you solve the mystery of the disappearing waves?
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?
this question, follow along with the experimental procedure below.
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
- 1) studying rocks
and other material on the surface
- 2) exploring
natural holes, canyons or caverns
- 3) drilling our
- 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
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.
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!
waves will not penetrate through all parts of 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.
can seismic waves tell you about the interior of the earth?
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
2: This data was gathered, collected, and presented on one graph.
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?
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?
4: Analyze Clues 1, 2, and 3 below. Extended learning is
for those students who want to be challenged even more.
1. This is how P and S waves would travel if all 8 cities
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.
3. The diagram in clue 1 above did not actually occur. Some
waves disappeared. Here is what really happened:
Do the diagrams
above tell you anything about the interior?
Do you want
more information? There is another major clue: Wave reflections
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:
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.