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CHAPTER 10. EARTHQUAKE <br />10-17 <br />The USGS has created ground motion maps based on current information about several fault zones. These <br />maps show the PGA that has a certain probability (2 percent or 10 percent) of being exceeded in a 50-year <br />period. Figure 10-9 shows the PGAs with a 10-percent exceedance chance in 50 years in Washington. <br />South-central Washington is a medium- to high-risk area. <br /> <br />Figure 10-9. PGA with 2-Percent Probability of Exceedance in 50 Years, Northwest Region <br />10.2.5 Warning Time <br />There is currently no reliable way to predict the day or month that an earthquake will occur at any given <br />location. Research is being done with warning systems that use the low energy waves that precede major <br />earthquakes. These potential warning systems give approximately 40 seconds notice that a major <br />earthquake is about to occur. The warning time is very short but it could allow for someone to get under a <br />desk, step away from a hazardous material they are working with, or shut down a computer system. <br />10.3. SECONDARY HAZARDS <br />Earthquakes can cause large and sometimes disastrous landslides and mudslides. River valleys are <br />vulnerable to slope failure, often as a result of loss of cohesion in clay -rich soils. Soil liquefaction occurs <br />when water-saturated sands, silts or gravelly soils are shaken so violently that the indi vidual grains lose <br />contact with one another and float freely in the water, turning the ground into a pudding-like liquid. Building <br />and road foundations lose load-bearing strength and may sink into what was previously solid ground. Unless <br />properly secured, hazardous materials can be released, causing significant damage to the environment and