Time-dependent Seismic Hazard Maps for the New Madrid Seismic Zone and Charleston, South Carolina, Areas

Abstract

“Apocalyptic claims do not have a good track record. And arguments that statistics support such claims—particularly arguments that simple, easily understood numbers are proof that the future holds complex, civilization-threatening changes—deserve the most careful inspection.”—Best (2004) Earthquake hazard maps that predict that portions of the central and eastern United States are more hazardous than California result from a number of crucial assumptions. One of these is that the recurrence of large earthquakes is time-independent, such that a future earthquake is equally probable immediately after the past one and much later. An alternative is to use time-dependent models in which the probability is small shortly after the past one and then increases with time. Applying such models to the New Madrid seismic zone and Charleston, South Carolina, areas predicts significantly lower hazards because these are “early” in their cycles. The reduction is greater than if we were to lower the assumed maximum magnitude within the range under discussion. The differences between the time-independent and time-dependent hazard maps bear out the point that estimating seismic hazard here or in other intraplate areas is a very uncertain enterprise. These seismic hazard maps predict the maximum earthquake ground motion expected at a specified probability level during a certain time interval, such that the larger the predicted motions, the higher the predicted seismic hazard. The maps are used, typically without consideration of their large uncertainties, to develop building codes that specify the levels of earthquake-resistant construction required in an area. The additional construction costs, which can be billions of dollars over hundreds of years, are incurred in hope of reducing property damage and loss of life from possible future earthquakes. Because these expenditures come at the expense of other possible uses, it is interesting to assess the range of possible hazard estimates for an …