Lateral variations of seismic intensity attenuation in Italy

A tomographic study of the attenuation of seismic intensity in the Italian territory has been carried on the basis of a felt report database including more than 50 000 macroseismic observations. The spatial variations of the attenuation coefficients have been computed on meshes of 50 and 25 km and compared with other geophysical observables. By checkerboard and restore tests using a Gaussian error with a realistic amplitude of one intensity degree we verified that a selected set including about 20 000 observations is able to reliably reproduce the imposed patterns. For the laterally varying attenuation model we also found a general reduction and a more uniform distribution of the average locality residual with respect to an isotropic attenuation law. The comparison of the inversion results with seismic velocity tomography of the crust and upper mantle shows fair correspondences between high-attenuation and low-velocity areas (Tyrrhenian slope of northern and central Apennines) as well as between low-attenuation and high-velocity ones (Po valley and Adriatic coast). The normalized attenuation functions computed for some areas of Italy also agree fairly well with empirical non-parametric attenuation functions determined by others, from accelerometer data. A clear correlation was found between the inferred behaviour of the slope of the attenuation function in the vicinity of the source (distance < 45 km) and the heat flow. In fact, the most attenuating zones almost coincide with the highs of heat flow located along the northern Tyrrhenian coast of Tuscany and Latium and in the other volcanic areas (Campi Flegrei, Mount Etna, Colli Euganei and Monti Lessini). This clear correlation represents a convincing confirmation of the physical grounds on which the use and interpretation of macroseismic data is based.