Nevertheless, the damage to structures located in the fault zone can be very high, especially where the land use is intensive. Surface faulting, in the case of a strike-slip fault, generally affects a long narrow zone whose total area is small compared with the total area affected by ground shaking. Although displacements of these kinds can result from landslides and other shallow processes, surface faulting, as the term is used here, applies to differential movements caused by deep-seated forces in the Earth, the slow movement of sedimentary deposits toward the Gulf of Mexico, and faulting associated with salt domes.ĭeath and injuries from surface faulting are very unlikely, but casualties can occur indirectly through fault damage to structures. Combinations of the strike-slip type and the other two types of faulting can be found. Surface faulting is the differential movement of the two sides of a fracture at the Earth's surface and can be strike-slip, normal, and reverse (or thrust). Geological Survey Professional Paper 1240B, 108 p. Taken from: Hays, W.W., ed., 1981, Facing Geologic and Hydrologic Hazards - Earth Science Considerations: U.S. Because amplitudes of low-frequency vibrations decay less rapidly than high-frequency vibrations as distance from the fault increases, tall buildings located at relatively great distances (60 miles) from a fault are sometimes damaged. Rayleigh and Love waves mainly cause low-frequency vibrations which are more efficient than high-frequency waves in causing tall buildings to vibrate. Buildings vibrate as a consequence of the ground shaking damage takes place if the building cannot withstand these vibrations. Compressional waves and shear waves mainly cause high-frequency (greater than 1 Hertz) vibrations which are more efficient than low-frequency waves in causing low buildings to vibrate. When a fault ruptures, seismic waves are propagated in all directions, causing the ground to vibrate at frequencies ranging from about 0.1 to 30 Hertz. The subjective numerical value of the Modified Mercalli Intensity Scale indicates the effects of ground shaking on man, buildings, and the surface of the Earth. These quantities can be determined from empirical (observed) data correlating them with the magnitude and the distribution of Modified Mercalli intensity of the earthquake, distance of the building from the causative fault, and the physical properties of the soil and rock underlying the building. In land-use zoning and earthquake resistant design, knowledge of the amplitude, frequency composition, and the time duration of ground shaking is needed. The objective of earthquake resistant design is to construct a building so that it can withstand the ground shaking caused by body and surface waves. Body and surface waves cause the ground, and consequently a building, to vibrate in a complex manner. The P and S waves mainly cause high-frequency vibrations whereas, Rayleigh waves and Love waves, which arrive last, mainly cause low-frequency vibrations. They are the most damaging waves, because buildings are more easily damaged from horizontal motion than from vertical motion. S waves arrive next and cause a structure to vibrate from side to side. P waves propagate through the Earth with a speed of about 15,000 miles per hour and are the first waves to cause vibration of a building. Although the physics of seismic waves is complex, ground shaking can be explained in terms of body waves, compressional, or P, and shear, or S, and surface waves, Rayleigh and Love. As a generalization, the severity of ground shaking increases as magnitude increases and decreases as distance from the causative fault increases. Ground shaking is caused by body waves and surface waves. Ground shaking is a term used to describe the vibration of the ground during an earthquake.
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