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SECONDARY HAZARDS CAUSED BY EARTHQUAKES

SECONDARY HAZARDS CAUSED BY EARTHQUAKES

LIQUEFACTION

Liquefaction is when saturated unconsolidated sediments (usually silt or sand) is liquefied from shaking. Shaking causes loss of cohesion between grains of sediment, reducing the effective stress resistance of the sediment. The sediment flows very much like the quicksand presented in movies. Liquefaction creates sand volcanoes, which is when liquefied sand is squirted through an overlying (usually finer-grained) layer, creating cone-shaped sand features. It may also cause buildings to settle or tilt.

Earthquake-induced tsunamis have caused many of the more recent devastating natural disasters. Tsunamis form when the sea floor is offset by earthquakes in the ocean subsurface. This offset can be caused by fault movement or underwater landslides and lifts a volume of ocean water generating the tsunami wave. Tsunami waves travel fast with low amplitude in deep ocean water, but are significantly amplified as the water shallows as they approach the shore. When a tsunami is about to strike land, the water in front of the wave along the shore will recede significantly, tragically causing curious people to wander out. This receding water is the drawback of the trough in front of the tsunami wave which then crashes on shore as a wall of water upwards of a hundred feet high. Warning systems have been established to help mitigate the loss of life caused by tsunamis.

LANDSLIDES

Shaking can trigger landslides (see landslide section for more information). One example is the 1992 magnitude 5.9 earthquake in St. George Utah. This earthquake caused the Springdale landslide, having a scarp that offset and destroyed several structures in the Balanced Rock Hills subdivision.

SEICHES

Seiches are waves on lakes generated by earthquakes, which cause sloshing of water back and forth and, sometimes, even changes in elevation of the lake. A seich in Hebgen Lake during the 1959 earthquake caused significant destruction to structures and roads around the lake.

LAND ELEVATION CHANGES

Significant subsidence and upheaval of the land can occur about the slippage that causes earthquakes. Land elevation changes are the result of the relaxation of stress and subsequent movement along the fault plane. The 1964 Alaska earthquake is an excellent example of this. Where the fault cuts the surface, elevation of one side causes a fault scarp that may be a few feet to 20 or 30 feet in height. The Wasatch Mountains represent an accumulation of fault scarps of a couple of dozen feet at a time over a few million years.

LICENSE

 

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Introduction to Physical Geography by R. Adam Dastrup is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

Kategoria: Moje artykuły | Dodał: kolo (2019-04-04)
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