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 databases recently made available. The observed attenuation pattern that i n the past was interpreted as a logarithmic or root (square or cubic) atten uation 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 inte nsity with distance is about one degree per 20 km, while at longer distance s the slope is about one degree per 50 km. This is in agreement with some r ecent findings of realistic modeling of seismic ground motion that has been explained as the transition from upper-crust direct Sg phases to waves ref lected at the Moho controlling the energy main release. The slope of the cu rve in the far field shows a regional dependence in agreement with recent w orks on the attenuation of Pn and Sn phases in Italy. If effective, this co rrelation might allow us to discriminate the contribution of crustal and su bcrustal paths in seismic intensity attenuation studies.