# Earthquakes

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## Earthquakes

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

## [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.

can someone help me with some logarithmic and exponential equations.
20/(×-6^2)
Salomon
okay, so you have 6 raised to the power of 2. what is that part of your answer
I don't understand what the A with approx sign and the boxed x mean
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
Salomon
I'm not sure why it wrote it the other way
Salomon
I got X =-6
Salomon
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
oops. ignore that.
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Commplementary angles
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Sherica
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Sherica
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Tamia
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Cied
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Stotaw
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Azam
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Azam
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At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
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