Guidelines for the Scientific Evaluation of an NSHM: The Legacy of the MPS19 Italian Model

In many countries, the acceptance of a new National Seismic Hazard Model (NSHM) to be used in risk reduction strategies usually comes after a deep scrutiny and analysis of the differences with the previous NSHM, in terms of their scientific reliability and of the practical applicability for building code purposes. These two requirements are fully legitimate, but they are conceptually very different and often mixed up in the scientific and public discussion. Although we acknowledge that any use of an NSHM in terms of risk reduction strategies ought to consider many additional nonscientific challenges, we claim that NSHM is a scientific model; hence, its credibility is intimately and exclusively related to its scientific reliability. Stimulated by how the last NSHM in Italy has been evaluated, in this article, we discuss in detail how to test the reliability of an NSHM with the available data, and the meaning of comparing the outcomes of the new and old NSHMs. Finally, we propose recommendations that may guide any scientists to navigate across the tumultuous process of assessing the scientific reliability of an NSHM and comparing, meaningfully, the outcomes of different NSHMs.