Download detecting the seismic shadow zone by using sound waves

APEC Youth Scientist Journal Vol. 5
DETECTING THE SEISMIC SHADOW ZONE
BY USING SOUND WAVES
∗
1
Kwang Pyo LEE1
Kangwon Science High School, 2242 Chi-Ak ro, Wonjoo city, Gangwon Province, ROK.
ABSTRACT
The Seismic waves is if it is not the only, the one of the most important part to
understand inner structure of the earth because we can see the structure of inner earth by
detecting the changes of velocity of seismic waves. So, many people deal with seismic waves
importantly, but it is too hard to understand knowledge visually about seismic waves in the
textbook and reference books associated with seismic waves. Especially, many students just
memorize a fact that the seismic shadow zone is a place which both p and s wave can’t reach
but they don’t understand it perfectly. So, making the device, that can detect sound waves
substitute to seismic waves, leads to more deep understanding about seismic waves.
Then, shock the sound wave, or seismic wave, to the device and detect it by a small
microphone so, detect the seismic shadow zone by analyzing sound waves.
1. INTRODUCTION
Seismic waves are main method to investigate the structure of inner earth. There is a
detailed explanation about seismic waves in textbook and reference books, and many geology
teachers and students think it is very important thing to study the structure of inner earth. But
it is very hard to understand many things dimensionally about seismic waves. It is easy for
many students to accept the facts associated with seismic waves but it is hard to understand it
perfectly and concretely. So I think it will be helpful for many students making devices which
can reproduce the situation that seismic waves pass through earth.
For this experiment, I think the thing which can substitute seismic waves. And by
analyzing this, it can help the seismic shadow zone in this experiment.
∗
Correspondence to : Kwang Pyo Lee ([email protected])
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APEC Youth Scientist Journal Vol. 5
2. RESAERCH METHOD
2.1. Making the seismic shadow zone checking device
The earth is divided into four parts: crust, mantle, outer core and inner core. The
density of crust is about 2.7
outer core is 12.2
, the density of mantle is about 3.3
, and the density of inner core is about 12.9
, the density of
. Make the model of
the earth, considering density of substances which are composed of the earth and their
diameter comprising the earth model, because velocity of seismic waves is influenced by
density of substances.
I hoped to make the earth model similar to the real earth however, it is not easy to
make this like the real earth. So, make it maximally similar to the real earth considering the
rate of density and semi-diameter.
Figure 1 : Prepared glue
Figure 3 : Tissues mixed with glue
Figure 2 : The acrylic sphere
Figure 4 : Add tissues to an acrylic sphere
First, make the outer core and inner core using billiard ball that will be the inner core
in the acrylic panel filled with water which can substitute outer core. In detailed, make the
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APEC Youth Scientist Journal Vol. 5
acrylic sphere using two half-sphere and attach by using scotch-tape, connecting these two
half-sphere after put into acrylic sphere billiard ball. Then fill the acrylic sphere with water
through small hole of acrylic sphere. Density of billiard ball is about 1.7
water is about 1
and density of
. A semi-diameter of billiard ball is 3.21cm and a semi-diameter of
acrylic sphere is 10cm.
Second, put tissues in the water densely.
Third, mix it, tissues and water, densely in the water, with glue. Density of dried
tissues with water and glue is 0.34
.
Fourth, add it(mantle and crust), wet tissues mixed with glue, to the prepared acrylic
sphere and, dry it in the constant-temperature oven. This is the perfect earth model and its
semi-diameter is 13.1cm. And set up a protractor on the earth model.
Fifth, give a short-time shock to the earth model, then detect a shock by using a small
microphone and analyzing results by Goldwave program.
2.2. Checking blind zone through sound wave instead of seismic wave
Velocity of sound waves follows upward formula. Although density of solid is bigger
than liquid, bulk strain of solid is bigger than liquid. Thus velocity of sound waves through
solid is bigger than through liquid. So, sound waves are able to substitute seismic waves
which are varying its velocity at boundary between crust and mantle, between mantle and
outer core, between outer core and inner core. I use sound wave instead of seismic wave
because of these reasons.
2.3. Shock the device and checking the blind zone
Give a shock during a short time to the acrylic sphere by the degree 0°, the seismic
shadow zone checking device. Prepared for finding sound waves, the small microphone is
installed on the device by the degree, in turn, 30°, 60°, 90°, 120°, 150°. By using the
Goldwave program which can detect a shock and analyze a wave, find waves by the degree,
in turn, 30°, 60°, 90°, 120°.
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APEC Youth Scientist Journal Vol. 5
3. RESULT AND DISCUSSION
3.1. Shapes of waves detected by the small microphone by the degree 30°, 60°, 90°, 120°.
Each of these four graphs below is wave pattern of shocks detected on the earth
model by the degree 30°, 60°, 90°, 120°. Table 2 is the enlarged pattern of waves of table 1.
Table 1. Detected waves
Figure 5. Degree 30°
Figure 6. Degree 60°
Figure 7. Degree 90°
Figure 8. Degree 120°
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APEC Youth Scientist Journal Vol. 5
Table 2. Detected waves enlarged
Figure 9. Degree 30°
Figure 10. Degree 60°
Figure 11. Degree 90°
Figure 12. Degree 120°
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APEC Youth Scientist Journal Vol. 5
The x-axis of these four graphs above means experiment time(time interval is 0.005s
at table 1), and the y-axis of these four graphs above means intensity of sound, decibel. On
the four graphs, green graph and red graph are the same.
4. CONCLUSION
The seismic shadow zone is an area of the Earth’s surface where seismic waves can’t
be detected. In this experiment, however, the small microphone detected waves because the
earth model is too small. So, I consider first-arrive wave as p wave and an area which detect
one type of seismic wave as the seismic shadow zone.
1) The shape of waves on the earth model by the degree 30° is different between the
first graph belonged in first rectangle and the second graph belonged in the rest of
rectangle. The first graph is faster than the second graph(Because the first wave reach
faster than the second wave.) and The first graph of amount of energy is larger than
the second graph. So the first graph is represented by a wave through the only mantle.
And the second graph is represented by a wave through both mantle and outer core.
Because by the decreasing critical angle, a graph through the only mantle is slower
than waves through both mantle and outer core. Thus, the first graph means a wave
through the only mantle and the second graph means a wave through both mantle and
outer core. And the zone on the earth model by the degree 30°, which both a wave
through the only mantle and a wave through both mantle and outer core pass by, is
not the seismic shadow zone.
2) The shape of waves by the degree 60° is not the same contrast to 30°. The pattern of
the waves above is always the same. It is expected that every waves by the degree 60°
passes only mantle, not outer core. If the small microphone detect the only one wave,
it must be a wave though the only mantle. Because the earth model is so small
contrast to real earth, so a wave through the only mantle must be detected however, a
wave through both mantle and outer core can’t be detected because of water. Thus, it
is possible to think that every waves on the earth model by the degree 60° passes
only mantle, not outer core. Thus, it must be the seismic shadow zone.
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APEC Youth Scientist Journal Vol. 5
3) The shape of waves by the degree 90° is the same compared to 60°. It is expected
that every waves on the earth model by the degree 90° passes only mantle, not outer
core like degree 60°. So it also must be the seismic shadow zone.
4) The shape of waves by the degree 120° is the same compared to 30°. It is also
different between a first graph belonged in first rectangle and the second graph
belonged in the rest of rectangle. The first graph is faster than the second graph, so
the first graph is represented by a wave through both mantle and outer core because
after the critical angle, a wave through both mantle and outer core is faster than a
wave through the only mantle. Also, because the inner a wave go through, the more
velocity of waves is increasing and the more a wave go through many changes, the
more the energy of a wave loses.
5) According to these results 1~4, the blind zone in this earth model is distributed by the
between degree 30° and 60° at least. It is different from the seismic shadow zone of
the real earth which is distributed to the between degree 102° and 140°. This
difference between them results from difference between the materials(These
materials used in this experiment are the best materials I can choose.) and semidiameter of the earth model and the materials of the real earth.
6) Also, I can expect the thing which has different density from the substances in the
earth model is in the acrylic sphere.
7) I can’t draw the fact that the seismic shadow zone of the real earth distributed
between the degree 102° and 140°. However, I can check the possibility that detect
the seismic shadow zone through the earth model.
8) If I make the model bigger than this model, I can detect the seismic shadow zone
precisely.
9) In this experiment, I analyzed only p wave. However if I analyze p and s waves, I
detect the blind zone of p wave and s wave, so I can detect the blind zone similar to
real one.
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APEC Youth Scientist Journal Vol. 5
5. ACKNOWLEDGEMENTS
Through this experiment, I can understand the seismic shadow zone more deeply
than other people. Although angle of generating pot of new seismic shadow zone is different
from original seismic shadow zone, it is not important where the seismic shadow zone
generates. It is possible to make the seismic shadow zone similar to the seismic shadow zone
of the real earth considering density of substances, diameter of these. From two differences
between the earth model and the real earth, a loss of energy and the velocity of the earth
model materials and the real earth are caused. Through these reasons, analyzing of sound
waves can check the existence of seismic shadow zone.
Kwang Pyo LEE
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