A methodology to estimate the hazard of earthquake ground motion at a site has been developed. The methodology consists of systematic procedures to characterize earthquake sources, the seismicity parameters of those sources, and functions for the attenuation of seismic energy, incorporating multiple input interpretations by earth scientists. Uncertainties reflecting permissible alternative inperpretations are quantified by use of probability logic trees and are propagated through the hazard results. The methodology is flexible and permits, for example, interpretations of seismic sources that are consistent with earth-science practice in the need to depict complexity and to accommodate alternative hypotheses. This flexibility is achieved by means of a tectonic framework interpretation from which alternative seismic sources are derived. To estimate rates of earthquake recurrence, maximum use is made of the historical earthquake database in establishing a uniform measure of earthquake size, in identifying independent events, and in detemining the completeness of the earthquake record in time, space, and magnitude. Procedures developed as part of the methodology permit relaxation of the usual assumption of homogeneous seismicity within a source and provide unbiased estimates of recurrence parameters. The methodology incorporates the Poisson-exponential earthquake recurrence model and an extensive assessment of its applicability is provided. Finally, themore » methodology includes procedures to aggregate hazard results from a number of separate input interpretations to obtain a best-estimate value of hazard, together with its uncertainty, at a site.« less