considered to have areas of high or very high seismicity (USGS, 2005). Current estimates suggest that there are $17.5 trillion worth of structures in the United States (FEMA, 2004)³—the value of structures in high and very high seismic risk states is about $5.8 trillion (33 percent of the nation’s total), and in all states prone to seismic damage the value is about $8.6 trillion (49 percent of the total). In population terms, 75 million people—including 46 million outside California—live in urban areas that have moderate to high earthquake risk.

Estimates of the extent of the seismic risk to the nation are based on three primary factors—the nation’s inventory of structures (buildings, highways, pipelines, etc.), the potential damage extrapolated from performance in past damaging earthquakes, and the seismic hazard as determined from the geologic record and from instrument recordings of earthquakes that have occurred over the past century. This risk is growing steadily because buildings and infrastructure systems nationwide are being constructed without an adequate understanding of the seismic hazards that are present. The variation in seismic hazard across the nation is only now beginning to be understood as a consequence of the monitoring programs that have been in place for the last 50 years. Much more must be learned to quantify hazard levels properly, so that communities can understand their hazard and take appropriate, cost-effective steps to reduce their risk. Although efforts to quantify the vulnerability of the built environment are under way, current estimates have a significant amount of uncertainty because many recent earthquakes have not yielded the coupled seismic monitoring data and damage information that would enable improved vulnerability analysis.

Uncertainty—both epistemic and aleatory—exists concerning the frequency with which potentially damaging earthquakes occur in various regions of the country, the level of seismic hazard that results, and the damage that these hazards will do to the natural and built environment. The epistemic uncertainty, reflecting inadequacies in understanding the true state of nature, can be reduced by gathering more data through seismic monitoring. For example, improved seismic monitoring has the potential to improve current estimates of earthquake frequencies, of the median level of ground motion attenuation models (relating earthquake magnitude and distance to ground shaking levels), and of the median level of fragility models (relating ground shaking levels to building