Chapter 5 - Chapter 5 Seismic Energy Stress/Strain diagram...

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Chapter 5 Seismic Energy · Stress/Strain diagram - the traditional sketch expression of the relationship between the stress energy imposed upon a rock and the strain (or deformation) of it. The diagram – the result of experiments in labs – tells us that rocks respond in two different manners when subjected to stress – prior to fracturing, that is… · Principle of elastic deformation/rebound - stress – prior to fracturing, that is. At low applications of stress, the rock will deform but as soon as you release the stress, the deformation disappears; that response is termed „elastic . If, however, you impose still greater stress, the rock will deform to the point that, when the stress is released, not all the deformation disappears; that s termed„plastic deformation . Eventually, obviously, even greater stress results in rock fracture – and, as we said above, that s what produces an earthquake because the stress is released suddenly. Bear in mind that cold and very brittle rocks (i.e. those very close to Earth s surface) tend to have virtually zero elasticity and break with the application of rather little stress. Rocks that are more deeply buried beneath surface (and thus warm or even hot) will normally survive more stress before rupturing. A good illustration of the release of energy across a major fault is seen in Figure 4 (illustration of principle of elastic rebound); here, the progressive deformation is obvious from the increasing curvature of the fence, and following rupture both sides of the fence again are straight but offset. · Definitions: focus, epicenter, fault Let s create a hypothetical earthquake – in a nice rural region where there ll be no dramatic damage – by release of energy that produces a reverse fault ( and the surface evidence is a scarp or cliff on the uplifted side) (Fig.5). It s pretty rare (impossible?) to find rock that s uniformly homogeneous, so some spot along the fracture plane is weaker than any other spots – and that s where the failure (or rupture) will start. That exact point is called the focus (sometimes called the hypocenter ) ; the point at surface exactly vertically above the focus is called the epicenter . The failure on the fault plane will be greatest right at the focus, and will gradually fade out in all directions along the plane as the energy of the release is consumed by moving the rock blocks. The greater the release of energy, the further the blocks move, the larger the fault, and the larger the earthquake. The seismic energy waves travel outward from the focus in all directions, but there are different kinds of motion to those waves, some travel through anything, some get totally bogged down in liquid, and others travel only along surface. · Body waves: P-waves, S-waves
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This note was uploaded on 11/01/2010 for the course EARTH SCIE 2010 taught by Professor Neil during the Fall '10 term at UWO.

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Chapter 5 - Chapter 5 Seismic Energy Stress/Strain diagram...

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