The Challenge of Defining Upper Bounds on Earthquake Ground Motions

Recent studies to assess very long-term seismic hazard in the United States and in Europe have brought the issue of upper limits on earthquake ground motions into the arena of problems requiring attention from the engineering seismological community. Few engineering projects are considered sufficiently critical to warrant the use of annual frequencies of exceedance so low that ground-motion estimates may become unphysical if limiting factors are not considered, but for nuclear waste repositories, for example, the issue is of great importance. The definition of upper bounds on earthquake ground motions also presents an exciting challenge for researchers in the area of seismic hazard assessment. This paper looks briefly at historical work on maximum values of ground-motion amplitudes before illustrating why this is an important issue for hazard assessments at very long return periods. The paper then discusses the factors that control the extreme values of motion, both in terms of generating higher amplitude bedrock motions and of limiting the values of motion at the ground surface. Possible channels of research that could be explored in the quest to define maximum possible ground motions are also discussed.