Earthquake Lab 15.1 Distance to Epicenter and Location of Epicenter Objectives: Materials: To learn how to determine distances from seismographs to epicenters To learn how to determine the location of epicenters using 3 or more seismographs Circle compass Map P-S Waves Velocity Chart/Graph (separate sheet) Scrap Paper Procedure Part A: Distance to Epicenter 1. Use the P-S Waves Velocity Chart/Graph to help answer the following questions (see Part A Questions) 2. For: a. P-wave arrival time (Question 1) i. Go to distance measurement on horizontal axis ii. Go up to O-wave curved line. Stop. iii. Now go left to time axis iv. Read time (each line is 10 seconds) b. S-wave arrival time (Question 2) i. Go to distance measurement on horizontal axis ii. Go up to S-wave curved line. Stop. iii. Now go left to time axis iv. Read time (Each line is 10 seconds) c. P-wave epicenter to seismograph distance (Question 3) i. Go to time measurement on vertical axis ii. Go right to P-wave curved line. Stop. iii. Go down to distance axis iv. Read distance (km) d. S-wave epicenter to seismograph distance (Question 4) i. Go to time measurement on vertical axis ii. Go right to S-wave curved line. Stop. iii. Go down to distance axis iv. Read distance (km) e. Distance from epicenter to seismograph using P-S arrival time differences (Question 5)
i. Using scrap paper, mark time difference from vertical time axis ii. Now take the scrap of paper and move it so the two mark for time difference fits exactly between the P-wave curve and the S-wave curve iii. Go down to distance axis iv. Read distance (km) f. Time from epicenter to seismograph if distance is known (Question 6) i. Go to distance measurement on horizontal axis ii. Go up to P-wave and S-wave curved line iii. Measure the vertical distance between the two curved lines with a scrap paper iv. Put scrap paper along time axis to get time reading Questions Part A: (10 marks) 1. How much time does it take for a P-wave to get to the seismograph from a distance of a. 1000km: min b. 3500km: min c. 8000km: min 2. How much time does it take for a S-wave to get to the seismograph from a distance of a. 2000km: min b. 4500km: min c. 8000km: min 3. How far is the epicenter away if the P-wave arrives in minutes? a. 3 min: km b. 5 min: km c. 7:48 min: km 4. How far is the epicenter away if the S-wave arrives in minutes? a. 3 min: km b. 10:12 min: km c. 22:24 min: km 5. How far is the epicenter away if the difference in the P-wave/S-wave arrival times is minutes? a. 2 min: km b. 3:24 min: km c. 5:30 min: km 6. What is the difference in the P-wave/S-wave arrival times if the epicenter is km away? a. 1800 km: min b. 500 km: min c. 3000 km: min Procedure Part B-1: Location of Epicenter #1 (7 marks) 1. Get a circle compass. What the sharp tip! Be very, very careful! 2. On the P-S Waves Velocity Chart/Graph you are given the time difference between P-wave and S-wave arrivals. Determine the distance to the epicenter from each seismograph (like Question 5 in Part A). (3 marks) 3. Go to the map of North America ( Part B-1 Location of Epicenter #1 ). Look at the bottom of the page and refer to the map scale 4. Now put the sharp, pointy, needle tip on the 0 of the scale.
5. Open the compass to the distance to epicenter you calculated for Chicago (until the pencil part is at the correct distance). 6. Lift compass and place the sharp, pointy, needle tip on Chicago. 7. Make a circle around Chicago (1 mark) 8. Repeat steps 3 to 6 for Saskatoon (1 mark) 9. Repeat steps 3 to 6 for Vancouver (1 mark) 10. The three circles should intersect at one point or area (a small triangle). This is your epicenter. (Give yourself a pat on the back if this is what happened. If not, repeat steps 2 to 9.) 11. Mark epicenter with a RED dot. (1 mark) Procedure Part B-2: Location of Epicenter #2 (9 marks) 1. Repeat all the steps (2-11) from procedure part B-1. Use the map called Part B-2 Location of Epicenter #2. Procedure Part B-3: Location of Epicenter #3) (10 marks) 1. Measure the time lag difference between P-wave and S-wave arrival times and record in table. 2. Repeat all the steps (2-11) from procedure Part B-1. Use the map called Part B-3 Location of Epicenter #3. 3. Return circle compass to rack and answer questions below. Questions Part C: (5 marks) 1. Where is the epicenter of the earthquake for B-1 (state or province)? 2. Where (west, east, south, north, above, or below) might the focus be then for B-1? 3. Why might all the circles not cross at the same point? 4. What possible errors (name at least two) might be encountered in determining the earthquake s epicenter? a. b. BONUS: Why do the P-wave and S-wave time-distance curves do just that, curve down suggesting an increase in wave velocity?