When is a site-specific PSHA worth the effort? The answer has less to do with project type and more to do with uncertainty. A site-specific hazard study is not simply a way to obtain a different spectrum—it is a way to better understand the assumptions, sources, site conditions, and uncertainties that control the seismic hazard. This article explores when that additional understanding can meaningfully influence engineering decisions.

If the USGS Multi-Period Response Spectrum (MPRS) has become so sophisticated, what does a site-specific PSHA actually do differently? This article explores the key ingredients of seismic hazard analysis, including source characterization, recurrence relationships, ground-motion models, logic trees, site conditions, and basin effects, and explains how site-specific studies differ from the national hazard model.

Since the adoption of MPRS in ASCE 7-22, I have occasionally heard the question: "Do we still need site-specific seismic hazard analyses?" It's a fair question. The modern USGS hazard model is more sophisticated than ever before, but it remains a national model. This article examines what MPRS improved, what limitations remain, and where site-specific studies continue to provide value.