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Social sciences: geography

Grade 7

Natural hazards

Module 4


1. The occurrence and cause of earthquakes

A scientist who had experienced a powerful earthquake told of how it seemed to him as if the whole world had become a carpet that was given a good shake and was laid down again. People, buildings, mountains – everyone and everything seemed to have become mere dust on the carpet.

[From: Die Huisgenoot - Jongspan , 21 May 1987]

You need to remember that the earth’s crust consists of large, flat plates of rock. They are kilometres thick and drift on the dense, warm mass of magma of the mantle. The movement is very slow, so it cannot be perceived. The layers of rock that are closer to the mantle are hotter and melt more readily. Nearer the surface of the earth, the layers of rock are cooler and therefore harder.

Where the plates of the earth meet, they collide or slide against one another. Because of the shifting of the layered rock, they may bend or fold and even break like a plank that is broken in two. The earth’s crust is able to withstand the stress and pressure that is built up because of the movement from within. When it reaches a point at which it cannot accommodate the pressure by bending or folding, it cracks and breaks. Then there is a shift in the layers of rock and people perceive it as an earthquake.A fault line occurs which is even visible at the earth’s surface.

An earthquake is felt for a few minutes only, but after-shocks may occur for a while afterwards.

The well-known San Andreas fault runs through San Francisco in the American state of California. Millions of people live in this area, and are aware of this threat.

Figure 5 shows how the large rock plates move past one another very slowly (5 – 7 cm per year). When the sides hook onto one another, the stress increases. The rocks suddenly move and start to break off. The result – an earthquake occurs.

Figure 6

The strength of an earthquake is measured by a seismograph

Activity 1:

To build a model of an earthquake

[lo 2.1, 2.3]

Making a model of an earthquake: Work in groups.

  1. Obtain two equally thick wooden boards. Place them alongside one another. Use cardboard to make building blocks that can be used to represent bricks, houses or buildings. Place these blocks in position over the fault line (where the two wooden boards touch one another). Now draw the boards away from one another or slide them past each other in opposite directions.

2. Explain what exactly happened to the blocks?

3. What are the names of the different parts of the earth’s crust (represented by the wooden boards)?

4. What would you suggest for making buildings more Earthquake proof? Discuss this in your group and report your suggestions to the class.

Where do most earthquakes occur?

Earthquakes mainly occur at the margins of ground-plates, which is where these plates are slowly moving towards or away from each other. The movement causes earthquakes to occur in the countries that lie above the margins of the plates, while countries that lie towards the centres of the plates are not seriously threatened by earthquakes.

Questions & Answers

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Source:  OpenStax, Geography grade 7. OpenStax CNX. Sep 09, 2009 Download for free at http://cnx.org/content/col11021/1.1
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