1. Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Earth Systems 3209 Unit: 4 The Forces Within Earth Reference: Chapters 4, 15, 16, 19; Appendix A & B

2. Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Locating an Earthquake Epicentre Unit 4: Topic 4.5 Focus on...  describing how seismogram records can be used to locate the epicentre of an earthquake.

3. Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Locating the Epicenter Text Reference: Pages 447 - 448  The source of an earthquake is called the focus, which is an exact location within Earth were seismic waves are generated by sudden release of stored elastic energy.  The epicenter is the point on the surface of Earth directly above the focus. This is the location that scientist calculate. The focus is directly below, however, scientist can not determine its depth.  To locate the position of an earthquakes epicenter, we need a seismogram reading from at least three different seismograph stations and a travel - time graph which shows the speed of both P- and S-waves.

4. Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Step 1: 3 Seismogram Records  Find the difference in arrival time between the P-wave and the S-wave.  To do this refer to each of the three seismogram readings and record arrival times for P-wave and S-wave.  One example is shown below: P-WavesS-Waves Arrival time of P-waves (23 Minutes) Arrival time of S-waves (27 Minutes) Locating the Epicenter

5. Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Step 2: Record Difference in Arrival Times  Subtract the arrival time of the S-wave from the arrival time of the P-wave.  27 minutes – 23 minutes = 4 minutes  Thus, the S-wave arrived 4 minutes after the P-wave.  Next you plot 4 minutes on the travel-time graph. Locating the Epicenter

6. Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Step 2: Record Difference in Arrival Times  Subtract the arrival time of the S-wave from the arrival time of the P-wave.  27 minutes – 23 minutes = 4 minutes  Thus, the S-wave arrived 4 minutes after the P-wave.  Next you plot 4 minutes on the travel-time graph.  Slide the red line representing 4 minutes up between the P-wave and S-wave lines until it fits between the two lines. Locating the Epicenter

7. Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Step 2: Record Difference in Arrival Times  Subtract the arrival time of the S-wave from the arrival time of the P-wave.  27 minutes – 23 minutes = 4 minutes  Thus, the S-wave arrived 4 minutes after the P-wave.  Next you plot 4 minutes on the travel-time graph.  Slide the red line representing 4 minutes up between the P-wave and S-wave lines until it fits between the two lines.  Read off the distance below. Distance = 2500 km. Locating the Epicenter

8. Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Step 3: Record Difference to the Epicenter  The S-P interval tells us the distance to the epicenter from each seismograph station where the earthquake was recorded.  The epicenter of the earthquake is located 2500 km from the first seismograph station.  You need to do the same procedure for two other seismograph stations.  Seismograph station 1 = 2500 km  Assume that the other readings are:  Seismograph station 2 = 3500 km  Seismograph station 3 = 4500km 3500 km4500 km Locating the Epicenter

9. Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Locating the Epicenter Step 4: Use Triangulation to Pin Point the Epicenter:  The distance to the epicenter from each seismograph station is:  Station #1 -- d 1 = 2500km  Station #2 -- d 2 = 3500km  Station #3 -- d 3 = 4500km  At each station we can draw a circle on a map that has a radius equal to the distance to the epicenter from each seismograph station.  Three such circles will intersect in a point that locates the epicenter of the earthquake.

10. Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Example 1: If the distance from two seismograph stations to the epicenter is known, then the epicenter is...? A)midway between the two stations. B)at one of three possible locations. C)at one of two possible locations. D)beneath one of the stations How many seismic stations are needed to locate the position of the epicenter? A)2C) 3 B)4D) 6

11. Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Example 2: As a result of an earthquake, the seismogram below was obtained. (i)What is the distance from the recording station to the epicentre?

12. Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Example 2: As a result of an earthquake, the seismogram below was obtained. (ii)A second recording station located 3500 km from the epicenter recorded the P-wave arrival time at 10:00 AM. At what time did the S-wave arrive at this station?

13. Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Your Turn... Take the time and complete the following questions... (Solutions to follow) Question: With reference to the graph, answer the next two questions: If the difference between the first P wave and the first S wave is 10 minutes how far away is the epicenter of the earthquake? A)about 3000 kmC) about 6000 km B)about 5000 kmD) about 9000 km

14. Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Solutions... Answer: With reference to the graph, answer the next two questions: If the difference between the first P wave and the first S wave is 10 minutes how far away is the epicenter of the earthquake? A)about 3000 kmC) about 6000 km B)B) about 5000 kmD) about 9000 km

15. Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Your Turn... Take the time and complete the following questions... (Solutions to follow) Question: With reference to the graph, answer the next two questions: If an earthquake is 5000 km from a seismic station, the first S wave would arrive at the seismic station in... ? A)about 6 minutesC) about 12 minutes B) about 8 minutesD) about 15 minutes

16. Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Solutions... Answer: With reference to the graph, answer the next two questions: If an earthquake is 5000 km from a seismic station, the first S wave would arrive at the seismic station in... ? A)about 6 minutesC) about 12 minutes B) about 8 minutesD) about 15 minutes

17. Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Summary... Overview of Points covered:  To find the location of an earthquake we use the method of Triangulation.  To do this you need three siesmogram readings from three stations:  Steps include: 1)Calculate the difference in arrival times between P and S waves. 2)Use a travel-time graph to find distances of each station. 3)Draw circles to pin-point the location where three circles intersect. 4)The intersection point is the location of the epicenter!!