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2025 Hazard Mitigation Plan <br />Kittitas Gounty, Washington <br />3.2.3. Slope Stability <br />Slope stability refers to the potential of land slippage due to factors such as steepness, composition of <br />materials, and water content within soils. Slopes that have been landscaped and altered from their natural <br />vegetated state or saturated by septic tanks are also subject to sliding. Slumping can also occur when <br />water infiltrates the soil and comes in contact with an impermeable layer. Although the upper layers of <br />soil may not become saturated, water perches on the impermeable layer and causes a slippery interface <br />resulting in the downward and outward movement of weak rock or unconsolidated material. Much of the <br />western and northern portions of Kittitas County contain slopes of 15% or greater. Slopes less than 15% <br />are generally found in the river basins in the eastern portions of the County..12 Refer to the Landslide <br />section (Chapter 4) of this Plan for further details on the hazard. <br />3.2.4. Seismic Features <br />Seismic events could pose limited landslides and liquefaction hazards in areas where steep or exposed <br />slopes occur. Landslides occur when the structural integrity of a geological formation is damaged. There <br />are known areas of landslide activity, which may or may not have resulted from seismic events, along <br />the Yakima River. Soil liquefaction occurs when soil loses its strength and bearing capacity during an <br />earthquake. This is most likely to occur on non-cohesive soils with high moisture content. These types of <br />soils are poorly compacted and combined with moist conditions, are subject to liquefying during an <br />earthquake. Structures built on liquefiable soil are subject to greater shaking and damage during an <br />earthquake, but this damage can be minimized by engineering and construction methods. Kittitas County <br />has little potential for seismic events other than secondary effects from activity occurring west of the <br />Cascades.13 Refer to the Earthquake section (Chapter 4) of this Plan for further details on the hazard. <br />Figure 3-3 illustrates the topographical map for Kittitas County. <br />12 lbid <br />13 lbid <br />Chapter 3: Community Profile 21