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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]) - 55 - 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 - 56 - 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°. - 57 - 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° - 58 - 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° - 59 - 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. - 60 - 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. - 61 - 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 - 62 -