1. PHYSICS EXERCISE "EARTHQUAKES"

2. 1.) Focus- This is the point, usually deep underground, where the initial dislocation and energy release occurs. Definitions 2.) Epicenter- This is the point on the Earth's surface directly above the earthquake's focus. It is the point where the waves will have their greatest amplitude. 3.) Primary(P) Waves- These are the longitudinal sound waves generated by the earthquake. They are the fastest moving of all earthquake waves and are capable of propagating through both solids and liquids. 4.) Secondary(S) Waves- These are transverse shear waves. They travel more slowly than "P" waves and can only propagate through solids. The inability of "S" waves to travel through the Earth's outer core has lead seismologists(geologists who specialize in the study of earthquakes) to conclude that this region is liquid. Together the "P" and "S" waves are called body waves since they travel through the Earth.

3. Both P-Waves and S-Waves Recorded Only P-Waves Recorded

4. 5.) Long(L) Waves- These are the waves that travel along the surface of the Earth and are responsible for most of the damage associated with an earthquake. They are the slowest traveling of the three earthquake waves. There are two types: Love and Rayleigh.

5. 6.) Magnitude- The magnitude of an earthquake is a measure of the amount of energy released by the earthquake. It is determined by measuring the largest amplitude of the waves arriving at a seismic recording station, adjusting for the distance between the station and the earthquake's epicenter and then plotting on a Richter Scale. Using the Richter Scale an earthquake is assigned a magnitude of 1 or greater. The Richter Scale is logarithmic so that an increase of one unit on the scale corresponds to a ten-fold increase in the amplitude of the waves detected. An increase of one unit on the scale also corresponds to a 30-fold increase in energy released.

6. For example, an earthquake with a magnitude of "7" released 30 times as much energy as an earthquake with a magnitude of "6" and 900 (30 x 30) times as much energy as one with a magnitude of "5". NOTE: An earthquake with a magnitude of 6.5 releases as much energy as the atomic bomb (20-kilotons) dropped on Hiroshima, Japan at the end of World War II.

8. Part One Determining the Distance to the Earthquake’s Epicenter

9. Because the three types of earthquake waves(P,S,L) travel at different speeds they will require different times to travel to a particular recording station. The "P" waves will arrive first, followed by the "S" waves and then the "L" waves. This difference in arrival times can be used to determine the stations distance from the epicenter of the earthquake. Consider the following analogy: Two cars, A and B are to travel down a long straight track. They will start at the same place and time, but car A will travel at 100 mph while car B will travel at only 50 mph. At certain points down the track observers are stationed and equipped with timers and communications equipment. Observer 1 is placed 100 miles from the starting line, observer 2 is 200 miles from the starting line and observer 3 is at an unknown distance.

12. 100 miles 200 miles x miles

13. 100 miles 200 miles x miles Car A arrives at 100 miles

14. 100 miles 200 miles x miles Difference in arrival times @ 100 miles = 1 hour Car A arrived at 100 miles Car A arrives at 200 miles Car B arrives at 100 miles

15. 100 miles 200 miles x miles Difference in arrival times @ 200 miles = 2 hour Car A arrived at 200 miles Car B arrives at 200 miles

16. 100 miles 200 miles x miles Car A arrives at x miles

17. 100 miles 200 miles x miles Car A arrived at x miles Car B arrived at x miles Difference in arrival times @ x miles = 4 hour x = 400 miles

18. The distance from a seismic recording station to the earthquakes epicenter can be determined by measuring the difference in arrival times of the "P" and "S" waves. Based on the speeds of "P" and "S" waves the following table has been calculated:

19. EXAMPLE: Suppose that a seismograph in Orangeburg detects the arrival of P waves at 11:05:09 am and the arrival of S waves at 11:05:27 am. The difference in arrival times is 18 seconds. According to the table above the earthquakes epicenter was 150 km from Orangeburg.

20. Listed below are the arrival times of the P and S waves at three other seismic recording stations: Problem 1. Determine the distance from each station to the earthquake's epicenter:

21. Part Two Determining the Location of the Earthquake’s Epicenter

22. The distances from the three stations to the epicenter can be used to determine the location of the epicenter. On a map where the positions of the stations are marked, draw circles centered on each station. The radius of each circle represents that station's distance from the epicenter. The region where the three circles intersect is the location of the epicenter. The circles are of course drawn to the same scale as used on the map, in this case 1 cm = 25 km. Problem 2 Using the map provided determine the location of the epicenter. Draw the smallest possible circle enclosing the region where the epicenter is located.

24. Part Three Determining the Earthquake’s Magnitude

25. The amplitudes of the waves reaching a certain seismic recording station will depend on the magnitude of the earthquake and that station's distance from the epicenter. The Richter scale takes these effects into account and provides a means of assigning a magnitude to an earthquake which is independent of the station doing the measurements. Shown below are the seismograph tracings recorded at stations 1,2 and 3.

26. Orangeburg Station Recording 150 km from Epicenter Amplitude = 50mm

27. The Richter Scale Orangeburg Station distance = 150km Amplitude = 50mm Magnitude = 7.4