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Landslide/Mass Movements 12.2.5 Warning Time Landslides can occur suddenly or slowly. The velocity of slide may range from a slow creep of inches per year to many feet per second, depending on slope angle, material, and water content. Generally accepted warning signs for landslide activity include the following:  Springs, seeps, or saturated ground in areas that have not typically been wet before  New cracks or unusual bulges in the ground, street pavements, or sidewalks  Soil moving away from foundations  Ancillary structures such as decks and patios tilting or moving relative to the main house  Tilting or cracking of concrete floors and foundations  Broken water lines and other underground utilities  Leaning telephone poles, trees, retaining walls or fences  Offset fence lines  Sunken or down-dropped road beds  Rapid increase in creek water levels, possibly accompanied by increased turbidity (soil content)  Sudden decrease in creek water levels though rain is still falling or just recently stopped  Sticking doors and windows and visible open spaces indicating frames out of plumb  A faint rumbling sound that increases in volume as the landslide nears  Unusual sounds, such as trees cracking or boulders knocking together. Some methods used to monitor landslides can provide an idea of the type of slide and the amount of time prior to failure. Assessing the geology, vegetation, and amount of predicted precipitation for an area can help in predictions of what areas are at risk during general time periods. Currently, there is no practical warning system for individual landslides, however. The standard operating procedure is to monitor situations on a case-by-case basis and respond after an event has occurred. 12.3 EXPOSURE 12.3.1 Population and Property A quantitative assessment of exposure to the landslide hazard was conducted using the landslide susceptibility mapping and the asset inventory developed for this plan, with an emphasis on zones with the highest degree of susceptibility (high and very high risk). Population exposure was estimated by calculating the number of buildings in each hazard area as a percent of total planning area buildings, and then applying this percentage to the estimated planning area population. Table 12-2 summarizes the estimated countywide population living in the mapped landslide susceptibility areas and the estimated property exposure. Detailed results by jurisdiction are provided in Appendix E. Figure 12-3 shows the occupancy class defined by Hazus for all buildings in three mapped landslide hazard areas. Some building uses are more vulnerable to landslides, such as single-family homes, while others are less vulnerable, such as agricultural land or parks. Residential properties make up 98 percent of this exposure. 12-7