Seismic hazard in Northeastern United States

Abstract Seismic hazard has been evaluated at a number of locations in the northeastern United States using probabilistic methods. The analysis was carried out to examine the effects of significant variations in various seismic parameters, reflecting primarily the uncertainty in our knowledge of tectonic processes, inadequate historical data base, and absence of strong ground motion records. Seismic sources were considered in two ways: • - Using the tectonic province approach that is consistent with the procedure followed by the US Nuclear Regulatory Commission to evaluate nuclear power plants, and in which earthquakes not related to a particular geologic structure are assumed to be associated with a tectonic province. • - Using seismic zones that were identified solely from the analysis of patterns of historical seismicity. The maximum earthquake for each province or zone was assumed to be either (a) the historical maximum intensity or (b) historical maximum intensity plus one unit. In both cases, several distant areas (in which earthquakes of intensity ⩾ VIII (MM) have occurred), were considered additional seismic sources. Several published attenuation relationships were also considered. The effects of uncertainty in activity rates were examined by computing rates for several time intervals. The uncertainty in attenuation relationships contributed significantly to the variation in seismic hazard estimates in the northeastern United States. For the sites studied, there was less than half order of magnitude increase in seismic hazard when the maximum magnitude earthquake was assumed to be historical maximum plus one unit rather than the historical maximum at lower site intensities.