Contemporary science issues Lesson 14: Journey to the centre of the Earth © 2006 Gatsby Technical Education Projects.

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Presentation transcript:

Contemporary science issues Lesson 14: Journey to the centre of the Earth © 2006 Gatsby Technical Education Projects

SEISMIC WAVES Can they tell us anything about the Earth’s structure?

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

Waves can change direction when they change speed Example – light can change direction when it enters glass from air and slows down

Earthquake waves are like sound waves They travel faster through denser materials Examples: speed of sound in air 330m/s speed in water 1500m/s speed in rock > 5000m/s

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

There are 2 types of earthquake waves P-waves (primary or pressure waves) S-waves (secondary or shear waves)

P-waves can travel through solids, liquids and gases S-waves can only travel through solids

P-waves travel faster than S-waves so reach their destination sooner, but.. S-waves do more damage to buildings

P-waves travel as a longitudinal wave; the particles vibrate in the same line as the waves travel Movement of particles wavelength Direction of travel of wave Particles close together Particles far apart

S-waves travel as a transverse wave The particles vibrate at right angles to the direction of travel of the wave

Earthquake waves follow curved paths through the Earth

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….

epicentre P & S waves detected here No S-waves here: P-waves arrive later than expected No waves at all here

The complete picture S-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