Scenario-based time-dependent definition of sesimic input: an effective tool for engineering analysis and seismic isolation design

For the protection of cultural heritage and the design of strategic buildings and critical infrastructures, when it is necessary to consider extremely long time intervals, the standard probabilistic estimates of seismic hazard (PSHA) are by far not applicable. The heuristic limitations are, indeed, a major limit of PSHA, whose results, extrapolating ground motion with an infinitely long return period from a few hundreds years of the available earthquake catalogues, may turn out to be a purely numerical artefact. A viable alternative, capable of minimizing the drawbacks of PSHA is represented by the use of a scenario-based methodology, named neo-deterministic approach (NDSHA), which relies on observable data and is based on the physical modelling of seismic waves generation and propagation processes. The NDSHA naturally supplies realistic time series of ground motion, which represent also reliable estimates of ground displacement readily applicable to seismic isolation techniques, useful to preserve historical monuments and relevant man made structures. In addition, an integrated NDSHA approach has been developed that allows for the operational definition of time-dependent scenarios of ground shaking, through the routine updating of formally defined earthquake predictions. The integrated NDSHA procedure, which is currently applied to the Italian territory, combines different pattern recognition techniques, designed for the space-time identification of strong earthquakes, with algorithms for the realistic modelling of ground motion. Accordingly, when an alarm is declared, a set of scenarios of expected ground shaking at bedrock, associated with the alarmed areas identified by means of the algorithms CN and M8S, can be readily computed by means of full waveform modelling, both at regional and local scale, considering all of the possible earthquake sources within the alerted areas. For the relevant sites, further investigations can be performed taking into account the local soil conditions, in order to compute the seismic input (realistic synthetic seismograms) for engineering analysis. The practical application of seismic input modelling for seismic isolation purposes, and particularly for the protection of historical buildings, has been already carried out for example for several sites, including the cities of Trieste (NE Italy) and Valparaiso (Chile) and the towns of Marigliano and Ercolano (Naples, Italy). A recent example of the practical advantages that can be provided by the time-dependent definition of ground-shaking scenarios is given by the Cathedral of Santa Maria di Collemaggio, which was severely damaged during the L’Aquila earthquake (6 April 2009). Based on the ongoing alert for the nearby CN region and the relevant ground shaking expected at the site, the restoration and isolation of the cathedral could have been timely completed, possibly limiting if not preventing the occurred damage. The relevance of the realistic modelling, which permits the generalization of empirical observations by means of physically sound theoretical considerations, is evident, as it allows for the optimisation of the structural design with respect to the site of interest. Moreover, the time information given by the intermediate-term middle-range earthquake prediction can enhance preparedness and planning of risk mitigation actions.