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Some questions about Earthquakes

Geol 1202 Planet Earth Assignment - Earthquake Hunters Seismologists study seismic waves not only so as to be able to locate earthquakes but to also better understand the internal structure of the planet. Seismic waves can be generated, not only from tectonic activity but also from man-made sources such as explosions. The petroleum industry spends millions of dollars each years on seismic surveys to help locate potential reservoirs. But we are not looking for oil, instead we are going to locate the epicentre of a hypothetical earthquake. Note: Before attempting this assignment you should have read the lecture and understand the concepts covered. Recall from the section on locating earthquakes that you can use the difference in arrival time between P and S waves to determine the distance of the recording station from the epicentre of the quake; and how, with three distances you can triangulate on the epicenter. This is just what you are going to do in this assignment. But first an example. Finding the time difference Seismic waves are recorded on a seismograph - these can be as simple as a pendulum and a sheet of paper, to large digital recorders. The traditional seismograph is simply a rotating drum with a recording stylus (although most recordings are digital now). The time difference is the interval of time between the first arrival of the P and S waves. P waves travel fastest and so will be the first wave to arrive at the recording station. The slower S waves arrive later and the difference in the arrival times is a reflection of the distance of the recording station from the epicentre. The figure below is an illustration of a section of paper from a seismograph (called a seismogram) The P-wave first arrival is at 9:00 and the S-wave is at 9:05. So the time difference is 5 minutes. paper moves this way increasing time recording stylus (pen) 9:10 UTC 9:05 UTC S-wave first arrival 9:00 UTC P-wave first arrival Finding the distance from the epicentre The easiest way is to mark on the edge of a piece of paper the distance represented by your time difference (5 minutes in our example). This line represents 5 minutes on our chart 3000 4000 14 13 12 11 S-wave curve 10 9 Find the location on the chart where the distance between the 2 curves is 5 minutes apart 8 7 6 5 P-wave curve 4 This is the distance of that particular recording station from the epicentre of the quake. 2000 16 Move the paper along, keeping it vertical, until the distance between the 2 curves matches the distance on your paper. Then draw a line straight down at this point to find the distance in miles. Distance (km) 15 Time (minutes) Next you look on the Distance Chart and find the distance at which the time between the P and S arrivals matches yours. 1000 17 3 2 1 This point 0 represents 500 1000 1500 2000 2500 3000 time zero. Distance (miles) The 2 waves are generated at the At 500 miles the S and P waves epicentre. are almost 2 minutes apart Find what distance from the epicentre this represents Locating the epicentre Suppose our example recording station was located in Halifax. Draw a circle centered on the station. The radius should be equal to the distance from the epicentre (2100 miles in our example). The scale bar at the bottom of the map figure allows us to set the correct radius. The circle then represents the locations where the seismic waves could have originated. radius = 2100 miles 0 Use the scale for the figure to find the distance of 2100 miles This will be the radius of your circle. 1000 2000 miles 3000 4000 Two other locations will give us three circles that intersect - and therefore the epicenter of the quake. The coordinates can be read from the map figure - in our example that is: Latitude 35 degrees and Longitude 100 degrees. 35 degrees radius = 2100 miles the point where three circles intersect is the epicentre of the quake 100 degrees 0 1000 2000 3000 4000 miles What do I do? I have provided three seismograms for an fictitious earthquake. Your task is to use this information to locate the epicentre. To accomplish this you will need a ruler, a drawing compass and a keen alert mind. (Ok, you don't need the last but it would be a help) What do I submit? ! ! ! ! ! the arrival times of the first P and S waves for each station. the elapsed time between the first P and S waves for each station. your calculation of the distance from the epicentre for each station. the approx. latitude and longitude of the epicentre of the quake. This will be one set of coordinates. your annotated map figure with the three circles - each centered on the appropriate city Note: We will be using miles rather than kilometres in this exercise to preserve the correct distances. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1500 2000 Distance (miles) 1000 2500 3000 Then use this table to determine the distances of each station from the epicentre. 500 P-wave curve S-wave curve 4000 Mexico Seattle New York increasing time 9:00 UTC 9:00 UTC 9:00 UTC 15 3000 9:05 UTC 9:05 UTC 9:05 UTC 16 2000 Distance (km) 9:10 UTC 9:10 UTC 9:10 UTC 17 Time (minutes) 1000 Use these seismograms, recorded at 3 different stations, to determine the arrival times of the P and S waves. Then calculate the time difference. 0 0 1000 miles 2000 2000 miles Mexico City 1000 Seattle 3000 3000 4000 4000 New York Finally use the map to locate the epicenter. Do this by drawing a circle, centered on each station, with a radius equal to the distance from the epicentre that you determined earlier, Your three circles should then intersect at one point - the epicentre. This is the point you record the coordinates for. You must also sumbit an image of this annotated map figure. Student Number: Name:

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The arrival time of first P wave for station 1...

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