Q: How do earthquakes cause structural damage? How does this relate to the geology of the affected area? (Submitted by fergus o'hare , dundee, scotland , united kingdom)A: Several factors determine the structural damage from an earthquake including, the proximity to active earthquake faults; the building construction (type of building, foundation, materials used, and quality of workmanship); the local site conditions (type and condition of soil, slope of the land, and fill material); and the geologic structure of the earth beneath the structure.
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Fig. 1 Â Â Relatively new reinforced concrete buildings that collapsed in the Marmara, Turkey, earthquake, August 17, 1999.(From C. Scawthorn, Earthquake engineering, in McGraw-Hill Yearbook of Science &Technology 2002) .
Generally, the severity of ground shaking increases as the magnitude increases, and decreases as the distance from the causative fault increases. Structures above or in the immediate vicinity of surface faulting can be severely damaged or destroyed.
Earthquakes generate seismic waves of varying lengths that travel along the surface and through the Earth at varying speeds, depending upon the material through which they move. Seismic waves will cause a structure to move irregularly and, as a result, vibrate. Structures are damaged when they experience vibrations that correspond to their natural period. Structures literally begin to tear themselves apart as they vibrate in response to seismic waves. The natural period of a structure indicates what wavelength and frequency of the earthquake ground-motion has the capacity to cause the building to vibrate. Short buildings have short natural periods and are damaged by short wavelength, high frequency seismic waves. Tall buildings, bridges, and other large structures have long natural periods and are damaged by long wavelength, low frequency seismic waves.
Local site conditions and subsurface geologic structure can increase the amount of damage from an earthquake. Structures located on materials that have a natural period of vibration similar to that of the building will suffer increased damage through resonance effects. In general, structural damage from earthquakes is greatest in areas underlain by soft soils, alluvium, or loose, water-saturated, unconsolidated soils, and least in areas underlain by bedrock. Ground motion can cause some types of soils (clay-free soil deposits, primarily sands and silts) to liquefy, which means they temporarily lose strength and behave as viscous fluids rather than as solids. The younger and looser the sediment and the higher the water table, the more susceptible a soil is to liquefaction. Liquefaction causes the ground surrounding or beneath structures to fail. When the soil supporting a building or some other structure liquefies and loses strength, large deformations can occur within the soil, allowing the structure to settle and tip. Large blocks of ground may move laterally because of liquefaction in a subsurface layer.
Kaye M. Shedlock
