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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

what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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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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