Seismic Hazard Analysis for Building Codes

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The purpose of seismic design is to reduce risk to life and property from future earthquakes. Risk is defined as the probability of a loss; normally, an increase in the seismic design load reduces the seismic risk. The current approach toward seismic design is to reduce the risk at individual locations (site-specific risk) to an acceptable level. This approach is reasonable from the perspective of a building owner, but it is not reasonable from the perspective of an entire community, because it does not ensure that the risk to multiple locations affected by the same earthquake (aggregate risk) is also reduced to an acceptable level. The site-specific and the aggregate risks do not always go hand-in-hand. For example, if a region is affected only by small but frequent earthquakes, the probability of significant loss at a given location is high due to the high probability of an earthquake happening close to that location. However, the probability of a significant aggregate loss is low because each small earthquake will affect only a few locations. On the other hand, if the same region is affected only by large but rare earthquakes, the probability of a significant loss at a given location is low due to the low probability of strong shaking at that location, but the probability of a significant aggregate loss is high because the large earthquake will affect …