Contemporary science issues

Lesson 14: Journey to the centre of the Earth

© 2006 Gatsby Technical Education Projects

SEISMIC WAVESSEISMIC WAVES

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

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

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

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

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 There are 2 types of earthquake wavesearthquake 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 particleswavelength

Direction of travel of wave

Particles close togetherParticles

far apart

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

Earthquake waves follow Earthquake waves follow curved paths through the Earthcurved 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

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

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