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CHAPTER 10. EARTHQUAKE <br />10-16 <br />Table 10-4. Earthquake Annual Probabilities within 50 Years <br />Peak Ground Acceleration (%g) Annual Probability of Occurrence within 50 Km of <br />Kittitas (Annual Frequency of Exceedance) <br />0.269 2.0957 % <br />.053 0.788 % <br />.0738 0.449 % <br />.103 0.243 % <br />.145 0.121 % <br />.203 0.0582 % <br />.284 0.0263 % <br />.397 0.0111 % <br />.556 0.00428 % <br />.778 0.00147 % <br />1.09 0.000433 % <br />a. PGA measured in percent of g, where g is the acceleration of gravity <br /> <br />10.2.4 Severity <br />The severity of an earthquake can be expressed in terms of intensity or magnitude. Intensity represents the <br />observed effects of ground shaking on people, buildings and natural features. Magnitude is related to the <br />amount of seismic energy released at the hypocenter of an earthquake. It is determined by the amplitude of <br />the earthquake waves recorded on instruments. Whereas intensity varies depending on location with respect <br />to the earthquake epicenter, magnitude is represented by a single, instrumentally determined value for each <br />earthquake event. The severity of an earthquake event can be measured in the following terms: <br />• How hard did the ground shake? <br />• How did the ground move? (Horizontally or vertically) <br />• How stable was the soil? <br />• What is the fragility of the built environment in the area of impact? <br />The severity of a seismic event is directly correlated to the stability of the ground close to the event’s <br />epicenter. The difference in severity between intensity ranges can be immense. A poorly built structure on <br />a stable site is far more likely to survive a large earthquake than a well -built structure on an unstable site. <br />Thorough geotechnical site evaluations should be the rule of thumb for new construction in the planning <br />area until creditable soils mapping becomes available.