If an earthquake occurs in Washington, D.C. (USA), will you feel the shock waves? If you are too far away, you will not be able to feel the ground shaking. However, a good seismograph will detect the earthquake. In fact, seismographs all over the world will measure the waves from this earthquake. How much time will these waves need to get from Washington to you? The chart below tells you.

At the moment the earthquake occurs, 'Primary' (P) waves and 'Secondary' (S) waves begin racing away from the epicenter in all directions. Since the P waves are faster, they reach cities around the world before the S waves can get there. For example, according to the chart, P waves will travel from Washington to San Francisco in 7.0 minutes, but the S waves will travel from Washington to San Francisco in 12.7 minutes. The S waves arrive 5.7 minutes later than the P waves (12.7 - 7.0). Which waves arrive first in Moscow: the P waves or the S waves? You can easily guess: the P waves are first to reach Moscow (11.3 minutes), and then the S waves finally show up at 20.7 minutes (9.4 minutes later than the P waves).

Now it is your turn to be the geologist:

1. Make a graph of the P-wave information in the table. Let distance from the epicenter (Washington) be the horizontal axis (sideways). Let travel time from the epicenter be the vertical axis (up and down). (Travel time is measured as the number of minutes after the quake originally occurred.)

2. On the same set of axes, plot the S-wave travel time in the same way. (Distances from the epicenter will be the same numbers as for P-waves.)

3. Connect the P-wave points to make a smooth curve. Then connect the S-wave points. Make both lines as smooth as possible without any bending.

All earthquakes send out waves like the P and S waves sent out from the Washington quake. These waves arrive in cities all around the world. If you graph the waves from any typical earthquake, your graph will have the same shape as the graph you saw from the Washington quake.

1.Think about the wave speeds:

- a) Which waves always seem to arrive first -- P waves or S waves?
- b) When are the S waves far behind the P waves -- when they arrive at a city close to an earthquake or a city far from an earthquake?

2. Suppose you have a seismograph 2000 kilometers away from an earthquake epicenter.

- a) How many minutes after the quake occurs will the P waves reach your seismograph?
- b) How many minutes after the quake occurs will the S waves reach your seismograph?
- c) Which waves will reach you earlier -- the P waves or the S waves?
- How many minutes earlier?

3. How many minutes apart will the P waves and the S waves arrive for an earthquake that is...

- a) 5000 km away from your seismograph?
- b) 8000 km away from your seismograph?

4. Now suppose your seismograph suddenly begins recording some P waves. Then exactly 4 minutes later it begins recording some S waves. How far are you from the epicenter of the quake? Explain how you found your answer.

Return to the main lesson ("Earthquake!").

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