explain how earthquake waves provide information about the interior part of the earth based on the above statement in this illustration
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explain how earthquake waves provide information about the interior part of the earth based on the above statement in this illustration
Answer:
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.
Tensional stress (or extensional stress), which stretches rock;
Compressional stress, which squeezes rock; and
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.
Elastic Deformation -- wherein the strain is reversible.
Ductile Deformation -- wherein the strain is irreversible.
Fracture -- irreversible strain wherein the material breaks.
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We can divide materials into two classes that depend on their relative behavior under stress.
Brittle materials have a small to large region of elastic behavior, but only a small region of ductile behavior before they fracture.
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:
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.
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 low confining stress, material will be brittle and tend to fracture sooner.
Strain rate -- Strain rate refers to the rate at which the deformation occurs (strain divided by time). At high strain rates material tends to fracture. At low strain rates more time is available for individual atoms to move and therefore ductile behavior is favored.
Composition -- Some minerals, like quartz, olivine, and feldspars are very brittle. Others, like clay minerals, micas, and calcite are more ductile This is due to the chemical bond types that hold them together. Thus, the mineralogical composition of the rock will be a factor in determining the deformational behavior of the rock. Another aspect is presence or absence of water.
In general, rocks near the surface of the earth behave in a brittle fashion, unless they are deformed slowly. Thus, when they are acted upon by differential stress, they tend to fracture.
For any inclined fault plane we define the block above the fault as the hanging wall block and the block below the fault as the footwall block
Normal Faults - are faults that result from horizontal extensional stresses in brittle rocks and where the hanging-wall block has moved down relative to the footwall block.
Reverse Faults - are faults that result from horizontal compressional stresses in brittle rocks, where the hanging-wall block has moved up relative the footwall block.
A Thrust Fault is a special case of a reverse fault where the dip of the fault is less than 45o. Thrust faults can have considerable displacement, measuring hundreds of kilometers, and can result in older strata overlying younger strata.
Strike Slip Faults - are faults where the displacement on the fault has taken place along a horizontal direction. Such faults result from shear stresses acting in the crust. Strike slip faults can be of two varieties, depending on the sense of displacement. To an observer standing on one side of the fault and looking across the fault, if the block on the other side has moved to the left, we say that the fault is a left-lateral strike-slip fault. If the block on the other side has moved to the right, we say that the fault is a right-lateral strike-slip fault. The famous San Andreas Fault in California is an example of a right-lateral strike-slip fault. Displacements on the San Andreas fault are estimated at over 600 km.
Oblique Slip Faults - If the displacement has both a vertical component and a horizontal component (i.e. a combination of dip slip and strike slip) it is called an oblique slip fault.
Blind Faults
A blind fault is one that does not break the surface of the earth. Instead, rocks above the fault have behaved in ductile fashion and folded over the tip of the fault.
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Active Faults
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