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For Teachers

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Can you solve the mystery of the disappearing waves?

Earlier activities explained that when an earthquake occurs seismic waves spread out in all directions and can be felt in places far away.  Many years ago geologists noticed that these seismic waves travel to the far away places by going deep through the interior of the earth.  Some of the P and S waves only travel a few thousand kilometers through the earth before arriving on the surface some distance away from the epicenter.  Other waves travel deeper towards the center of the earth and seem to disappear.  Why do these waves disappear never to reach the surface?

To answer this question, follow along with the experimental procedure below.


OBSERVATIONS

What happens to earthquake waves inside the earth?
To determine why these earthquake waves are disappearing, scientists have to make observations about the earth.  Some possible suggestions are:

  • 1) studying rocks and other material on the surface
  • 2) exploring natural holes, canyons or caverns
  • 3) drilling our own holes
  • 4) studying lava or other material that comes up from underground
  • 5) taking X-ray pictures of the earth
  • 6) studying meteorites, planets or other space objects

All of the methods listed above may help a little.  If the people that think the Earth is like an apple are correct, the deepest holes ever drilled or explored on the earth's surface would only be a tiny scratch on the apple peel.  Additionally, the lava, water and other materials that comes up from underground are not very deep in the earth and would be just under the apple peel. 

To solve the mystery, very deep observations inside the earth must be made.  Would X-rays give a good picture of the inside of the earth?  Probably not since X-rays cannot travel through solid rock.  However, another type of ray or wave can penetrate solid rock.  Can you name some waves that are good at traveling through rock? You guessed it -- waves from an earthquake!


HYPOTHESIS

Earthquake waves will not penetrate through all parts of the earth.


PREDICTION

The earth is made of both solid and liquid layers.  Some earthquake waves will not penetrate non solid or liquid portions of the Earth.  When seismic waves encounter non solid or liquid portions of the earth only P waves will continue to the other side of the earth because S waves do not travel through liquids.


What can seismic waves tell you about the interior of the earth?

Sep 1: Using electronic seismographs, scientists from around the world record data from seismic waves that were generated by the same earthquake.  The same procedure was followed when collecting the data

Step 2: This data was gathered, collected, and presented on one graph.

Step 3: Make some observe from the seismograms collected during the earthquake.  Here are some hints to help you with your observations.

 
June 2, 2007gn="left">Observation 1: How many cities received all three types of waves?  In how many cities did the waves seem to disappear?  

Observation 2: Locate the various cities on a globe or map.  What do you notice about the cities that received all three types of waves compared to the cities that did not receive all the waves?

Step 4: Analyze Clues 1, 2, and 3 below.  Extended learning is for those students who want to be challenged even more.

Clue 1. This is how P and S waves would travel if all 8 cities received them: Image of Eight Cities

Clue 2. Remember that P waves are pressure waves (push-pull) and S waves are shear waves (side-to-side). Both types of waves travel well in solid rock.  When they hit liquid/melted material, P waves continue to travel, but S waves die out completely.

Clue 3. The diagram in clue 1 above did not actually occur. Some waves disappeared. Here is what really happened:
         
P waves    



S waves

Do the diagrams above tell you anything about the interior?

 

EXTENDED LEARNING
Do you want more information? There is another major clue: Wave reflections or echoes.

If a P wave tries to dive straight through the center of the earth, some of the wave energy is able to penetrate the center and arrive on the opposite side of the globe. However, not all of it makes it through. Some of the wave energy bounces backwards before it gets to the center, like a sound wave that echoes off of a wall. By studying these P-wave echoes, geologists think that the P waves are reflecting off of several different layers inside the earth that act like walls. What would make the waves bounce back? This is the next major clue:

         Shadow Zone

Extended Learning Clue. This diagram shows that the four largest echoes seem to happen at depths of about 30 km, 150 km, 400 km, 2900 km, 4600 km.


 



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Last updated on July 4, 2004 2002