The attenuation of seismic intensity in Italy: A bilinear shape indicates the dominance of deep phases at epicentral distances longer than 45 km

The attenuation of seismic intensity with distance in Italy is analyzed by using felt intensity report data obtained from two comprehensive historical data-bases recently made available. The observed attenuation pattern that in the past was interpreted as a logarithmic or root (square or cubic) attenuation law shows quite clearly two different linear trends in the near and in the far field. At distances shorter than 45 km, the decrease of the intensity with distance is about one degree per 20 km, while at longer distances the slope is about one degree per 50 km. This is in agreement with some recent findings of realistic modeling of seismic ground motion that has been explained as the transition from upper-crust direct Sg phases to waves reflected at the Moho controlling the energy main release. The slope of the curve in the far field shows a regional dependence in agreement with recent works on the attenuation of Pn and Sn phases in Italy. If effective, this correlation might allow us to discriminate the contribution of crustal and subcrustal paths in seismic intensity attenuation studies.