Download How do we measure Earthquakes?

How do we measure Earthquakes?
So far we have seen how and where earthquakes happen. In this lesson we will
be finding out how they are measured.
Read the following extract from the Nottingham Evening Post.
“The earthquake, measuring 4.2 on the Richter scale, struck mid-afternoon
yesterday and was felt most strongly by some residents the of Long Clawson
near Melton Mowbray. Witnesses described the feeling as a brief shuddering of
buildings as if a lorry had crashed somewhere close by. The only reports of
damage so far are chimney stacks collapsing in Melton Mowbray and Long
Eaton.”
ƒ Write down any words or phrases in the article which tell you about the
strength of the ‘Long Clawson ‘Quake’.
There are two ways of measuring the strength of earthquakes;
1 The Richter Scale.
This is measured using a ‘seismograph’. The movements in the earth’s crust
cause a pen to move from side to side on a rolling sheet of paper. The larger the
‘waves’ created, the stronger the earthquake, (see below).
Time
6.00:30
Period of main shock.
After tremor.
6.00:40
6.00:50
6.01:00
6.01:10
6.01:20
After the earthquake, the ‘seismograph’ is analysed and the earthquake is given
a value on the Richter scale. Each additional value on the scale is equal to a x10
increase in earthquake strength. An earthquake measuring 6 on the Richter scale
is ten times greater than one measuring 5 and 100 times greater than one
measuring 4. Earthquakes are rarely greater than 9 on the Richter scale.
2 The Mercalli Scale.
This involves looking at the effect which the earthquake has had and measures
earthquakes on a 12 point scale.
ƒ What is the name of the machine on which an earthquake is measured?
ƒ In the example shown approximately how long did the main shock last?
ƒ How much stronger is an earthquake measuring 7 than one measuring 5 on the
Richter scale?
ƒ Now complete the sheet on the Mercalli scale.
ƒ Where would you put the ‘Long Clawson ‘Quake’ on the Mercalli Scale?
ƒ Why do you think that the Richter scale is used more often than the Mercalli for
measuring Earthquakes?
The Mercalli Scale
2
4
Cannot be felt – just able to read
on Seismograph.
Sleeping persons wake –
hanging items swing
5
7
Felt by nearly everyone, things
indoors fall over.
Many houses suffer damage e.g.
chimneys fall.
10-12
Houses collapse, landslides, cracks open in the ground.