Lab_8_transparent.pdf

Lab #8: Epicenter and magnitude of an Earthquake A) Purpose of the assignment: In Lab #7, you have learned how to use differences in time of arrival to get distances and found the house of Polly and Salim without knowing where they were coming from. In this lab, you will use measured data to find the epicenter of a real earthquake using the fact that P waves are travelling faster than S waves. You will also calculate the magnitude of the earthquake using amplitude measured on the seismograms. B) Learning objectives and skills • carefully read instructions • read graphs and extract data from them • apply basic graphical and mathematic skills • provide clear explanation of reasoning and method used C) Tasks Part I: Locating the Epicenter There are hundreds of seismic stations throughout the world. In order to locate the epicenter of an earthquake, you need to estimate the time interval between the arrival of the P and S waves, called the S-P interval, on the seismograms (e.g., Figure 1) from at least three different stations (remember Lab#7). The S-P interval is then converted into a distance to the epicenter. Figure 1: Example of seismogram. Here vertical blue and black lines are spaced at 2-second intervals so that the S-P time interval is about 36 seconds.

You can now determine the distance from each seismic recording station to the earthquake's epicenter using the known times of travel of the S and P waves. Figure 2: S and P wave travel-times graphed versus distance, as well as the variation in distance with the difference of the S and P travel times. 1) Approximately, how long does it take for an S wave to travel 300 kilometers? (2 points) 2) Given your answer to question 1), what is the average velocity of S waves in kilometers per second? Explain your calculation. What would be the velocity of S waves in miles per hour (1 mile = 1.61 km)? (2 points) 3) How many times faster are the P waves compared to the S waves? Explain your calculation. (2 points)

4) You determined that the S-P interval is 36 seconds on the seismogram shown in Figure 1. Using Figure 2, what does this S-P interval correspond to in terms of distance? (2 points) 5) Figures 3 and 4 show a map of the Southwestern part of the United States as well as three seismograms recorded during an earthquake that occurred in the area. Using these seismograms together with what you have learned in Lab #7 and in question 1, and the expanded part of Figure 2 provided in Figure 3, locate the epicenter of the earthquake on the map shown in Figure 4. Explain the different steps of your reasoning. (6 points)

Las Vegas, NV seismograph station Eureka, CA seismograph station Elko, NV seismograph station Figure 3: (Top) Three seismograms that recorded the earthquake and will allow you to locate the epicenter on the map provided in Figure 4. (Right) expended part of Figure 2 to give you better precision to determine the location of the earthquake. 6) Using just the amplitude (i.e., the vertical axis), which seismograph station is probably closest to the epicenter? Assume all three stations are located on the same type of rocks and that all rocks and Earth’s layers traversed by the seismic waves between the earthquake and all three stations are homogeneous. (2 points)

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