faults - Geological Maps 3: Faulted Strata Brittle...

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1 Geological Maps 3: Faulted Strata Brittle deformation in rocks is characterized by fractures, joints and faults. Fractures and joints can be of any size, orientation or pattern. Some joints form regular patterns in rocks that are large enough to be seen in aerial or satellite photographs (e.g., rectangular joint patterns in granite bedrock of the Canadian Shield). However, as far as geological maps are concerned, most joints and fractures are minor "players" and can be ignored. Faults are another matter as they strongly influence geological maps. A fault can be defined as any brittle deformation-induced fracture where there has been movement of the blocks on either side of the plane defining the fault (the fault plane ). Fault nomenclature can be rather cumbersome. The fault plane is the actual surface where the strata has been broken (Figure 1). The fault line is the line made by the intersection of the fault plane and the surface of the Earth (Figure 1). The fault blocks are the strata on either side of the fault plane and the fault scarp is the cliff formed on the "uplifted" fault block where the fault plane rises above the surface of the Earth (Figure 1). Faults are among the most studied of the geological features on the planet because they are where earthquakes occur. Earthquakes kill more people than any other natural process with the possible exception of hurricanes. As a matter of record, 1999 was a terrible year for earthquakes. Tens of thousands of people were killed in only three earthquakes (two in Turkey; one in Taiwan). Earthquakes occur due to the build-up of Figure 1: Schematic diagram illustrating key components of fault-systems. The fault illustrated is of a normal dip-slip variety.
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Geological Maps 3-Faults 2 stress past the point of natural resistance (friction) of rock strata. The stress can be compression, tension or shear. All will produce earthquakes, but as you will see shortly, each of these forces produces different types of faults. Stress tends to build up slowly, but steadily in rocks. Sometimes there will be precursor evidence of an impending earthquake (soil creep, surface tilting, increase in emission of subterranean gases). More often than not however, the ground just starts shaking. The point on the fault plane where the slippage occurs is the point where all seismic waves are emitted. This is called the earthquake focus (Figure 1). The seismic waves travel exceptionally fast (4 to 7 km/second) in all directions from the focal point including straight up. The point on the Earth's surface directly above the focus is the
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Geological Maps 3-Faults 3 point where the seismic waves are the strongest and the shaking is the greatest. This is called the epicenter (Figure 1). The intensity of an earthquake is directly proportional to the amount of stress released and inversely proportional to the depth of the focus. The absolute worst thing that can happen to a city is to be situated atop a fault line and then to have a shallow earthquake along the fault plane (e.g., focus less than 50 km deep) that releases a tremendous amount of energy. This is exactly what happened beneath
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This note was uploaded on 02/04/2012 for the course GLY 111 taught by Professor Haywick during the Fall '11 term at S. Alabama.

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faults - Geological Maps 3: Faulted Strata Brittle...

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