Earth's Core |
(Think about these carefully before you consult the answers below)
QUESTIONS
1) Although similar in some ways, seismic probing of the Earth's interior is different from radar and sonar. How is it most different? <answer>
2) Can you think of any analogies between seismic methods and the spectroscopy discussed in chapter 3? <answer>
3) On of the big problems with locating the epicenter of earthquakes is that we must know the speed of the P and S waves through the various kinds of materials they pass through. It would greatly facilitate things if earth materials were the same everywhere -- but of course they are not. Nor is it likely that we will ever find such a situation on another planet. Why not? <answer>
4) Most seismic instruments (at least those that measure sudden Earth movements) utilize a property of nature studied extensively by Galileo and Newton in particular. Do you know what it is? <answer>
5) In an earthquake, which type of wave, P or S, causes the most damage at the surface of the Earth? <answer>
ANSWERS
1) Radar uses electromagnetic waves, which are transverse waves analogous to the S waves of seismic research. Sonar uses acoustic waves, which are analogous to the P waves of seismic research. All techniques use reflected waves, but radar and sonar rely on waves reflected back to the sending apparatus, whereas seismic research relies primarily on seismic waves transmitted to a distant station. Also, both sonar and radar rely primarily on artificially events or signals generated by the equipment, whereas seismic research relies mostly on natural events. However, seismic prospecting for oil and mineral deposits often does rely on signals returned to the sending apparatus, and on artificial events (usually small explosions on or just under the surface of the Earth.) Seismic prospecting is simply sonar for shallow Earth layers.
2) One similarity is that both techniques use waves. Spectroscopy uses electromagnetic waves (light) and seismic research uses waves through solid material. Thus the waves employed are of completely different types. However, both processes utilize the concept of refraction, in which the waves are bent as they pass through different mediums. (For more information, research "Snell's Law.") When we estimated the size of the Earth's core, we made the simplifying assumption that the waves passes straight through the Earth, but in fact they are refracted somewhat.
3) Any object large enough to seriously be considered a planet will most likely be differentiated with the densest materials at the core and least dense materials near the surface. The smaller and less massive the object, the less differentiation is likely to have occurred, and thus perhaps the less complicated the interior structure. Some small asteroids likely are not significantly differentiated at all, whereas larger planets such as earth have several distinct layers, and intermediate objects such as the Moon probably likely have fewer.
4) Although several principles are involved, the dominant one is that of inertia, or the property of material objects to resist changes in their state of motion. When an earthquake motion occurs, the recording mechanism (such as a pen at one end of a pivot arm to which a mass is attached) resists the motion in proportion to the strength of the earthquake. Other types of earthquake motion detectors use other techniques, but the original and most common method is to utilize the inertia of a mass.
5) The greatest damage usually is from the sideways motion of the S waves, which can rip houses, tear large buildings off their foundations, and even alter the course of rivers. P waves cause mostly up and down jostling, which typically is less damaging.
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