earthquake virtual

Transcription

1 Activity: Elasticity of the crust Elasticity of the crust. The earth s crust is slightly elastic, it can be stretched or compressed. Without this elasticity all movements along faults would be slow and continuous. This elasticity is simulated by using elastic bands. 1. Choose the number of elastic bands 2. Press the start button. This will start the virtual winch turning. The stress will increase until the upper block moves. Record the actual movement which is shown on the screen. 3. Press next to make a second movement. There is considerable variation in the amount of displacement so you will need to repeat this at least 100 times to get a reliable average. 4. Fill in the table. 5. Vary the number bands and repeat instructions 1 to Plot a graph of your data. 1

2 Activity: Friction between the sides of the fault This depends on the roughness of the sides of the fault and is simulated by using different grades of sandpaper. Grade 120 is fine with a grain size of 0.125mm, grade 80 is intermediate with a grain size of 0.201mm and grade 60 coarse with a grain size of 0.269mm. In any one activity only one grade of sandpaper is used. 1. Choose the sandpaper grade. 2. Press the start button. The elastic band will begin to stretch. The stress will increase until the upper block moves. Record the actual movement which is shown on the screen. 3. Press next to make a second movement. There is considerable variation in the amount of displacement so you will need to repeat this at least 100 times to get a reliable average. 4. Fill in the table. 5. Vary the sandpaper grade and repeat instructions 1 to Plot a graph of your data. 2

3 Activity: Pressure acting at right angles to the fault plane These forces are due to the weight of the overlying rock and to other pressures in the earth. This pressure is simulated by adding weights to the top of the wooden block. 1. Choose the weight. 2. Press the start button. The elastic band will begin to stretch. The stress will increase until the upper block moves. Record the actual movement which is shown on the screen. 3. Press next to make a second movement. There is considerable variation in the amount of displacement so you will need to repeat this at least 100 times to get a reliable average. 4. Fill in the table. 5. Vary the weight repeat instructions 1 to Plot a graph of your data. 3

4 Activity: Stress acting parallel to the direction of movement This is the stress which causes the movement. To measure this the elastic band is replaced by a stress meter and readings of the stress taken immediately before and after each movement. 1. Press the start button. This will start the winch turning. The stress will increase until the upper block moves. Record the displacement, the maximum stress which will be just before the slip takes place and the minimum stress which will be just after the slip. 2. You should record the details of about 100 movements. 3. Calculate the difference in stress for each movement. 4. Plot a graph of maximum stress against the displacement. 5. Plot a graph of the difference between the maximum and minimum stress against the displacement. 6. Plot a graph of the maximum stress against the displacement. 4

5 Activity: Speed of movement of the crust The speed at which the crust of the earth is moving should affect eith the frequency of earthquakes or the size of displacement. 1. Choose the speed. 2. Press the start button. The elastic band will begin to stretch. The stress will increase until the upper block moves. Record the actual movement which is shown on the screen. 3. Press next to make a second movement. There is considerable variation in the amount of displacement so you will need to repeat this at least 100 times to get a reliable average. 4. Fill in the table. 5. Change the speed and repeat instructions 1 to Plot a graph of your results. 5

6 Activity: Variability of displacement The size of the maximum displacement at each movement on a given fault varies greatly. In this activity all the variables, the sandpaper grade, the number of elastic bands, and the weight are kept constant and the displacement at each movement is measured.. 1. Press the start button. The elastic band will begin to stretch. The stress will increase until the upper block moves. Record the actual movement which is shown on the screen. 2. Press next to make a second movement. There is considerable variation in the amount of displacement so you will need to repeat this at least 500 times to acquire enough data to plot a good graph. 3. Record each displacement. 4. Record the number of movements and the total displacement after the last movement so you can calculate the average displacement. 5. Make a tally of the number of displacements of each length. 6. Plot a frequency distribution for the displacements. 7. Record the range and the mode 6

7 Results Activity 1 Elasticity There is a clear relationship between the number of elastic bands and the displacement. The more bands the smaller the average displacement. So the less elastic the crust is the smaller the displacement should be. Activity 2 Friction The coarser the sandpaper the larger the average displacement (60 grade is course, 120 is fine). So the rougher the fault plane the larger the displacement should be. Activity 3 Pressure at right angles to the fault plane, = weight There is no clear relationship between weight added and displacement. Activity 4 Stress acting parallel to the direction of movement A plot of change in stress against displacement shows that there is a good correlation. The greater the change in stress the greater the displacement. If the stress in the rocks could be measured this would give a some idea of the size of next earthquake. Activity 5 Speed of movement The greater the speed the greater the average displacement. Activity 6 Variability of displacement The range of displacements varies from about 4mm to about 45mm. The mode will be about 14mm. The frequency of displacements increases rapidly up to the maximum of about 14mm and then decreases gradually and exponentially. 7

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