**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:
- Can the graph you have just drawn be used to calculate
the distance to any earthquake? Explain why or why
not.
- Which city is closest to the earthquake epicenter?
How far, in kilometers is this city from the epicenter?
- 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.
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