How Well Do Earthquake Hazard Maps Work and How Good Do They Have to be?

Abstract:

Earthquake hazard maps seek to describe the level of earthquake hazards in a region and provide a scientific foundation for earthquake preparation and mitigation. In many cases these maps do reasonably well. However, recent large earthquakes that did great damage in areas predicted to be relatively safe illustrate the need to assess how well these maps are actually performing and how good they need to be to be useful. Economic analysis comparing the cost of mitigation to the expected reduction in loss shows that an inaccurate hazard estimate still is useful as long as it is not too much of an overestimate. Because better hazard forecasts can yield better mitigation policy, we need agreed ways of assessing how well a map performed and thus whether one map performed better than another. The metric implicit in current maps, that during the chosen time interval the predicted ground motion will be exceeded only at a specific fraction of the sites, is useful but permits maps to be nominally successful although they significantly underpredict or overpredict shaking, or to be nominally unsuccessful but do well in terms of predicting shaking. We explore some possible metrics that better measure the effects of overprediction and underprediction and can be weighted to reflect the two differently and to reflect differences in populations and property at risk. Although no single metric alone fully characterizes map behavior, using several metrics can provide useful insight for comparing and improving hazard maps.