SUMMARY
In this study we examine uncertainty and parametric sensitivity of Peak Ground Acceleration (PGA) and 1-HzSpectral Acceleration (1-Hz SA) in probabilistic seismic hazard maps (10% probability of exceedance in 50years) of Northern and Central Italy. The uncertainty in hazard is estimated using a Monte Carlo approach torandomly sample a logic tree that has three input-variables branch points representing alternative values for bvalue,maximum magnitude (Mmax) and attenuation relationships. Uncertainty is expressed in terms of 95% confidenceband and Coefficient Of Variation (COV). The overall variability of ground motions and their sensitivityto each parameter of the logic tree are investigated. The largest values of the overall 95% confidence bandare around 0.15 g for PGA in the Friuli and Northern Apennines regions and around 0.35 g for 1-Hz SA in theCentral Apennines. The sensitivity analysis shows that the largest contributor to seismic hazard variability is uncertaintyin the choice of ground-motion attenuation relationships, especially in the Friuli Region (?0.10 g) forPGA and in the Friuli and Central Apennines regions (?0.15 g) for 1-Hz SA. This is followed by the variabilityof the b-value: its main contribution is evident in the Friuli and Central Apennines regions for both 1-Hz SA(?0.15 g) and PGA (?0.10 g). We observe that the contribution of Mmax to seismic hazard variability is negligible,at least for 10% exceedance in 50-years hazard. The overall COV map for PGA shows that the uncertaintyin the hazard is larger in the Friuli and Northern Apennine regions, around 20-30%, than the Central Apenninesand Northwestern Italy, around 10-20%. The overall uncertainty is larger for the 1-Hz SA map and reaches 50-60% in the Central Apennines and Western Alps.