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What does the behaviour of P and S waves tell geologists about the structure of the Earth's interior?

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Introduction

What does the behaviour of P and S waves tell geologists about the structure of the Earth’s interior?

Vibrations are produced in the Earth’s crust when rocks in which elastic strain has been building up suddenly rupture, and then rebound. The vibrations can vary from barely noticeable to catastrophically destructive. Earthquakes can release energy thousands of times greater than the world’s first atom bomb.

Six types of shock waves are generated in the process. Two are classified as body waves-that is, they travel through the Earth’s interior-and the other four are surface waves. The waves are further differentiated by the kinds of motions they impart to rock particles. Primary or compressional waves (P waves) send particles oscillating back and forth in the same direction as the waves are travelling, whereas secondary or transverse shear waves (S waves) impart vibrations perpendicular to their direction of travel. P waves always travel at higher velocities than S waves, so whenever an earthquake occurs, P waves are the first to arrive and be recorded at geophysical research stations throughout the world.

An earthquake will occur when rocks, (usually at plate boundaries) are put under immense strain or pressure. This pressure will eventually fracture the rock, similar to when bending a brittle object such as a wafer, or a ruler.

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Middle

When a quake hits an urban area, other risks stem from the quake itself. Fires start, but cannot be put out because the water mains are fractured. Landslides can also be a major hazard after a quake; they are unexpected and simply drown people in thick mud, similar to the effect of an avalanche. In some instances these may cause more destruction than the quake itself.

If a quake occurs at a plate margin under the ocean, the huge gap that is formed fills with water very rapidly, this causes huge waves to form, these can devastate coastal areas- they are known as tsunamis, meaning tidal wave.

There is little to tell geologists about the structure of the Earth, meteorites that come from the asteroid belts in space, formed at the same time as the Earth, during the ‘big bang’. They are useful because they tell us the exact age of Earth. Ocean trenches have also been extensively surveyed, primarily by the Americans. They can reach up to depths of 11-12 KM into the Earth’s crust. But when put into proportion of the crust being up to 40 KM deep in places, this seems a relatively small distance.

Geologists have calculated that the density of the Earth’s crust is 5.

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Conclusion

 waves are recorded, but their energy is very small.

 At locations over 103º no S waves are recorded, this indicates that the outer core could be liquid, as S waves cannot travel through liquids. An increase is recorded in P wave velocity at the boundary between the outer and inner core discontinuity.

See diagram 2-Shadow zones on the Earth’s surface (p263)

One problem that does exist is that as P waves travel from the crust into the mantle, they speed up. The density of the crust is 2.7-4.0g/cm³, and the density of the mantle is 2,3g/cm³. This is a problem because P wave velocity should decrease as the density decreases. The answer is that the mantle must consist of more incompressible material.

To conclude-What does the behaviour of P and S waves tell us about the interior of the Earth?

S waves- Behave identically to P waves in the crust and mantle. Both waves decrease in velocity at the boundary between the crust and low velocity zone, despite their being an increase in density in the rocks. We can also say that the outer core must be a liquid because the S waves cannot travel through it, indicating low rigidity levels.

P waves- A low velocity in the outer core indicates that the rocks are less incompressible, therefore are liquid. The increase in velocity at the outer core/inner core boundary indicates that the inner core is solid, and not a liquid.

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