Inhomogeneity of Macroseismic Intensities in Italy and Consequences for Macroseismic Magnitude Estimation

We show that macroseismic intensities assessed in Italy in the last decade are not homo-
geneous with those of the previous periods. This is partly related to the recent adoption
of the European Macroseismic Scale (EMS) in place of the Mercalli–Cancani–Sieberg
(MCS) scale used up to about one decade ago. The underestimation of EMS with respect
to MCS is about a half of a degree on average and, even more significant, if the MCS
intensities are estimated according to the approach developed for the quick evaluations
of damage by macroseismic seismologists of the Italian Department of Civil Protection.
We also show the inhomogeneity over time of the average differences between instru-
mental and macroseismic magnitudes computed from intensity data, indicating an aver-
age overestimation of magnitudes of about 0.3 units for the instrumental ones before
year 1960 and of about 0.2 units for the macroseismic ones after such date. This is con-
sistent with previous studies that hypothesized the incorrect calibration of mechanical
recording seismometers operating in Italy and in the surrounding countries before the
introduction of the standard electromagnetic seismometers from the beginning of
1960s. For such reasons, the magnitudes of preinstrumental earthquakes in the
Catalogo Parametrico dei Terremoti Italiani seismic catalog, used for the most recent
seismic hazard assessment in Italy, might be overestimated, on average, by about
0.1–0.2 magnitude units.