2024 EERI Annual Meeting (2024AM)

Helen Crowley, Secretary-General, Global Earthquake Model (GEM) Foundation; Editor, Earthquake Spectra

The Joyner Lecture honours the distinguished career of William B. Joyner at the U.S. Geological Survey and his abiding commitment to continuing communication and education at the interface between research findings of earthquake science and the practical realities of earthquake engineering.

One such interface that has been the source of much debate in recent years relates to the use of probabilistic seismic hazard assessment (PSHA) models as the basis for seismic design and assessment. Since the publication of the landmark paper by Cornell in 1968, PSHA has become the standard approach for defining the seismic actions in design codes. Before then, following the Messina (Italy) earthquake in 1908, seismic zonation maps based on observed macroseismic intensity from past earthquakes were used to define where, and to what level, buildings should be designed to withstand the lateral forces from earthquakes. These zonation maps were often, and somewhat inevitably, updated after damaging earthquakes.

Despite the clear improvements that the use of a PSHA-based approach brought, criticism was frequently placed on the outputs of these models, such as when the ground motions from earthquakes were seen to exceed those underlying the design code, suggesting a widespread misunderstanding of the meaning of the latter. Since the 2000’s, there has been a move towards explicitly discussing the levels of risk that are being accepted by the code, together with the consequences expected under the design levels. This has led, in the United States, to the adoption of the so-called risk-targeted approach for defining the seismic actions, which aims at harmonising the probability of collapse of buildings across the region of interest. This methodology has not yet been widely adopted by design regulations in other parts of the world, though in Italy, for instance, a significant effort has been made to evaluate the underlying spatial variation of risk to buildings designed to the latest standards. Whilst a more explicit recognition of the level of risk associated with seismic design codes has been an important step forward in Italy, the latest update to the PSHA model, developed in 2019 by the Italian Geological Survey (INGV), has continued to receive criticism and has even been rejected as the basis for an update of seismic actions in the design code. In this lecture, the argument will be made that the onus should now be on structural engineers to demonstrate the impact of these changes in terms of the levels of risk to the building stock, and how resilience has been, and can continue to be, built into the code to accommodate such changes resulting from advances in earthquake science.