Seismic Hazard Assessments: current issues and advanced approaches for effective Disaster ReductionA. Peresan (1,2), G.F. Panza (1,2), F. Romanelli (1,2), G. Kossobokov (3,4) Operational issues are a pressing concern in seismic hazard assessment (SHA), as fatally evidenced by the recent most destructive events, including the Tohoku (2011) and Haiti (2010) earthquakes. A reliable and comprehensive characterization of expected seismic ground shaking is essential to improve building codes, particularly for the protection of critical infrastructures and for land use planning. Nowadays it is well recognized by the engineering community that standard hazard indicator estimates (e.g. seismic PGA) alone are not sufficient for the adequate design, mainly for special buildings and infrastructures. Moreover, any effective tool for SHA must demonstrate its capability in anticipating the ground shaking related with large earthquake occurrences, a result that can be attained only through rigorous verification and validation process. So far, the major problems in classical probabilistic methods for seismic hazard assesment (PSHA) consisted in the adequate description of the earthquake recurrence, particularly for the largest and unfrequent events, and of the attenuation models, which may be unable to account for the complexity of the medium and of the seismic sources and are often weekly constrained by the available observations. Current computational resources and physical knowledge of the seismic waves generation and propagation processes allow nowadays for viable numerical and analytical alternatives to the use of attenuation relations. Accordingly, a scenario-based approach to SHA at different scales - regional, national and metropolitan – is proposed, which allows considering a wide range of possible seismic sources as the starting point for deriving scenarios by means of full waveforms modelling. The method does not make use of attenuation relations and permits to carry on parametric analysis and stability tests that may contribute to characterise the related uncertainties, as well as to fill in the unavoidable gaps in available observations. Besides the standard estimates, the flexibility of the scenario-based method permits to account for earthquake recurrence and eventually allows for the generation of ground shaking maps, which provide the hazard in terms of probability of exceedance of a given threshold of ground motion at a specific site. The comparative analysis of different seismic hazard estimates and cross-checking against available observations, performed for the Italian territory, clearly evidenced one of the basic limits of PSHA estimates, that is the overly dependency of ground shaking on earthquakes recurrence (i.e. on the probability threshold selected for the maps). In view of the critical dependence of classical PSHA estimates on earthquake recurrence, which is poorly characterized particularly in areas with a low level of current seismicity and a limited time span of available observations, it appears preferable to resort to a scenario-based approach to seismic hazard assessment. From an anthropocentric perspective, buildings and other critical structures should be designed capable to resist future earthquakes. When an earthquake with a given magnitude M occurs, it causes a specific ground shaking that certainly does not take into account whether the event is rare or not; thus ground motion parameters for seismic design should not be scaled depending on earthquake recurrence. |
