Seismic - GEOL 114 The Earth's Dynamic Interior Lecture...

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GEOL 114 The Earth's Dynamic Interior Lecture Notes (Copyright © 2005 by Jeffrey S. Barker) 2. Seismic Waves Introduction The Earth is our laboratory, but its interior is basically inaccessible. The deepest boreholes penetrate to less than 12 km, which is less than 0.2% of the distance to the center of the Earth. Therefore, we must resort to remote sensing methods (measurements made at the surface that suggest or imply what is happening at depth), such as: The Shape and Rotation of the Earth; Gravity; the Magnetic Field; Heat Flow; Geology; and Seismology Seismology involves the propagation of seismic waves through the Earth, and provides the most detail of any of these measurements. Therefore, we will spend a bit of time on seismology, then use the results as our first information on each portion of the Earth's interior. There are many sources of seismic waves, including: earthquakes, underground nuclear explosions, volcanic eruptions, meteorite impacts, hammers, trucks and cars, weather and ocean waves. Energy travels away from the hypocenter by temporarily deforming the surrounding medium (rock). This deformation eventually reaches the Earth's surface, where it may be recorded on a seismograph . A seismograph is simply a pendulum which records the relative motion of the ground. We usually think of a pendulum swinging, but in the case of a seismograph, the case or frame moves with the GEOL 114 1 2. Seismic Waves
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ground, while the inertia of the pendulum keeps it still. The relative motion is the same. There are usually three seismographs at each station: one recording up-down motion, one recording north-south motion, and one recording east- west motion. The records of ground motion through time at the station are called seismograms . GEOL 114 2 2. Seismic Waves Seismograms recorded at station BNY in the ground floor of the Science I building, Binghamton University.
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There are three types of waves in a solid material: P waves - Primary waves or "Pressure waves", travel by compressing and dilating the material (like sound waves). S waves - Secondary waves or "Shear waves", travel by deforming the shape of the material. Surface waves - These are actually due to the interference of P waves and S waves. They travel along the surface of the Earth, and their motion is similar to that of ocean waves. Seismic Wave Velocities The speed or velocity at which a seismic wave travels depends on the resistance of the material to being deformed . For example, a P wave in a fluid (like air or water) is caused by a change in pressure (for example, your vocal cords compress the air when you speak). The fluid resists this pressure and presses on the neighboring volume of fluid, compressing it. That volume resists being compressed and presses on the next volume, and so forth so that a pressure wave travels outward in all directions. The speed at which this process occurs depends on the incompressibility of the material, which is a
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This note was uploaded on 11/02/2010 for the course GEOLOGY 11 taught by Professor Howardr.naslund during the Spring '10 term at Binghamton University.

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Seismic - GEOL 114 The Earth's Dynamic Interior Lecture...

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