<< Chapter < Page Chapter >> Page >

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

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 to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get the best Algebra and trigonometry course in your pocket!

Source:  OpenStax, Geography grade 7. OpenStax CNX. Sep 09, 2009 Download for free at http://cnx.org/content/col11021/1.1
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Geography grade 7' conversation and receive update notifications?