M A S T E R P A G E
Because it is not natural to the area where it has been put, landfill in one spot is likely to be of a different
composition from landfill in another spot nearby. When seismic waves are transmitted through landfill, they are
amplified and their period is lengthened. Long earthquake waves are particularly destructive to some types of
surface structures. Landfills commonly will settle and sink during a strong earthquake.
Liquefaction
Whenever poorly consolidated soil or fine sand becomes saturated, an earthquake is likely to cause soil
liquefaction. Earthquake vibrations compact the soil, causing water mixed with sand to flow upward. Structures
may settle several feet or even topple, causing considerable damage. In a related phenomenon, sandy or muddy
soils may behave like liquids, flowing out onto the surface as sand boils or mud boils.
Slopes and Landslides
Structures on cliffs and ridges are also at high risk for earthquake damage, even if they are built on strong
bedrock. Earthquake waves appear to be reflected and amplified by topographic highs like cliffs and ridges.
Earthquakes also dramatically increase the potential for landslides in areas where landslides are common, such as
those where sedimentary rocks lie just under the soil. The probability of an earthquake-related landslide depends
on the strength of the slope materials, the steepness of the slope, and the extent and duration of ground shaking.
Structures on cliffs and ridges need to be designed to the highest earthquake standards, and should be fully
insured.
Tsunami
Tsunami are caused by faulting and the abrupt movement of the ocean floor during an underwater earthquake. A
wave generated by this movement can travel as fast as 640 km/hr (400 mph) on the open ocean, where it may not
be much above normal height. When it approaches the shore, however, it may attain a height of 15-20 m (50
feet)—in some cases, even 32 m. Tsunami present a distinct hazard to low-lying coastal areas, particularly the
west and northwest shorelines along the western North American coast and the northerly facing coast of Hawaii.
Low-lying waterfront properties in these areas are at high risk from tsunami.
Human-Made Structures
Human-made structures, such as dams, reservoirs, water tanks, and tall buildings, can present special earthquake
hazards, and need to be considered during site selection. Every building decision needs to consider the exposure
to geologic hazard and the probability of an earthquake, bearing in mind that earthquakes are possible anywhere
in the world at any time.
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