Codes, models and reality: Reductionism vs. holism in a review of microzonation studies in the Umbria-Marche region

Abstract

In the 10 years since the Umbria-Marche earthquake, several microzonation studies were carried out in the two regions. In the immediate aftermath of the event, the focus was on the epicentral area and toward emergency intervention and reconstruction plans. In the following years, regional and national projects aimed to transfer the lessons learned to other towns. Usually, those two kinds of microzonation studies are referred to as «simplified» and «detailed». The difference is more subtle, and leads to the question of whether a microzonation study can be tackled following a reductionist approach, i.e. leaving different experts taking care of a limited subject (geology, geophysics, seismology, geotechnics, structural engineering). The impression looking back at 10 years of studies is that a holistic approach would be more appropriate to describe a system (structure-soil-bedrock) that is non-linear, inhomogeneous, and presenting feedback among its components. A second problem that emerges is the link with codes and practitioners. During the past 10 years the seismic code has been changed and a new version is on arrival. The last proposed version of the code is based on a parameter (Vs30) that is discussed in the same country where it was first adopted, and introduces a parameter (acclivity) that appears to be a second-or third order problem with respect to others that are completely disregarded (e.g., 2-d site effects). A possible explanation for this mismatch between codes, models and reality is that our knowledge of distribution and amplitude of site effects is biased by selective under-sampling. Being driven by damage, and paying less attention to a uniform distribution of studied sites and situations, we act like a drunk man looking of his lost keys under a street lamp, not because he is sure that he lost them there, but because the light is there.