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CHAPTER 10. EARTHQUAKE <br />10-4 <br />Maps of PGA values form the basis of seismic zone maps that are included in building codes such as the <br />International Building Code. Building codes that include seismic provisions specify the horizontal force <br />due to lateral acceleration that a building should be able to withstand during an earthquake. PGA values are <br />directly related to these lateral forces that could damage “short period structures” (e.g. single-family <br />dwellings). Longer period response components determine the lateral forces that damage larger structures <br />with longer natural periods (apartment buildings, factories, high-rises, bridges). Table 10-1 lists damage <br />potential and perceived shaking by PGA factors, compared to the Mercalli scale. <br />Table 10-1. Mercalli Scale and Peak Ground Acceleration Comparison (USGS 2008, 2010) <br />Modified Mercalli <br />Scale Perceived Shaking <br />Potential Structure Damage <br />Estimated PGAa <br />(%g) Resistant <br />Buildings <br />Vulnerable <br />Buildings <br />I Not Felt None None <0.17% <br />II-III Weak None None 0.17%—1.4% <br />IV Light None None 1.4%—3.9% <br />V Moderate Very Light Light 3.9%—9.2% <br />VI Strong Light Moderate 9.2%—18% <br />VII Very Strong Moderate Moderate/Heavy 18%—34% <br />VIII Severe Moderate/Heavy Heavy 34%—65% <br />IX Violent Heavy Very Heavy 65%—124% <br />X—XII Extreme Very Heavy Very Heavy >124% <br />a. PGA measured in percent of g, where g is the acceleration of gravity <br /> <br />10.1.4 Effect of Soil Types <br />The impact of an earthquake on structures and infrastructure is largely a function of ground shaking, <br />distance from the source of the quake, and liquefaction, a secondary effect of an earthquake in which soils <br />lose their shear strength and flow or behave as liquid, thereby damaging structures that derive their support <br />from the soil. Liquefaction generally occurs in soft, unconsolidated sedimentary soils. A program called <br />the National Earthquake Hazard Reduction Program (NEHRP) creates maps based on soil characteristics <br />to help identify locations subject to liquefaction. Table 10-2 summarizes NEHRP soil classifications. <br />NEHRP Soils B and C typically can sustain ground shaking without much effect, dependent on the <br />earthquake magnitude. The areas that are commonly most affected by ground shaking have NEHRP Soils <br />D, E and F. In general, these areas are also most susceptible to liquefaction.