Lesson 8 - Seismic Waves (Set 1)

Focus, epicentre, seismometer, refract, absorb

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Focus, epicentre, seismometer, refract, absorb

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• Start Print slides 3 and 4
• Recall questions
• Main
• Discussion about earthquakes and video
• Introduce the fact that seismology has provided evidence for the structure of the Earth
• Waves and the Earth video
• Discuss P, L and S waves and compare them
• Discuss how the findings provided evidence for the structure of the Earth
• Discuss shadow zones and the reasons for them - Annotate diagram on slide 4.
• How seismometers work - video
• Learning check questions
• Plenary
• Exam question 13 marks
• Exam question 8 marks (Same as the first 8 marks of the 13 mark question - Choose whichever suits your group)

Focus, epicentre, seismometer, refract, absorb

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Practical / Demo - No demos or practical in this lesson

Safety

• Key safety point / equipment required

Aim

• Key teaching points

Top tips

• Things to do to make sure it works well

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Method

• Step by step instructions

Focus, epicentre, seismometer, refract, absorb

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Lesson 8 - Seismic Waves

Answer in your book:

• Give 3 uses of ultrasound.
• Give the equation to calculate the depth of a boundary below the surface.
• Calculate the depth of the sea bed if an ultrasound pulse took 0.4 s to return to the ship which emitted it. The speed of sound in seawater is 1350 m/s.

Focus, epicentre, seismometer, refract, absorb

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Lesson 8 - Seismic Waves

Answer in your book:

• Give 3 uses of ultrasound. Echo location, ultrasound scanners to image inside the body, industrial imaging (non-destructive testing) to detect flaws in metal
• Give the equation to calculate the depth of a boundary below the surface.

Depth of boundary below surface (m) = ½ x speed of ultrasound wave (m/s) x time (s)

• Calculate the depth of the sea bed if an ultrasound pulse took 0.4 s to return to the ship which emitted it. The speed of sound in seawater is 1350 m/s. 0.5 X 1350 X 0.4 = 270 metres

Focus, epicentre, seismometer, refract, absorb

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Lesson 8 - Seismic Waves

Learning Objectives:

Describe the internal structure of the Earth.

Compare the three types of seismic waves (P, S, L), including speed and type of wave

Explain how the internal structure of the Earth can be determined by waves passing through it.

Focus, epicentre, seismometer, refract, absorb

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What can you tell me about the structure of the Earth?

Stick in the diagram

Focus, epicentre, seismometer, refract, absorb

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How do we know what is inside the Earth?

This is the the top of the Kola Superdeep Borehole, the deepest hole that has ever been dug on Earth. It took Russia 36 years to dig and is about 12 km deep.

It does not go into the mantle.

So how do we know what is below the crust?

Focus, epicentre, seismometer, refract, absorb

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What does this diagram show?

Geography

Focus, epicentre, seismometer, refract, absorb

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Earthquakes

Focus, epicentre, seismometer, refract, absorb

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Seismology

Seismology is the study of earthquakes.

There are about half a million earthquakes each year but only a few cause serious damage

Focus, epicentre, seismometer, refract, absorb

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Seismology

Earthquakes predominantly occur at tectonic plate boundaries when huge stores of elastic energy, built up as the plates interact, is released very quickly.

The energy is dissipated via pressure waves that travel through the earth. These are called seismic waves.

Focus, epicentre, seismometer, refract, absorb

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Seismic waves

Focus, epicentre, seismometer, refract, absorb

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Seismic waves

There are three types of seismic waves:

• Primary (P) waves
• Secondary (S) waves
• Long (L) waves

Focus - Centre of the earthquake

Epicentre - Point on the Earth’s crust above the focus.

Seismometer - Device used to measure seismic waves

Focus, epicentre, seismometer, refract, absorb

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Seismic Waves

These waves cause most of the damage

Focus, epicentre, seismometer, refract, absorb

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Seismic wave tracks

The waves follow a curved path because the density of the Earth increases with depth below the surface, changing the speed the waves travel at.

• S-waves can’t travel through the liquid outer core because they are transverse waves.
• P-waves are refracted as they cross the boundary between the mantle and outer core.

Due to the changes in density as the waves travel through the Earth, seismic waves are deflected or absorbed.

Focus, epicentre, seismometer, refract, absorb

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Shadow zones

When an earthquake occurs some seismometers record only long waves.

These seismometers are in the shadow zone of the earthquake.

The existence of the shadow zone shows that there is a liquid outer core under the mantle.

Annotate your diagram

Focus, epicentre, seismometer, refract, absorb

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Why is there a shadow zone?

P-waves are refracted at the boundary between the mantle and the outer core, both when they enter and leave the core.

Because the second refraction is further around the waves can’t reach the shadow zone.

Weak P-waves detected in the shadow zone show that the core has a solid inner part that refracts P-waves at the boundary between the outer core and the inner core and into the shadow zone.

Focus, epicentre, seismometer, refract, absorb

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Seismometers

Focus, epicentre, seismometer, refract, absorb

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Learning check:

• Describe what the shadow zone it and what it tells us about the structure of the Earth.
• Explain why P-waves and S-waves are not detected in the shadow zone.

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You can use pages 186 & 187 of the physics textbook to help you.

Focus, epicentre, seismometer, refract, absorb

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Check your answers:

• Describe what the shadow zone it and what it tells us about the structure of the Earth.

The shadow zone is an area on the Earth’s crust where no P-waves or S-waves are detected. The existence of the shadow zone tells us that there is a liquid outer core below the mantle.

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• Explain why P-waves and S-waves are not detected in the shadow zone.

P-waves are refracted at the boundaries between the mantle and the core, causing the waves to bend towards the crust and creating a shadow zone.

S-waves cannot travel through the outer liquid core because they are transverse waves.

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Focus, epicentre, seismometer, refract, absorb

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Check your notes:

Focus, epicentre, seismometer, refract, absorb

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Exam Question

13 marks = 13 minutes

Focus, epicentre, seismometer, refract, absorb

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Exam Question

8 marks = 8 minutes

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Focus, epicentre, seismometer, refract, absorb

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Seismic Waves

These waves cause most of the damage

Focus, epicentre, seismometer, refract, absorb

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Logos / symbols

Focus, epicentre, seismometer, refract, absorb

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