ERSC 1F90 Geology and the environment

Assignment 3 Locating Earthquake Epicentres Introduction An earthquake involves the sudden release of energy that is stored when rocks undergo deformation. When the “strength” of the rock is exceeded by the forces causing deformation the rock breaks and the energy is released in the form of seismic waves or shock waves that radiate out from the center of energy release (a point below the Earth’s surface that is called the focus of the earthquake). The point on the land surface, immediately above the focus is called the epicentre of the earthquake. Three types of seismic wave are produced by an earthquake: Primary Waves (P-Waves) that travel very quickly through rock. Secondary Waves (S-Waves), slower than P-waves, also travel within the crust. Surface Waves, adulatory waves that roll across the ground surface causing damage to structures. P- and S-waves can be detected by seismographs that record the time of arrival of the various waves types and the amplitude of the waves at the location of the seismograph. Because P-waves travel faster than S-waves they arrive at a seismograph first, followed by the S-waves. The difference in time between the first arrival of each wave type depends on the distance of the seismograph from the earthquake focus; the greater the distance the greater the difference in times of arrival at a location away from the epicentre. The amplitude of both P- and S-waves depends on the magnitude of the earthquake (a measure of the energy released by the Earthquake) and the distance from the focus. For a given distance, as the earthquake magnitude increases the amplitude of the waves increases and for a given earthquake magnitude, the amplitude diminishes with increasing distance from the epicentre. In this assignment you will use the time of arrival of P- and S-waves at three seismograph locations to determine the location of the epicentre of the earthquake and the amplitude of the S-waves (along with the distance) to determine the magnitude of the earthquake. Materials needed: Coloured pencils (red, black, blue) A compass for drawing circles. A copy of the Seismographic Charts (available as PDF files at the Assignment 3 web site) A copy of the Assignment 3 Answer Sheet (also available at the Assignment 3 web site) Instructions: Before beginning the assignment you would be wise to go through the tutorial that is available on the Assignment 3 web page from which you downloaded this document.

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1. Download and print the “Seismographic Charts” and the “Assignment 3 Answer Sheet” from the directory at the course web site (both are PDF files that should provide the correct scales). When you print it out make sure that the scale on the map is 5 cm long (1 cm = 100 km). If you cannot print it out at this scale pick up an answer sheet in class. 2. Using the seismic charts, determine the duration of time between the arrival of the first P-wave and the first S-wave for each of the three locations. 3. For each seismic chart, determine the amplitude of the S-waves for each of the locations. You should cut out the scale from the top right corner of the page to use to measure the amplitude. Using this scale ensures the correct measurement just in case the scale of your printout is not quite the same as the original. 4. Plot the value for the time difference (S – P) for each of the three locations on the appropriate scale on the top portion of the “Assignment 3 Answer Sheet”. Use a red point for St. Catharines, a black point for Toronto and a blue point for Owen Sound. Using the same colour scheme, plot the amplitude of the S-waves for each location on the appropriate scale (labeled “Amplitude”). 5. Join the two points (one on the S-P scale and the other on the Amplitude Scale) for each location (with a line of the same colour as the points) to determine the Magnitude of the Earthquake. 6. From the plot on the S-P scale determine the Distance to the Epicentre (from the “Distance” scale) for each location. 7. On the map that appears on the bottom part of the Assignment 3 Answer Sheet, use a compass to draw a circle around each location (St. Catharines, Toronto and Owen Sound). The radius of each circle must be the distance from the epicentre that was determined from Step 6 (so that the diameter of the circle is, obviously, twice that distance). Use the scale on the map to estimate the diameter of the circle or use the conversion of 1 cm = 100 km on the map (which equates to a scale of 1 mm = 10 km). 8. On the same map, draw a red dot at the earthquake’s Epicentre and write the magnitude to the right of that dot (e.g., if the magnitude were 4.2, write M = 4.2). 9. Hand in only the Assignment 3 Answer Sheet by1:00pm on August 6, 2009..