BACKGROUND INFORMATION: Racing Waves

How far did the waves race?

An earthquake has just hit. Can you tell how far away the epicenter is located? You first need to know how to interpret a seismogram.

A good seismograph can detect seismic waves from an earthquake thousands of kilometers away. In this activity, you will create a data table and plot the speed of the waves time delay.

The first vibration detected by a seismograph is the primary, or P, wave.  These waves vibrate causing rock particles to move in a back and forth motion.  P waves travel at a speeds between 6 and 13 kilometers per second.  The next wave to reach the seismograph is the secondary, or S, wave.  These waves cause rocks to vibrate at right angles to the direction the wave is traveling (sideways like a slithering snake).  S waves travel at half the speed of a P wave; a speed of between 3.5 and 7.5 kilometers per second.  Although wave speeds vary, the ratio between the average speeds of a P wave and of its following S wave is quite constant at approximately 10 kilometers per second.  This fact enables seismologists to simply time the delay between the arrival of the P wave and the arrival of the S wave to get a quick and reasonably accurate estimate of the distance of the earthquake from the observation station.

Observe your seismogram:  Although the seismic waves that created the zig-zag lines left the epicenter at exactly the same moment, they arrived at different times in different locations. Cities that were close to the epicenter received waves first; distant cities received waves last.

The first wave to arrive is always the Primary (P) wave, (unless the P wave is deflected and never arrives at all).
Each city started recording its seismogram at the exact moment the P wave first arrived there. Thus each city started recording at a time of day different from any of the other cities.  For example, if the quake began at 12:00 p.m., Tokyo received the first P waves at about 12:05 (and the first S waves at about 12:09), but Rio received these same P waves at about 12:20.

Determine on your seismogram precisely how many minutes the first S waves arrived after the first P wave arrived. This is the time delay for S and P waves.  Record this information in the second column of the table.

4.  Set up a graph.  Each line on the y-axis should represent 20 seconds.  Record values up to 660 seconds.  Each line on the x-axis should represent 200 kilometers.  Record values up to 10,000 kilometers.

5.  Graph the following points:  (0 km, 0 min), (3100 km, 4.2 min); (6000 km, 7.5 min); and (8,000 km, 9.5 min).  HINT: (x, y) and remember to convert minutes to seconds before you plot your points!

4.  Using the graph you just created determine how far each city is from the epicenter of the earthquake.

 

Question:

1. Can the graph you have just drawn be used to calculate the distance to any earthquake?  Explain why or why not.
    
2. Which city is closest to the earthquake epicenter?  How far, in kilometers is this city from the epicenter?
    
3. Which of the cities seismographs would be first to detect the earthquake?

All of these waves in all of the cities came from the same epicenter.  In the next activity, Locating the Quake, you will use your table and graph to locate the epicenter of the earthquake.

Work is being done to improve the learning objectives of this site.