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Finding the Epicenter of an Earthquake (student copy) 1. The graph on page 188 shows how the difference in arrival times between P waves and S waves depends on the distance from the epicenter of the earthquake. Find the difference in arrival time for Denver on the y-axis of the graph. Follow this line across to the point at which it crosses the curve. To find the distance to the epicenter, read down from this point to the x-axis of the graph. Enter this distance in the data table. 2. Repeat Step 1 for Houston and Chicago. 3. Use the scale to set your compass at a radius equal to the distance from Denver to the earthquake epicenter that you recorded in your data table. 4. Draw a circle on the map with the radius determined in Step 3 using Denver as the center. (Hints: Draw your circles carefully. You may need to draw some parts of the circles off the map.) 5. Repeat Steps 3 and 4 for Houston and Chicago. Data Table City Difference in P and S Wave Arrival Times Denver, Colorado 2 min 40 sec Houston, Texas 1 min 50 sec Chicago, Illinois 1 min 10 sec Distance to Epicenter (kilometers) 0 0 200 200 400 400 600 600 800 1,000 1,200 800 1,000 1,200 1,400 1,600 1,800 2,000 1,400 miles kilometers Seattle San Francisco New York Chicago Salt Lake City Denver Savannah Houston Miami Analyze and Conclude Answer the following questions on a separate sheet of paper. 1. Drawing Conclusions: Observe the three circles you have drawn. Where is the earthquake's epicenter? 2. Measuring: Which city on the map is closest to the earthquake epicenter? How far, in kilometers, is this city from the epicenter? 3. Inferring: In which of the three cities listed in the data table would seismographs detect the earthquake first? Last? 4. Estimating: About how far from San Francisco is the epicenter that you found? What would be the difference in arrival times of the P waves and S waves for a recording station in San Francisco? 5. Interpreting Data: What happens to the difference in arrival times between P waves and S waves as the distance from the earthquake increases? 6. Communicating: Review the procedure you followed in this lab and then answer the following question. When you are trying to locate an epicenter, why is it necessary to know the distance from the epicenter for at least three recording stations? More to Explore You have just located an earthquake's epicenter. Find this earthquake's location on the map of Earthquake Risk in the United States in you textbook (pg 197). What is the risk of earthquakes in the area of this quake? This area is not at a boundary of earth's lithospheric plates (pg 159). Why is this area a risk for earthquakes? Finding the Epicenter of an Earthquake (teacher's copy with solutions and answers) 1. The graph on page 188 shows how the difference in arrival times between P waves and S waves depends on the distance from the epicenter of the earthquake. Find the difference in arrival time for Denver on the y-axis of the graph. Follow this line across to the point at which it crosses the curve. To find the distance to the epicenter, read down from this point to the x-axis of the graph. Enter this distance in the data table. 2. Repeat Step 1 for Houston and Chicago. 3. Use the scale to set your compass at a radius equal to the distance from Denver to the earthquake epicenter that you recorded in your data table. 4. Draw a circle on the map with the radius determined in Step 3 using Denver as the center. (Hints: Draw your circles carefully. You may need to draw some parts of the circles off the map.) 5. Repeat Steps 3 and 4 for Houston and Chicago. Data Table City Difference in P and S Wave Arrival Times Distance to Epicenter (kilometers) Denver, Colorado 2 min 40 sec 1,600 Houston, Texas 1 min 50 sec 1,000 Chicago, Illinois 1 min 10 sec 600 0 0 200 200 400 400 600 600 800 1,000 1,200 800 1,000 1,200 1,400 1,600 1,800 2,000 1,400 miles kilometers Seattle San Francisco New York Chicago Salt Lake City Denver Savannah Houston Miami Analyze and Conclude Answer the following questions on a separate sheet of paper. 1. Drawing Conclusions: Observe the three circles you have drawn. Where is the earthquake's epicenter? The epicenter is located on the border of western Tennessee and Kentucky, close to the border and "boot" of Missouri. 2. Measuring: Which city on the map is closest to the earthquake epicenter? How far, in kilometers, is this city from the epicenter? Chicago is closest to the epicenter, approximately 600 kilometers away.. 3. Inferring: In which of the three cities listed in the data table would seismographs detect the earthquake first? Last? The first of the three cities to detect the earthquake would be Chicago; the last city to detect the earthquake would be Denver. 4. Estimating: About how far from San Francisco is the epicenter that you found? What would be the difference in arrival times of the P waves and S waves for a recording station in San Francisco? San Francisco is approximately 2,900 kilometers from the epicenter. The difference in arrival times would be approximately 4 minutes 25 seconds. 5. Interpreting Data: What happens to the difference in arrival times between P waves and S waves as the distance from the earthquake increases? The difference in the arrival times increases as the distance from the epicenter increases. 6. Communicating: Review the procedure you followed in this lab and then answer the following question. When you are trying to locate an epicenter, why is it necessary to know the distance from the epicenter for at least three recording stations? The earthquake epicenter can be anywhere on the circumference of the circle drawn for the first city. The earthquake detected by the second city, combined with the detection by the first city, narrows the epicenter to the two common points shared by each circumference. The detection by the third city has only one of the two points in common and identifies the epicenter. More to Explore You have just located an earthquake's epicenter. Find this earthquake's location on the map of Earthquake Risk in the United States in you textbook (pg 197). What is the risk of earthquakes in the area of this quake? This area is not at a boundary of earth's lithospheric plates (pg 159). Why is this area a risk for earthquakes? The earthquake occurred in an area of moderate risk. (This area is known as the New Madrid fault.) Scientists believe that this area of the United States has faults, or fractures, in the earth's lithosphere that never turned into a complete lithospheric plate. You can think of these as partial, or incomplete, "cracks". Difference in Arrival Time of P and S waves (min) 9 8 7 6 5 4 3 2 1 1000 2000 3000 4000 5000 Distance to Epicenter (km) 6000 7000 Difference in Arrival Time of P and S waves (min) 9 8 7 6 5 4 3 2 1 1000 2000 3000 4000 5000 Distance to Epicenter (km) 6000 7000