![]() ![]() When the elastic limit is reached (point X), if stress continues to accumulate as strain, the rocks will deform plastically, and will not return to their original shape if the stress is released. As stress and strain increase, rocks first experience elastic deformation, and will return to their original shape if the stress is released. Figure 8.16 | A stress and strain diagram. Rocks tend to deform in a more plastic manner at depth, and in a more brittle manner near Earth’s surface. Rocks behave very differently at depth than at the surface. The deformation that results from applied stress depends on many factors, including the type of stress, the type of rock, pressure and temperature conditions (e.g., rocks deeper in the crust will be subject to higher pressures and temperatures), and the length of time the rock is subjected to the stress. With plastic deformation, the rocks do not return to their original shape when the stress is removed. An example of brittle behavior is a hammer hitting glass, which of course shatters the glass. Plastic deformation may lead to the rocks bending into folds, or if too much strain accumulates, the rocks may behave in a brittle manner and fracture. ![]() This elastic behavior continues until the rocks reach their elastic limit (e.g., point X on Figure 8.16), at which point the rock will begin to deform plastically. Initially, as rocks are subjected to increased stress which begins the process of strain, they behave in an elastic manner, meaning they return to their original shape after deformation ceases (e.g., Figure 8.16). ![]() Source: Randa Harris (2015) CC BY-SA 3.0 view sourceĪpplying stress creates a deformation in the rock, known as strain. This beam is experiencing tensional stress, and rocks have very little strength when exposed to such stress. Why did the Romans use so many vertical columns to hold up the one horizontal beam? If the horizontal beam spanned a long distance without support, it would buckle under its own weight. Rocks can withstand much more compressional stress than tensional stress (e.g., Figure 8.15). Simple shear force is created when rocks move horizontally past each other in opposite directions. Tensional forces operate when rocks pull away from each other. When compressional forces are at work, rocks are pushed together. There are three main types of stress: compression, tension, and shear. If stress is not concentrated at one point in a rock, the rock is less likely to change (break or bend) because of that stress. The stress is more spread out in an athletic shoe. In the high heeled shoe heel, the area is very small, so much stress is concentrated at that point. For example, imagine the stress that is created at the tip of the heel of a high heeled shoe and compare it to the bottom of an athletic shoe. Since stress is a function of area, changing the area to which stress is applied will change the resulting stress. Stress is a force applied to a given area. Rake>0 means the hanging wall moved up (thrust or reverse fault).Rocks change as they undergo stress. Rake=0 means the hanging wall, or the right side of a vertical fault, moved in the strike direction (left lateral motion) Rake = +/-180 means the hanging wall moved in the opposite direction (right lateral motion). Rake - the direction the hanging wall moves during rupture, measured relative to the fault strike (between -180 and 180 decimal degrees). For a vertical, strike slip fault (for which “hanging wall” has no physical meaning) we still call the right-side block the hanging wall to distinguish between right lateral and left lateral motion.ĭip - the angle of the fault in decimal degrees (0 to 90, relative to horizontal). This is important because rake (which gives the slip direction) is defined as the movement of the hanging wall relative to the footwall. This means that the hanging-wall block is always to the right. That is, the fault always dips to the right when moving along the trace in the strike direction (from one point to the next). Strike - the fault-trace direction in decimal degrees (0 to 360, relative to North), defined so that the fault dips to the right side of the trace. (adapted from page 106 of Aki & Richards (1980), Quantitative Seismology - Vol 1 ) ![]()
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