GNGTS 2013 - Atti del 32° Convegno Nazionale

magnitude, epicentral location and average annual rate of occurrence (Gruppo di Lavoro MPS, 2004). In this regard, it is usually assumed that the evolution of seismic activity is a Poisson process, characterized by complete independence among seismic sources (e.g., Stucchi et al. , 2011). In order to estimate the consequences of the various “seismic scenarios” in the sites of interest, the third phase of PSHA concerns the definition of a suitable ground motion model. It is usually represented by a set of empirical relationships between the earthquake magnitude, hypocentral distance and parameters of ground shaking such as the PGA (e.g., Ambraseys et al. , 1996; Sabetta e Pugliese, 1996). In the fourth and last phase, the amplitude of ground motion is computed for all possible “seismic scenarios” related to the various sources of the adopted seismogenic model. This elaboration is performed by using specific resources , as the SeisRisk III code (Bender e Perkins, 1987 and further developments). The results obtained are the so-called “hazard functions”, from which the seismic hazard maps are then obtained. Such maps report the ground motion expected within a given time interval in each point of the study area, with a given exceedence probability. A number of maps can be obtained by the procedure, in relation to different probability values (e.g., 81%, 63%, 50%, 39%, 30%, 22%, 10%, 5% e 2%) and different time intervals (e.g., 30 or 50 years, Gruppo di lavoro MPS, 2004). Moreover, the estimated ground motion parameter may be the PGA or the spectral acceleration, which defines the response of the soil-building system at different natural period of oscillation (e.g., 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, 0.75, 1.0, 1.5 e 2.0 seconds, Gruppo di lavoro MPS, 2004). However, it should be noted that current Italian regulations about seismic classification (e.g., Gruppo di Lavoro MPS, 2004; Ordinanza PCM/3519, 2006) require the map of PGA values expected in 50 year for a 10% exceedence probability. Furthermore, the PSHA may provide hazard maps in term of expected macroseismic intensity, usually in the scale Mercalli-Cancani-Sieberg or MCS (e.g., Gomez Capera et al. , 2010). This last kind of elaboration will be discussed later , in relation to the probabilistic estimates proposed for central-northern Italy. Main problems of the PSHA. An outstanding problem of the PSHA is the reliability of the hypotheses underlying this kind of approach. For instance, seismic activity is considered a steady-state process. This implies that the knowledge of past earthquakes may be used to predict some aspects of the future evolution of the seismicity. However, earthquake catalogues are reliable and satisfactorily complete for short time intervals only (the last centuries at most, e.g., Gruppo di Lavoro MPS, 2004). Of course, such intervals are much shorter than the periods in which seismogenic fault systems have developed in the study area (of the order of a million years). Moreover, several investigations (e.g., Castanos e Lomnitz, 2000; Stein et al. , 2013 and references therein) point out that concepts often involved in the probabilistic hazard assessments, such as “seismic cycle”, “characteristic earthquake” and “seismic gap”, can not easily be applied to the interpretation of the real seismic activity. Finally, there is a growing awareness that seismogenic faults may have long-term and long-range interactions, in contrast with the hypothesis of complete independence of seismic sources (e.g., Scholz e Gupta, 2000; Lou e Liu, 2010, Mantovani et al. , 2012a,b, 2013; Viti et al. , 2012, 2013). Another major problem is testing the predictions provided by PSHA. Indeed, a rigorous verification of hazard maps would require many records of strong motion in a very long time interval, i.e. several consecutive periods of 50 years (Meletti et al. , 2012; Frankel, 2013). A preliminary analysis has revealed a considerable discrepancy between the strong motion values, recorded at various Italian accelerometric stations in the period 1975-2005, and the PGA values predicted in the same sites by the PSHA (Albarello and D’Amico, 2008). In particular, the probabilistic assessment has underestimated the level of seismic shacking in 19 out the 68 accelerometric sites considered. Several authors (e.g., Reyners, 2011; Stein et al. , 2012) have shown that the PSHA estimates performed at the end of the last century have not adequately predicted the ground motion 475 GNGTS 2013 S essione 2.3