The impact on probabilistic ground‐motion hazard of different definitions of the horizontal component of ground shaking is examined. The scope is to highlight how such a minor detail within the complex computation chain of a probabilistic seismic‐hazard assessment can play a crucial role on final results. This is achieved by comparing hazard maps produced for Italy considering different definitions of the ground‐motion component at different spectral periods. In our exercise, special attention is paid to the treatment of the aleatory variability of ground motion (sigma) when one switches from one metric to another. The results show that differences in the definition of the horizontal component could imply differences in the hazard results as large as 40%.
Online Material: Color maps of geographical distributions of hazard value ratios.