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Gill Sans Bold
Senior Science
HSC Course
Stage 6
Disasters
Part 4: Earthquakes
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Number: 43171
Title: Disasters
This publication is copyright New South Wales Department of Education and Training (DET), however it may contain
material from other sources which is not owned by DET. We would like to acknowledge the following people and
organisations whose material has been used:
Diagrams showing floor plans, courtesy of Rod Bashford, Division of Life and Environmental
Science, Macquarie University
Part 6 pp 49-50
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© State of New South Wales, Department of Education and Training 2008.
Senior Science Stage 6 HSC Course
Lifestyle chemistry
Medical technology – bionics
Information systems
Option – Disasters
•
Disaster – natural or human?
•
Air pressure
•
Cyclones
•
Earthquakes
•
Bushfires
•
Detection and prevention
Contents
Introduction ............................................................................... 3
What is an earthquake? ............................................................ 4
What causes earthquakes? .................................................................6
The moving Earth ...................................................................... 8
Mercalli scale........................................................................................9
Seismographs.....................................................................................10
Richter scale.......................................................................................12
Predicting earthquakes ......................................................................13
Earthquake waves................................................................... 14
Making waves.....................................................................................14
Finding the epicentre............................................................... 18
Comparing P and S waves ................................................................18
Locating the epicentre........................................................................22
Suggested answers................................................................. 25
Exercises – Part 4 ................................................................... 27
Part 4: Earthquakes
1
Introduction
This part is the first of two dealing with how, even with current
technology, disasters such as earthquakes and bushfires are not easy to
predict. Specifically, in this part you will be looking at earthquakes
which occur globally, but with emphasis on the Australian setting.
In Part 4, you will be given opportunities to learn to:
•
outline differences in P, S and L energy waves produced by an
earthquake
•
identify energy transfers and transformations involved in L waves as
they travel along the Earth’s crust
•
explain how the difference in time of arrival of P and S waves can be
used to locate an earthquake’s epicentre
•
describe the difficulties of monitoring and predicting earthquakes.
In Part 4, you will be given opportunities to:
•
gather and process information from secondary sources to determine
the location of an earthquake’s epicentre
•
gather and process information from secondary sources on the use of
•
–
seismographs
–
Richter scale
–
Mercalli scale
to record and monitor earthquakes.
Extracts from Senior Science Stage 6 Syllabus © Board of Studies NSW,
Amended October 2002. The most up-to-date version is to be found at
http://www.boardofstudies.nsw.edu.au/syllabus_hsc/index.html
Part 4: Earthquakes
3
What is an earthquake?
An earthquake is a ground movement caused by shock waves.
These movements result from stresses built up by the rocks in the
Earth’s crust.
What causes earthquakes? Before we can answer that we need to look
more closely at the layered structure of the Earth.
lithosphere
(rigid solid)
oceanic crust
100 km
continental crust
700 km
upper mantle
inner core
1216 km
outer core
asthenosphere
(capable of flow)
mantle
2270 km
2185 km
upper
mantle
700 km
6371 km
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Disasters
The Earth’s crust consists of about twelve rigid plates. The convection
currents in the mantle cause the plates to move.
Some plates move together, some apart, others move sideways past each
other. The way these plates can move past each other is shown in the
diagrams below.
•
Divergent. Plates move away from each other creating
new lithosphere.
•
Transform. Plates slide past each other.
•
Convergent. Plates move towards each other, thereby
destroying lithosphere.
Relative plate motion.
The plates are relatively stable. Earthquakes occur mainly at the plate
boundaries. For example, the Anatolian Fault in Turkey and the
San Andreas Fault in California in the United States.
Before people scientifically studied earthquakes there were all sorts of
reasons and causes proposed for them. These were not very useful in
preparing people to take the necessary steps and lessen the effects of
Part 4: Earthquakes
5
earthquake disasters. But over time, as the scientific knowledge of them
increased, people found out what causes them.
What causes earthquakes?
The forces in the Earth’s crust result in elastic deformation of rocks.
Stresses build up and rocks break or else move along a fracture.
The sudden movement and release of energy establishes shock waves
which form an earthquake. The aftershocks are caused by the transfer of
strain to other blocks of rock.
Blocks of rocks can be displaced along fault lines in different directions.
The point where the rocks snap or fracture is called the focus. The point
directly above the focus on the Earth’s surface is called the epicentre.
Focus and epicentre.
6
Disasters
Earthquake waves are also called shock waves or earth tremors.
These waves travel through different substances at different speeds.
Waves travel faster through a solid than they do through a liquid.
As the waves move away from the source they become smaller and less
violent. This means the closer you are to the source of an earthquake the
more damage you can expect to sustain or experience.
Features of a wave.
Few earthquake waves are generated in the world’s oldest continent.
Darwin
Brisbane
Sydney
Perth
Adelaide
Canberra
Melbourne
= earthquake location
Hobart
There are questions on earthquakes in Exercise 4.1 at the end of this section.
Part 4: Earthquakes
7
The moving Earth
What are the chances of an earthquake? Well, that depends on where on
the globe you live. At some locations the chances are far greater than at
others. In spite of all the advances in seismology (study of earthquakes)
scientists still can’t say for certain when and where exactly an earthquake
will happen.
The chance of earthquakes (and volcanoes for that matter) occurring is
much greater where two plates come together. These places are shown
on the map below.
0
2000 4000 6000 8000
km
23 12 ∞ N
0∞
23 12 ∞ S
180∞
Volcanoes
Earthquake
belt
0∞
Location of volcanoes and earthquakes.
On the 28th December 1989, at 10:27 am, Newcastle (Australia’s sixth
most populated city) was severely damaged by a moderate earthquake.
This was the first earthquake with fatalities in Australia since European
settlement, claiming 13 lives. The devastation to buildings and other
structures was extensive.
8
Disasters
In other countries it is not uncommon to have regular earthquakes killing
hundreds or thousands of people at a time.
The Australian Seismological Centre in Canberra estimates that, on
average, the Australian region experiences an earthquake the size of the
Newcastle earthquake, or larger, about every ten months. Most of these,
however, occur in areas of low population so we don’t tend to pay
much attention.
Many people in Australia thought major earthquakes just didn’t happen
here. The Newcastle experience dispelled this myth! Still, the chance of
an earthquake like the Newcastle one devastating a major town or city in
this country is very low.
However other places may not be so lucky. American scientists predict
the chance a major earthquake striking somewhere in the San Francisco
Bay area in the next three decades is about 90 percent.
We could draw a probability line to compare the chance of a major
earthquake at Sydney with that at San Francisco in the near future.
impossible
0
certain
0.5
Sydney
1
San Francisco
Scientists study the Earth and all the conditions which might contribute
to earthquakes. They also look at how many earthquakes, and their
intensities, have occurred in a particular region. From this information
they are in a position to say how likely, or unlikely, it is for an
earthquake to occur.
So earthquakes can be a potential hazard to Australians.
Mercalli scale
The Mercalli scale assigns an intensity to an earthquake that is based on
the effects of the earthquake on people’s homes. After the earthquake
surveys ask people to rate the effects of the earthquake on their location.
Earthquakes are rated with Roman numerals from I (not felt) to XII (total
destruction). The rating of the amount of damage will depend on the
geology of the foundations, methods of building construction and
population density. A location built on sand will experience more effects
than a location built on granite.
Part 4: Earthquakes
9
Mercalli was an Italian who telephoned various locations after
earthquakes to find out the intensity of effects. His scale was modified in
1931 to fit Californian building conventions. This Modified Mercalli
scale is the one that is most used today.
Modified Mercalli scale of earthquake intensities
Intensity
rating
Description
Effects
I
Negligible
Not felt, only detected by instruments
II
Lightest
Suspended objects swing
III
Light
Many people indoors but few outdoors notice
IV
Moderate
Windows and dishes rattle; parked cars rock
V
Rather strong
Windows and dishes break
VI
Strong
Everyone feels; furniture moves
VII
Very strong
Most people run outdoors; felt in moving cars
VIII
Destructive
Drivers have trouble steering; furniture overturned
IX
Ruinous
Foundations damaged; underground pipes break
X
Disastrous
Ground cracks; railways lines bent
XI
Very disastrous
Most buildings collapse; large cracks in ground
XII
Catastrophic
Ground moves in waves; objects thrown into air
Seismographs
How can you tell if an earthquake is ‘the biggest’, the ‘most destructive’,
or where the earthquake originated, and so on? Sophisticated recording
devices take measurements and the information, when analysed, can
provide answers to these questions.
A seismograph is an instrument that ‘feels’ the shaking of the Earth.
These earth movements are recorded by a seismograph. The seismic
records produced are useful mainly for working out the focus or source
of the earthquake.
10
Disasters
A seismograph.
This kind of seismograph is designed to measure horizontal ground
motion. Seismographs that measure vertical ground motion have the
rotating drum aligned vertically.
The records obtained from seismographs, called seismograms, provide a
great deal of information concerning the behaviour of seismic
(earthquake) waves.
Describe how this seismograph works.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answers.
Part 4: Earthquakes
11
Richter scale
The Richter magnitude scale based on seismograph measurements is
more quantitative than the Mercalli scale that describes shocks felt in a
particular location.
The Richter scale devised by the American seismologist
Charles F Richter in 1935 measures the magnitude of an earthquake.
The magnitude is a measure of the maximum amplitude of S waves.
Earthquake magnitude is measured from 0 to about 9. Each number step
represents an increase of ten times in measured amplitude. For example
an earthquake of magnitude 6 is ten times larger than those of 5, and
100 times (10 ¥ 10) larger than one of magnitude 4, and so on. As an
estimate of energy, however, each whole number step in the magnitude
scale corresponds to the release of about 31 times more energy than the
amount associated with the preceding whole number value.
Richter
magnitude
< 2.5
Effect of earthquake
Approximate
number per year
recorded, although generally not felt
900 000
2.5 – 5.4
generally only minor damage detected
30 000
5.5 – 6.0
slight damage to structures
500
6.1 – 6.9
can be destructive in populated regions
100
7.0 – 7.9
serious damage inflicted; major earthquake
20
≥ 8.0
total destruction to populated areas; great
earthquakes
about one every
year or so
Earthquakes with magnitude of about 2.0 or less are usually referred to as
micro-earthquakes; they are not commonly felt by people and are
generally recorded only on local seismographs. Events with magnitudes
of about 4.5 or greater are strong enough to be recorded by sensitive
seismographs all over the world.
Great earthquakes have magnitudes of 8.0 or higher. On the average, one
earthquake of such size occurs somewhere in the world each year.
Although the Richter Scale has no upper limit, the largest known shocks
have had magnitudes in the 8.8 to 8.9 range. Recently, another scale
called the moment magnitude scale that measures energy released has
been devised for more precise study of great earthquakes.
12
Disasters
Predicting earthquakes
Where an earthquake could occur is easier to predict than when an
earthquake could occur.
Prediction is sometimes based on the historical record of earthquakes in
an area. If two severe earthquakes have occurred each century then one
could be expected every 50 years.
Measurements of the build up of strain (eg. elongation) in rocks across
faults using lasers or Global Positioning Satellites could be useful.
Particularly if the extent of strain reached when previous earthquakes
occurred has been recorded.
Japan has underwater seismometers off it’s east coast to measure warning
shocks and estimate the size of resulting tsunamis. Russia has used
measurements of a P-wave velocity change of 10–15% just before the
earthquake as a prediction tool.
Ground tilting, increased emission of radioactive radon gas from
underground and changes in electrical conductivity of rocks have all been
monitored to try to predict earthquakes.
Part 4: Earthquakes
13
Earthquake waves
Earlier you saw that an earthquake occurs at a point below the Earth’s
surface called the focus. In some 90% of cases the focus is within about
8 km from the surface. In only 2% of cases is it further than 30 km deep.
The point directly above the focus, on the surface, is the epicentre.
Generally the most intensity of the earthquake is felt at, or near,
the epicentre.
The intensity of the earthquake decreases with increasing distance from
the focus. On the surface this appears as concentric circles (or ellipses)
radiating from the epicentre. Each of these (almost) circles joins all
points having the same earthquake intensity and is called an isoseismal
line (iso- same; seismal- earthquake).
The focus and epicentre of an earthquake.
Making waves
Seismologists have discovered there are different kinds of waves
produced during an earthquake. There are basically two kinds of waves
which are created at the focus and travel through the Earth’s interior.
14
Disasters
These two body waves are called primary or P waves, and secondary
or S waves.
P waves travel the fastest and are compression waves. That is, they
push-pull the rocks in the direction the wave is travelling. They can
travel through both liquids and solids. P waves cause the particles in the
material to move back and forth in the same direction the wave moves.
direction of particle vibration
direction of movement
Compression waves produced by push-pull on the end of a slinky spring.
S waves travel slower than P waves and are transverse waves.
They shake or shear the rock particles at right angles to the direction of
travel. They can travel through solids, but not through liquids. S waves
oscillate at right angles to the direction the wave moves. This is similar
to creating a wave by flicking a rope.
S waves are transverse waves.
There is a third group of complex waves, collectively known as L waves.
They are made up of waves which move in elliptical or horizontal
transverse motions. They are generated at the surface when P and
S waves reach there.
L waves travel only on the surface of the Earth and are responsible for
most damage done. Because these surface waves are confined to a
narrow region near the surface, they are not spread throughout the Earth
as are P and S waves. They therefore maintain their maximum amplitude
Part 4: Earthquakes
15
longer. L wave energy transfers through greater distances than either P
and S waves.
L wave energy transformations produce kinetic energy and potential
energy of stress in structures on the Earth’s surface. The L wave energy
can destroy natural and human structures on the Earth’s surface.
Because the surface L waves have longer periods (time between crests or
troughs) they are sometimes referred to as long waves.
Relative motion of P, S and L waves.
1
The speed of P waves through granite in the crust is about 6 kms–1;
S waves travel at 3.5 kms–1.
Use the formula, time =
distance
speed
a) How long does it take P waves to travel through 1000 km of
granite? ___________________________________________
b) How long does it take S waves to travel through 1000 km of
granite? ___________________________________________
c) What is the difference in time between P and S waves travelling
a distance of 1000 km? _______________________________
2
The speed of P waves through water is 1.5 kms–1. How long does it
take P waves to travel through 1000 km of water?
______________________________________________________
______________________________________________________
______________________________________________________
16
Disasters
3
After an earthquake, the difference between the arrival times for
P and S waves at a seismic station is 2 min 30 s. It is known that
through this portion of the Earth P waves travel at 7.5 km/s, while
S waves travel at 4.4 kms–1.
a) How much further than an S wave can a P wave travel in 1 s?
_________________________________________________
b) How much further than an S wave can a P wave travel in
i)
1 min? _______________________________________
ii) 2 min? _______________________________________
iii) 5 min? _______________________________________
c) Looking at your answers to b, what can you say about the
distance between a P wave and an S wave as time increases?
_________________________________________________
d)
With a time difference of 2 min 30 s between the arrival of the
two waves, it is known that a P wave can travel 465 km further
than an S wave. Which of the values marked x, y, or z on the
diagram corresponds to this distance? __________________
seismic detection
station
focus
P wave
S wave
x
y
z
Check your answers.
There are some questions on earthquake waves in Exercise 4.2.
Part 4: Earthquakes
17
Finding the epicentre
In the previous section you saw that an earthquake is a vibration in the
Earth caused by the rapid release of energy. This energy radiates in all
directions from its source, the focus. It does this through P and S waves,
generated at the source and through L waves generated on the surface of
the Earth. Even though the energy dissipates rapidly with increasing
distance from the focus, instruments located around the world can
record the event.
In this section you will explore some techniques seismologists use to
determine the location of the epicentre of an earthquake.
Comparing P and S waves
Seismologists use the differing speed and arrival times of P and S waves
to calculate how far away an earthquake occurred.
Although wave speeds vary by a factor of ten or more in the Earth
(depending on the material they are passing through), the ratio between
the average speeds of a P wave and of its following S wave is quite
constant. This fact enables seismologists to simply time the delay
between the arrival of the P wave and the arrival of the S wave to get a
quick and reasonably accurate estimate of the distance of the earthquake
from the observation station.
To find the difference in arrival times of P and S waves scientists
examine a seismogram drawn by a seismograph. A seismogram is just a
record produced on paper or film of an earthquake.
It takes scientists plenty of experience to be able to interpret such
records. For this course you will consider only simple cases.
18
Disasters
A seismogram.
Notice there are three conspicuous pulses shown on such a seismograph.
The first pulse begins with the arrival of the P wave. Then comes the
S wave (almost 5 min later on the diagram above). Now there are two
waves being recorded, both the P and S. The size of the S waves should
indicate to you the amount of energy they carry and the amount of
damage they can do.
The L wave arrived just 7 min after the P wave. In this region of the
seismograph all three waves are shown. However, in this region the
L waves are dominant as P and S waves have expended much of their
energy by the time they arrive at the seismic station.
Only P and S waves are required to locate an earthquake.
Because L waves can be generated at a number of points on the
surface and are complex they are not used.
Can you see that the time difference between P and S wave arrival on this
seismogram is about 4 min 45 s?
Now a travel-time graph is needed to translate this time difference
into a distance.
Travel-time graph
A travel-time graph (sometimes called a time-distance curve) is used to
turn the P-S time difference found from the seismogram into a distance
from the earthquake’s epicentre.
From the seismogram above, a time difference of 4 min 45 s was found
between when the first P waves arrived and when the first S waves arrived.
Part 4: Earthquakes
19
Take a ruler, or piece of paper, and measure the length represented by
this time on the vertical axis. Now move this ruler or paper across until
the time difference between the P wave and S wave also measures this
amount. Dropping your vertical line down to the horizontal axis gives
the distance the earthquake is from the recording station.
As you can see, from the diagram below, this distance is about 3200 km
for a time difference of 4 min 45 s.
20
Disasters
1
Below are three seismograms.
a) Which seismogram is closest to the recording station? How do you
know?
_________________________________________________
_________________________________________________
b) Which seismogram is furthest from the recording station?
How do you know?
_________________________________________________
_________________________________________________
Part 4: Earthquakes
21
2
Each of the seismograms in question 1 has been drawn to the
following time scale.
0
100
200
300 400 500
time (seconds)
600
700
For each of the seismograms measure the time difference between
the arrival of the P and S waves, then use the travel-time graph to
determine how far each is from the recording station.
Seismogram
Time difference (s)
Distance (km)
X
Y
Z
Check your answers.
Locating the epicentre
It is one thing to know that an earthquake occurred so many kilometres
away, but in which direction? Suppose an earthquake occurred 1000 km
from Sydney. On a map this could be anywhere on the rim of the
circle shown.
It could have occurred in western NSW, or somewhere in Victoria or
Tasmania, or even out under the ocean.
22
Disasters
If we knew that the earthquake occurred 980 km from Sydney, 2560 km
from Perth, and 2450 km from Darwin, it now becomes easier to locate
the epicentre of the earthquake.
Generally three different seismological stations pool their results to find
the epicentre. Each draws a circle around its location. Where the three
circles intersect is the earthquake’s epicentre.
Suppose the three seismograms provided in the previous set of questions
were recorded at Brisbane (seismogram X), Perth (seismogram Y) and
Adelaide (seismogram Z).
The earthquake’s epicentre is located about 3300 km from Brisbane.
The scale for the Map of Australia on the next page shows that 1000 km
is represented by 2.8 cm. So, 3300 km is represented by
3.3 ¥ 2.8 = 9.2 cm .
a)
Use your compass to draw a circle (or part of a circle) having this
radius on the map of Australia on the next page.
b) Calculate the radius on the map around Perth and Adelaide for this
earthquake. Draw arcs (part circles) on the map.
_____________________________________________________
_____________________________________________________
Part 4: Earthquakes
23
c)
Mark the position of the epicentre of this earthquake.
Check your answers.
There is another exercise in locating epicentre in Exercise 4.3 at the end of
this section.
24
Disasters
Suggested answers
Seismographs
The inertia of the suspended mass tends to keep it motionless.
The recording drum is anchored to the bedrock and vibrates in response
to the seismic waves.
Thus, the stationary mass provides a reference point from which to
measure the amount of displacement occurring as the seismic wave
passes through the ground below.
Another way of describing this is to think of a seismograph as a simple
pendulum. When the ground shakes, the base and frame of the
instrument move with it, but inertia keeps the pendulum bob in place.
It will then appear to move, relative to the shaking ground. As it moves
it records the pendulum displacements as they change with time, tracing
out a record called a seismogram.
Earthquake waves
1
2
3
1000
= 167 s
6
1000
b) t =
= 286 s
3◊5
a) t =
c) difference in time = 286 – 167 = 119 s (just under 2 minutes).
1000
time =
= 667 s = 11 min 7 s. (Notice how P waves travel
1◊ 5
slower through water than granite.)
a) In one second, a P wave can travel 7.5 km while an S wave
travels 4.4 km. So a P wave can travel 3.1 km further.
b) i
ii
3.1 ¥ 60 = 186 km
3.1 ¥ 120 = 372 km
iii 3.1 ¥ 300 = 930 km
Part 4: Earthquakes
25
c) Distance between two wave fronts increases as time increases.
d) y
Travel-time graph
1
a) Y is closest as the difference in time between the arrival of
P and S waves is least.
b) X is furthest as the difference in time between the arrival of
P and S waves is greatest.
2
(Your values may differ slightly from the ones given here to small
inaccuracies in reading the seismograms and travel-time graph.)
Seismogram
Time difference (s)
Distance (km)
X
300
3300
Y
70
700
Z
190
1800
Locating the epicentre
On your map, the radius around Perth is 2.0 cm and Adelaide, 5.0 cm.
26
Disasters
Exercises – Part 4
Exercises 4.1 to 4.3
Name: _________________________________
Exercise 4.1: What is an earthquake?
1
What is an earthquake?
_____________________________________________________
_____________________________________________________
_____________________________________________________
2
Describe how an earthquake may occur.
_____________________________________________________
_____________________________________________________
_____________________________________________________
3
With the aid of a diagram explain the difference between the
earthquake’s focus and epicentre.
Part 4: Earthquakes
27
Exercise 4.2: Earthquake waves
1
Briefly describe the differences between P, S and L waves.
______________________________________________________
______________________________________________________
______________________________________________________
2
In any solid material P waves travel about 1.7 times faster than
S waves, while L waves can be expected to travel 0.9 times the speed
of S waves.
a) Through a certain material the velocity of an S wave has been
measured to be 5 kms–1. What is the estimated speed of
i)
a P wave?
_______________________________________________
ii) an L wave?
_______________________________________________
b) Through a particular part of the upper mantle of the Earth, the
speed of P waves was measured at 8.1 kms–1. What is the
estimated speed for:
i)
an S wave?
_______________________________________________
ii) an L wave?
_______________________________________________
c) After an earthquake it takes 4 min 50 s for the P wave to reach a
seismic station, 2500 km away.
i)
How many seconds in 4 min 50 s?
_______________________________________________
ii) Calculate the speed of this P wave.
_______________________________________________
iii) What is the speed for the corresponding S wave?
_______________________________________________
iv) How long would it take the S wave to arrive?
_______________________________________________
v) Approximately how much later is the S wave expected to
arrive?
_______________________________________________
28
Disasters
Exercise 4.3: Finding the epicentre
Suppose an earthquake occurred somewhere in the Pacific Ocean.
Three seismic stations located at San Francisco, Tokyo and Canberra
recorded the following seismograms for it.
This is the time scale for each of the seismograms.
0
Part 4: Earthquakes
2
4
6
8
10
time (minutes)
12
14
29
Use the time scale and the travel-time graph from the notes to complete
the table.
Seismogram
location
Time difference (min)
of P-S wave arrival
Distance (km)
(nearest 100 km)
San Francisco
Tokyo
Canberra
Now use the information in the table to locate the earthquake epicentre
on the map.
30
Disasters