GNGTS 2017 - 36° Convegno Nazionale

32 GNGTS 2017 S essione 1.1 epicentral intensity is half degree lower than the observed one ( I 0 – b , data in Tab. 1). For the ESI scale, the trend is opposite and predicted epicentral intensity is about half degree higher than the observed one. Intensity attenuation with distance: generalization. In order to properly compare events with different I 0 , we follow the approach used by Gasperini (2001) on the database of macroseismic observations provided by Monachesi and Stucchi (1997). The difference between epicentral and local intensity has been computed for each MDP, in the form: ΔI = I 0 - I i (2) where I 0 is the epicentral intensity and I i the local intensity at the i-th point. Hypocentral distances were calculated from epicentral distances and assuming a fixed depth of 10 km, in analogy with the work by Gasperini (2001). Fig. 1 shows ΔI as a function of the hypocentral distance, where a distance binning of 5 km is adopted. For MCS data, we found the same result of Gasperini (2001), i.e., a difference in slope around 45 km, that we define as the boundary between near- and far-field. ESI data show indeed a linear trend up to 75 km. Fig. 1 - Differences between hypocentral and local intensity (Δ I ) as a function of distance, for the MCS and ESI database. Mean values averaged in 5 km bins are shown as black circles. Only the intervals with at least 5 occurrences are considered.