specified area integrated over the post-earthquake period. Risk depends on the hazard, but it is compounded by a community’s exposure—its population and the extent and density of its built environment—as well as the fragility of its built environment, population, and socioeconomic systems to seismic hazards. Exposure and fragility contribute to vulnerability. Risk is lowered by resiliency, the measure of how efficiently and how quickly a community can recover from earthquake damage.
Risk analysis seeks to quantify the risk equation in a framework that allows the impact of political policies and economic investments to be evaluated, to inform the decision-making processes that contribute to risk reduction. Risk quantification is a difficult problem, because it requires detailed knowledge of the natural and the built environments, as well as an understanding of both earthquake and human behaviors. Moreover, national risk is a dynamic concept because of the exponential rise in the urban exposure to seismic hazards (EERI, 2003b)—calculating risk involves predictions of highly uncertain demographic trends.
Estimating Losses from Earthquakes
The synoptic earthquake risk studies needed for policy formulation are the responsibility of NEHRP. These studies can take the form of deterministic or scenario studies where the effects of a single earthquake are modeled, or probabilistic studies that weight the effects from a number of different earthquake scenarios by the annual likelihood of their occurrence. The consequences are measured in terms of dollars of damage, fatalities, injuries, tons of debris generated, ecological damage, etc. The exposure period may be defined as the design lifetime of a building or some other period of interest (e.g., 50 years). Typically, seismic risk estimates are presented in terms of an exceedance probability (EP) curve (Kunreuther et al., 2004), which shows the probability that specific parameters will equal or exceed specified values (Figure 1.1). On this figure, a loss estimate calculated for a specific scenario earthquake is represented by a horizontal slice through the EP curve, while estimates of annualized losses from earthquakes are portrayed by the area under the EP curve.
The 2008 Great California ShakeOut exercise in southern California is an example of a scenario study that describes what would happen during and after a magnitude-7.8 earthquake on the southernmost 300 km of the San Andreas Fault (Figure 1.2), a plausible event on the fault that is most likely to produce a major earthquake. Analysis of the 2008 ShakeOut scenario, which involved more than 5,000 emergency responders and the participation of more than 5.5 million citizens, indicated that the scenario earthquake would have resulted in an estimated 1,800 fatalities, $113 billion in damages to buildings and lifelines, and nearly $70 billion in busi-