Comment on “Review: Strong Ground Motions—Have We Seen the Worst?” by Fleur O. Strasser and Julian J. Bommer

Strasser and Bommer (2009) raise some important issues on maximum amplitudes of strong ground motion parameters. Any method of risk assessment operates under uncertainty and there are trade-offs between costs, benefits, and risks to be considered. Peak ground acceleration (PGA) has been shown to be a very poor indicator of building damage, yet it is commonly used to predict earthquake risk for want of a better one. The credibility of a model of risk management that uses PGA as input depends primarily on its performance, meaning its impact on future actions adopted by decision-makers today. Strasser and Bommer conclude that “it is improbable that the worst possible ground motion has been recorded and that in all probability this situation will not change in the foreseeable future.” The factual basis for this statement remains unclear. Earth materials have a finite strength, and it is not clear why this strength should increase with the period of observation or with the quantity of observations. We are skeptical that “the short length of time for which instrumental recordings are available … implies …