Seismic location model
The string of truth
Material
Assembly
Tape one string between the legs of a person so that the person can make 1/2 meter long steps. (Use the ape losely so if the person needs to step to the side in an emergency they can easily break free of the string.
Tape another string between the legs of a second person so that they can take 1 meter long steps.
To Do and Notice
Start the people side by side.
The third person claps once per second.
At each clap the two string people take one step forward toward the fourth person standing at some distance. Perhaps 10 meters away.
The person with the longer string has a higher velocity than the person with the shorter string. The long string person goes 1 meter per second, the shorter string person goes 1/2 meter per second.
When the first string person arrives at the observer location the observer starts counting claps.
When the second string person arrives the observer stops countng.
In the case where the observer is 10 meters away the observer will count 10 claps between the arrival times of the walkers.
What's Going On?
Seismologists use seismometers to measure the time of arrival of seismic waves. P waves travel faster than S waves. Just like our 1 meter string person walks faster than ourt 1/2 meter string person.
By knowing the velocity of P and S waves and timing the difference between their arrival times the seismologist can estimate the distance to the epicenter of the earthquake.
Math Root
If the speed of the P wave is vp, the speed of the s wave is vs, and the distance to the epicenter is d then the time interval, t, between the P and S wave arrival is
t = d/vs - d/vp
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Scientific Explorations with Paul Doherty |
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10 July 2002 |