrate at right angles to the direction of travel of the wave
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Earthquake waves follow Earthquake waves follow curved paths through the Earthcurved paths through the Earth
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•there is always a region on the opposite side of the Earth from the quake where s-waves cannot be detected
•P-waves reaching the other side of the world take longer than expected, even when their curving path is accounted for
P-waves can be detected nearly all over the world but….
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epicentre
P & S waves detected here
P & S waves detected here
No S-waves here: P-waves arrive later than expected
No waves at all here
No waves at all here
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The complete pictureThe complete pictureS-waves stop suddenly here;
P-waves change direction suddenly
P-waves change direction suddenly again here
Both P and S waves follow curving paths here
Only P-waves following curving paths here..
…and here
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