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9.8 Option - Disasters: 3. Disasters are not easy to predict.
Extract from Senior Science Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.
[Edit: 22 Aug 08]
Prior learning: Science Stages 4-5 syllabus: 5.9.2a, 5.9.4 f), 4.10d).
Background: Earth tremors are constantly detected by geologists around the world using a seismograph but despite this the precise time and place of major earthquakes are not easy to predict. A general predictor of bushfires is a previous dry season followed by a hot summer. Many bushfires are started by arsonists and when these people are going to strike is not easy to predict. Also the direction a fire is going to take is unpredictable as the wind can change very quickly. Fires can now be detected in remote areas by satellites.
outline differences in P, S and L energy waves produced by an earthquake
- An earthquake is a movement in the Earth's crust. The crust is made up of a number of plates which float on the material beneath. While the continents feel like 'solid ground' they are in fact rafts that are afloat. Where these plates move into, past or away from each other, the crust is very unstable. As the plates move into different positions, they cause tremors or earthquakes.
- An earthquake produces a number of waves in the earth's crust. There are three types, P, S and L waves
- P (Primary) waves are compression waves. They travel fastest and can travel through both liquids and solids.
- S (Secondary) waves are transverse waves. They travel slower than the P waves. Observation stations can use the difference between the arrival of the P and S waves to work out how far away the epicentre of the earthquake was from the observation station.
- L waves are the slowest waves. They are surface waves, which travel along the surface of the crust as a ripple. They are the waves that cause most of the damage in an earthquake. The damaging effect of these waves dies out quite rapidly as the waves move out from the centre of the quake.
identify energy transfers and transformations involved in L waves as they travel along the earth's crust
- Earthquake waves belong to two different families of waves, compression (or longitudinal waves) and transverse waves.
- P waves are compression waves, while S and L waves are transverse waves.
- To understand transverse waves, think of a spring or rope. If the spring is pulled reasonably tight, and you tap down on the spring at one end, then the effect of the tap will travel along the spring. The effect of the tap was to move the coils of the spring up or down (or left to right) a few millimetres. The tension in the spring quickly pulled the coils back into line. However, the first few coils pulled their neighbours out of line so they moved up, then down... and so on. Notice that the tap made the coils move at right angles to the spring while the ripple moved along the spring. The two motions are at right angles to each other. For this reason, we call this a transverse wave.
- As L waves travel along the surface of the Earth, they move the ground from side to side. Any buildings standing on the ground move side to side with it. If the building does not have the strength or flexibility to cope with the movement, it will develop cracks and may even collapse.
gather and process information from secondary sources on the use of
- seismographs
- Richter scale
Mercalli scale
to record and monitor earthquakes
- Use a CD ROM, the Internet, a text book or an encyclopaedia to gather information on how a seismograph works and what it is used for. You could also check the word 'seismometer' as some books refer to this instrument as a seismometer.
- At the same time as you investigate seismograph you will find references to the Richter Scale and the Mercalli scale. Find out when they were invented, by whom and how the scales work.
- Process this information
by gathering together what you have found from the different sources and organising
it for the seismograph under the headings, Instrument, How it works, What
it does. For the Richter and Mercalli scales the headings could be:
What the scale measures, How it is calculated, How it is useful.
gather and process information from secondary sources to determine the location of an earthquake's epicentre
- Most of the information you need for this activity has already been gathered in the above dot point of the syllabus but you might want to search further for more information. Clark and Cook's Perspectives of the Earth from the Academy of Science has information on it on p. 345 or look in the usual places such as books, Internet, journals and magazines.
- Once you understand how geologists determine the location of an earthquake's epicentre you could try to determine one yourself, using data from different seismographs in various locations around the epicentre.
explain how the difference in time of arrival of P and S waves can be used to locate an earthquake epicentre
- In many places around the world, scientists have set up observation posts which record movement in the Earth's crust. The equipment for this is called a seismograph. It produces a picture called a seismogram which represents the movement of the Earth's crust at the observation station.
- The seismogram shows two important quantities, the P-S time and the amplitude.
- The amplitude is a measure of the amount of movement of the Earth's crust at the observatory.
- The P-S time is the amount of time that passed between the arrival of the P-waves and the arrival of the S-waves.
- The speed of the two
waves is well known and so the P-S time can be used to calculate how far the
centre of the earthquake was from the observatory,
- Imagine two cars leave a spot and travel towards you with different speeds.
- You do not know how far away they were when they started, but you do know that one travelled at 10 metres per second (m/s) and the other travelled at 15 m/s. If one car arrives 3 seconds after the other, how far away from you were they when they started?
- Let the distance be 's' metres, then the time taken by the two cars will be s/10 and s/15 respectively using the equation Time = distance/velocity
- The difference in times will be s/10 -s/15
- We know this difference is 3 seconds
- Therefore s/10 - s/15 = 3
- Multiply both sides by 150 (10 x 15) to get 15 x s - 10 x s = 3 x 150
- 5s = 450
- s = 90 metres
- This same method is used to calculate the distance of the epicentre of an earthquake from the observatory.
- If the distance of the epicentre from three or more different observatories can be calculated, then the epicentre can be pinpointed in the Earth's crust.
describe the difficulties of monitoring and predicting earthquakes
- It is very difficult to monitor and predict earthquakes. This is so because we are trying to monitor the movement of the ground we stand on. However, we move with the ground as it shakes. So, we need equipment that can measure the movement of the earth without moving with it. In most seismographs, this is achieved by attaching the seismograph securely to the ground. Inside the seismograph is a heavy object that is suspended so that it will barely move when the rest of the seismograph moves. When an earthquake strikes, the seismograph will move with the ground, but the heavy object inside will remain stationary. As the seismograph shakes with the ground, the object then uses a marker of some sort to record the motion of the seismograph relative to the heavy object.
- Another difficulty in monitoring earthquakes is the fact that they happen deep underground, usually impossible to reach with measuring equipment.
- Although we do know that certain areas are likely to experience earthquakes, predicting exactly where and when they will occur is almost impossible. As a result we cannot get measuring equipment into the right place. Furthermore, the earthquake may be so severe that it will actually destroy the equipment set up to measure it.
- In order to predict where an earthquake will strike next, people monitor all the parts of the Earth that are on Plate Margins. University geology departments and government geology departments all around the world do this with equipment such as seismograms.
- A lot of study has gone into predicting where earthquakes will strike and our methods and technology are improving. Having this sort of knowledge is important, as it will help save lives and minimise damage to property.
identify some of the conditions that can combine to trigger a bushfire including dry weather, high temperatures and flammable vegetation
- Many parts of Australia are at risk from bushfire.
- You should remember the
three things that are needed to keep a fire going:
- fuel
- air (or rather oxygen)
- heat to start the fire or keep it going.
- Take away any one of these three and the fire will stop.
- Conditions in our country
can produce situations where all three requirements for fire are abundant:
- Periods of hot dry weather can leave plants dead and dry; excellent fuel.
- In some cases, materials left in and around homes provide a fuel source. Usually this includes leaves and twigs in house gutters and litter and rubbish left around the home.
- Hot weather is often associated with strong winds. This wind provides a great oxygen supply for the fire and can carry sparks, starting new fires in other places.
- While the high temperatures of hot days are not enough to make fuel materials burst into flames, thunderstorms often associated with hot dry weather can start fires. Some fires are started by accident, through cigarette butts being thrown away carelessly, campfires not being put out properly or by 'burning off ' operations going wrong. Some major bushfires have been lit on purpose by vandals.
- Many of our native trees and shrubs contain chemicals in their sap which are very flammable.
- Some areas of our country are hilly or mountainous. A fire started at the bottom of a hill will quickly race uphill since hot air rises.
gather, process and analyse information to identify types of native vegetation that promote the spread of bushfires
- To gather information
you could go to the
Forests NSW
website on
State Forests of NSW: Bushfires, Trees and Houses. Some more information on bush
fire research is available from the CSIRO.
Two books you might be able to access that have information on native plants and bushfires are Luke, R.H. and Macarthur, A.G. (1978) Bushfires in Australia. AGPS, Canberra. 359 pp. and Vines, R.G. (1981) Physics and chemistry of rural fires. In Gill, A.M., Groves, R.H. and Noble. I.R. (eds) Fire and the Australian Biota. Australian Academy of Science, Canberra. pp129-149. You could also try an Australian encyclopaedia. - You probably already know that eucalyptus trees burn well in fires because of the oil in their leaves and because the bark sometimes hangs down, so flames on the ground may reach the bark and then burn up the tree. Any bush will burn easily if it has been very dry and there is a lot of dead fuel (branches and leaves) on the ground.
- Process the information by classifying the vegetation. One way could be according to chemicals in the plant that promote burning. Another thing to consider is the size of the plants.
- Analyse this information and arrive at conclusions about the plants you should have if you live in a fire prone area.
gather, process and analyse secondary information to identify precautions that can be taken to minimise the likelihood of damage by bushfire including the removal of flammable material and shrubs
- To gather information
look on the Internet. One source is the NSW Rural Fire Service. Fire
Safety Information NSW Rural Fire Service. If you can't access the Internet
speak to the education officer by calling 1800 654443 which is a free call
any where in the state or call 02 9684 4411. They will also send you out material.
The address is The Education Officer, NSW Rural Fire Service, Locked Mail
Bag 17, Granville 2142.
- You could also contact your local government to find out what their Risk Management Plan for Fire is
- One way to process this information is to put it in a table. Some headings could be: Materials of the house, Clearing around the house, Action to be taken when a bushfire is in the area.
- Analyse this information by considering the effectiveness of different precautions under different circumstances, e.g. a small localised fire compared to a very hot fire that moves through the tree tops at a rapid speed.
describe the effect of the slope of the land and intensity of the wind on the speed of the bushfire
- Any fire needs heat, fuel and an air supply to keep going.
- Our countryside can provide all these in large amounts.
- Two features that contribute to bushfire hazards are the wind and the landscape.
- The wind:
- supplies oxygen for the fire
- blows the fire across the landscape. On a still day, it is easy to contain a fire. The hot air from the fire rises straight up and so the fire stays fairly stationary. Once the fuel in that spot has burnt up, the fire will go out. However a wind can blow the flames and or sparks into other areas with more fuel. Even if it does not get blown into an area of dry wood or leaves, the heat from the fire will quickly dry out leaves, so they will catch fire.
- The landscape can be
flat or hilly. A hilly landscape is more dangerous:
- It makes access for fire fighters more difficult
- Hot air rises so that the heat from a fire at the foot of a gorge will rise and dry out vegetation higher up possibly igniting it.
- Fires, which have reached the top of a ridge, can 'jump' from one ridge to the next, sometimes hundreds of metres away, as sparks are carried across by wind.
- As a result, hilly country and wind can make fires spread rapidly across the countryside and change direction without warning. This change of direction is particularly dangerous as it may trap people, including fire fighters in a spot where they cannot get out of the way of the oncoming fire.
- In some fortunate cases, a wind change has helped fire fighters when a wind change has blown the fire back into an area where it had already burnt all the fuel and the fire can be brought under control. However this is quite rare.
gather, process and analyse information on the use of natural resources to retard the progress of fire including
- water
- natural plants
- Gather information
from the Internet as it will be difficult to find information on this dot
point from books. This Sydney Morning herald article reports on fire
bombing with water in November 2003 . You could also contact the Rural
Fire Service. You can access them on the internet at www.bushfire.nsw.gov.au
or speak to the education officer by calling 1800654443 which is a free call
any where in the state or call 02 9684 4411. They will also send you out material.
The address is The Education Officer, NSW Rural Fire Service, Locked Mail
Bag 17, Granville 2142.
Before Elvis arrived on the scene, the aerial attack on New South Wales' bushfires was led by a modestly-dimensioned Bell helicopter, similar to a New Zealand Air Force Iriquois, with a specially fitted 1400 litre belly tank.
The Bell has been effective but small compared to the Helitanker - a beast which can carry a massive 9000 litres and can load water through its snorkel in just 35 to 45 seconds, making it perfect for taking multiple loads of water to a bushfire.
Look on the Internet for other information on plants that retard the progress of fire. - Process the information you have gathered by classifying it under 'Water' and 'Plants". Cross reference the material you have to see if the material from different sources agrees.
- Analyse the information. You could consider which is more effective, water or plants. Water may not always be as easy to access. Does it make any difference what the intensity of the fire is?
identify and describe some of the energy transfers and transformations involved in bushfires
- When a bushfire rages, it generates a huge amount of heat. This heat produces a large amount of hot air which rises. As a result of this rising air, a large convection current is produced, which is noticed as a very strong wind, in front of the fire, blowing back towards the fire.
discuss the reduction of fuel by controlled burns and backburns in reducing the risk of bushfires
- There are two ways in which fire fighters fight fire with fire.
- These are controlled burns and backburns. Both methods remove the fuel for the bushfire.
- A controlled burn:
- should only be done on days of little or no wind
- is lit in an area where natural growth has produced plant material which may become a fire risk in hot weather when it has dried out.
- The fire is lit and watched carefully as it burns. It is kept quite small so it does not generate enough heat to do major damage to other plant life. The job of the fire is to burn up all the plant material which may pose a risk in summer so that there will be less fuel around when summer comes.
- Unfortunately even controlled burns can be dangerous as seen in June 2000 when a number of National Parks and Wildlife Service fire fighters were killed in Sydney during a burnoff.
- A backburn:
- is intended to help put out a bushfire
- is lit to make the bushfire run out of fuel faster.
- is lit in front of the bushfire as the bushfire burns.
- A fire is lit a small distance in front of the fire. This fire is meant to use up fuel as the major fire approaches. The back burn must be lit far enough away so that it has enough time to burn up the fuel but not so far that it can get away.
- A backburn is successful if it is lit close enough to the main fire so that, as the main fire approaches, the wind it generates draws the flames and heat of the backfire into itself. This will then mean that, as the main fire and backburn join together, the fires run out of fuel and can be brought under control.
- The risk with backburning is that the backburn itself gets out of control making the fire worse.
perform an investigation to compare the flammability of dry and fresh leaves
- Perform the investigation by choosing what sort of leaves you want to test. You may want to test several different types of leaves but you can only compare dry and fresh leaves of the same type. Should you have the same number of leaves in each test or the same weight of leaves?
- Make sure you do the investigation safely so as not to let any thing else catch fire and make sure no student gets burnt.
gather and process information to explain what steps should be taken if caught in a bushfire
- Gather information
from the Internet or from brochures available from the Rural Fire Service
(see below). Information that may assist you with the above investigation
can be found on the NSW
Rural Fire Service website at Holiday Fire Safety or speak to the education
officer by calling 1800654443 which is a free call any where in the state
or call 02 9684 4411. They will also send you out material. The address is
The Education Officer, NSW Rural Fire Service, Locked Mail Bag 17, Granville
2142.
- Process the information you have by keeping the relevant information and discarding the rest. The class might decide to make a poster on the steps to take if caught in a bushfire and display them on the wall.
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