GNGTS 2021 - Atti del 39° Convegno Nazionale

233 GNGTS 2021 S essione 2.1 INHOMOGENEITY OF MACROSEISMIC INTENSITIES IN ITALY AND CONSEQUENCES FOR MACROSEISMIC MAGNITUDE ESTIMATION G. Vannucci 1* , B. Lolli 1 , P. Gasperini 1,2 1 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Bologna 2 Dipartimento di Fisica e Astronomia, Università di Bologna We show that macroseismic intensities assessed in Italy in the last decade are not homogeneous with thoseof thepreviousperiods (Fig.1). This ispartly related to the recent adoptionof theEuropean Macroseismic Scale (EMS) in place of theMercalli Cancani Sieberg (MCS) scale used up to about one decade ago. The underestimation of EMS with respect toMCS is about a half of a degree on average and evenmore significant if theMCS 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 instrumental and macroseismic magnitudes computed from intensity data, indicating an average 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 consistent 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 pre-instrumental earthquakes in the CPTI15 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. Corresponding author: gianfranco.vannucci@ingv.it Fig. 1 - Average difference between macroseismic (Mw macro ) and instrumental (Mw instrum ) magnitudes (solid line) and number of earthquakes used for the comparison (grey bars) over five-year time intervals from the CPTI15 catalog. The dotted lines indicate the average differences in the intervals 1905-1959 and 1960- 2009. Error bars indicate the standard errors (1s) of the mean differences. Earthquakes in volcanic areas of Mt. Etna, Mt. Vesuvius, Ischia Island and Campi Flegrei are excluded (see text).