2019_KittitasCountyHMP_Volume 1_For Adoption

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CHAPTER 10. EARTHQUAKE 10-18 people. Earthen dams and levees are highly susceptible to seismic events and the impacts of their eventual failures can be considered secondary risks for earthquakes. 10.3.1 Seiche A seiche is a standing wave in an enclosed or partly enclosed body of water, normally caused by earthquake activity, though also possibly caused by other factors such as wind. The effect is caused by resonances in a body of water that has been disturbed. Vertical harmonic motion results, producing an impulse that travels the length of the basin at a velocity that depends on the depth of the water. The impulse is reflected back from the end of the basin, generating interference. Repeated reflections produce standing waves with one or more nodes, or points, that experience no vertical motion. The waves in a seiche are stationary in the horizontal plane; they move up and down, but not forward like wind waves at sea. That is why these waves are called standing waves. The frequency of the oscillation is determined by the size of the basin, its depth and contours, and the water temperature. Seiches can occur in harbors, bays, lakes, rivers and canals. They are often imperceptible to the naked eye, and observers in boats on the surface may not notice that a seiche is occurring due to the extremely long wavelengths. These events usually do not occur near the epicenter of a quake, but often hundreds of miles away. This is due to the fact that earthquake shock waves close to the epicenter consist of high-frequency vibrations, while those at much greater distances are of lower frequency, which can enhance the rhythmic movement in a body of water. The biggest seiches develop when the period of the ground shaking matches the frequency of oscillation of the water body. With three large reservoirs and a risk of seismic events, there is potential for seiches to occur in Kittitas County. The degree of vulnerability to this secondary hazard is difficult to gage without hazar d mapping that illustrates extent, location and potential severity of probabilistic events. 10.4. CLIMATE CHANGE IMPACTS The impacts of global climate change on earthquake probability are unknown. Some scientists say that melting glaciers could induce tectonic activity. As ice melts and water runs off, tremendous amounts of weight are shifted on the earth’s crust. As newly freed crust returns to its original, pre-glacier shape, it could cause seismic plates to slip and stimulate volcanic activity according to research into prehistoric earthquakes and volcanic activity. NASA and USGS scientists found that retreating glaciers in southern Alaska may be opening the way for future earthquakes (NASA, 2004). Other research indicates that geologic events such as earthquakes are sensitive to changes on the earth’s surface, such as shifts in water or atmospheric pressure. Other scientists have expressed doubts that earthquakes are significantly impacted by climate change. Secondary impacts of earthquakes could be magnified by climate change. Soils saturated by repetitive storms could experience liquefaction during seismic activity due to the increased saturation. Dams storing increased volumes of water due to changes in the hydrograph could fail during seismic events. There are currently no models available to estimate these impacts. 10.5. EXPOSURE 10.5.1 Population The entire population of Kittitas County is potentially exposed to direct and indirect impacts from earthquakes. The degree of exposure is dependent on many factors, including the age and construction type of the structures people live in, the soil type their homes are constructed on, their proximity to fault location,