How earthquakes affect humans, buildings, and bridges depends on many factors. The most important factors are earthquake magnitude, the distance from the earthquake source, and the geologic conditions at a site. The most important hazard, however, is strong ground-motion – the primary hazard – directly generated by earthquakes . The ground motion can be assessed based on geological and seismological information. For example, Street and others (1996) 1) estimated that PGA of about 0.1 to 0.2g could be expected in the Henderson-Owensboro area in the next 500 years. The U.S. Geological Survey also estimated that a ground shaking level of about 0.3 to 0.4g for the Henderson-Owensboro area could be expected in the next 2,500 years (Frankel and others, 2002) 2) . The primary hazard has direct policy implication. For example, the ground motion of 0.3 to 0.4g PGA estimated by USGS has been used for policy decisions such as building codes and highway bridge seismic designs.
Seismic hazards are also affected by local geologic, hydrologic, and topographic conditions – the secondary hazards . Three phenomena generally will be induced by strong ground motion during earthquakes under certain local conditions: (1) amplification of ground shaking by a soft soil column, (2) liquefaction of water-saturated sand, silt, or gravel, creating areas of “quicksand,” and (3) landslides, including rock falls and rock slides, triggered by shaking, even on relatively gentle slopes. In order to differentiate the cause (primary) and effect (secondary), these secondary hazards are called relative seismic hazards.
These relative seismic hazards can be assessed based on the estimated primary hazard and available local geologic, hydrologic, topographic, and geotechnical data. Three maps (ground motion amplification potential, liquefaction potential, and earthquake-triggered landslide potential) will be produced in this project. These maps, in combination with the ground-motion (primary) hazard maps, can be used to develop a variety of hazard mitigation strategies such as seismic risk assessment, emergency response plans, and land-use planning. These maps should not be used for site-specific purpose, however (A Technical Note on Seismic Microzonation in the Central United States)