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Unformatted text preview: This page last updated on 15-Feb-2011 EENS 1110 Physical Geology Tulane University Prof. Stephen A. Nelson Earthquakes and the Earth's Interior Earthquakes Earthquakes occur when energy stored in elastically strained rocks is suddenly released. This release of energy causes intense ground shaking in the area near the source of the earthquake and sends waves of elastic energy, called seismic waves, throughout the Earth. Earthquakes can be generated by bomb blasts, volcanic eruptions, sudden volume changes in minerals, and sudden slippage along faults. Earthquakes are definitely a geologic hazard for those living in earthquake prone areas, but the seismic waves generated by earthquakes are invaluable for studying the interior of the Earth. In or discussion of earthquake we want to answer the following questions: 1. What causes earthquakes? 2. How are earthquakes studied? 3. What happens during an earthquake? 4. Where do earthquakes occur? 5. Can earthquakes be predicted? 6. Can humans be protected from earthquakes? 7. What can earthquakes tell us about the interior of the earth? Causes of Earthquakes Within the Earth rocks are constantly subjected to forces that tend to bend, twist, or fracture them. When rocks bend, twist or fracture they are said to deform. Strain is a change in shape, size, or volume. The forces that cause deformation are referred to as stresses. To understand the causes of earthquakes we must first explore stress and strain. Stress and Strain Recall that stress is a force applied over an area. A uniform stress is where the forces act equally from all directions. Pressure is a uniform stress and is referred and is also called confining stress or hydrostatic stress. If stress is not equal from all directions then the stress is a differential stress. Three kinds of differential stress occur. Earthquakes & Earth's Interior 2/21/2011 Page 1 of 24 1. Tensional stress (or extensional stress) , which stretches rock; 2. Compressional stress , which squeezes rock; and 3. Shear stress , which result in slippage and translation. When a rock is subjected to increasing stress it changes its shape, size or volume. Such a change in shape, size or volume is referred to as strain . When stress is applied to rock, the rock passes through 3 successive stages of deformation. z Elastic Deformation-- wherein the strain is reversible. z Ductile Deformation-- wherein the strain is irreversible. z Fracture-- irreversible strain wherein the material breaks. We can divide materials into two classes that depend on their relative behavior under stress. z Brittle materials have a small to large region of elastic behavior, but only a small region of ductile behavior before they fracture....
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