Une raison suffisante pour redéfinir l’intensité sismique: éviter des erreurs dans l’estimation de l’aléa sismique

The object of the paper is represented by an attempt to contribute to an adaptation of the concept of seismic intensity to the needs of engineering activities. A first condition in this connection is to pay the attention due to the spectral contents of ground motion. A case where rough approaches have led to erroneous estimates of ground motion severity, resulting in erroneous hazard estimates is presented. The main conclusion derived is represented by the strong need to specify the spectral interval for which macroseismic data are relevant and, as a consequence, to consider (and even to define) seismic intensity derived from post-earthquake surveys as related to a certain spectral interval. A methodology proposed in order to define and determine seismic intensity on the basis of instrumental data and, moreover, to define and determine seismic intensity spectra is then presented. Some illustrative examples are given.