eqcauses

# eqcauses - EQ Causes and Measurements Page 1 of 14 EENS 204...

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This page last updated on 20-Jan-2006 EENS 204 Natural Disasters Tulane University Prof. Stephen A. Nelson Earthquakes: Causes and Measurements Most earthquakes occur along zones where the Earth's crust is undergoing deformation. Deformation results from plate tectonic forces and gravitational forces. The type of deformation that takes place during an earthquake generally occurs along zones where rocks fracture to produce faults. Before we can understand earthquakes, we first must explore deformation of rocks and faulting. 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 or strain (change shape or size). The forces that cause deformation are referred to as stresses. To understand rock deformation we must first explore stress and strain. Stress and Strain Stress is a force applied over an area. One type of stress that we are all used to is a uniform stress, called pressure. A uniform stress is where the forces act equally from all directions. In the Earth the pressure due to the weight of overlying rocks is a uniform stress and is referred to as confining stress . If stress is not equal from all directions then the stress is a differential stress. Three kinds of differential stress occur. 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. Page 1 of 14 EQ: Causes and Measurements 1/20/2006

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Stages of Deformation 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. z Ductile materials have a small region of elastic behavior and a large region of ductile behavior before they fracture. How a material behaves will depend on several factors. Among them are: z Temperature - At high temperature molecules and their bonds can stretch and move, thus materials will behave in more ductile manner. At low Temperature, materials are brittle. z Confining Pressure - At high confining pressure materials are less likely to fracture because the pressure of the surroundings tends to hinder the formation of fractures. At Page 2 of 14 EQ: Causes and Measurements 1/20/2006
low confining stress, material will be brittle and tend to fracture sooner. z

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eqcauses - EQ Causes and Measurements Page 1 of 14 EENS 204...

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