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

Abstract

We show that macroseismic intensities assessed in Italy in the last decade are not homogeneous 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 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 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.

A theoretical comparison among macroseismic scales used in Italy

“…Even if such theoretical argument appears absolutely sound, it is contradicted by the empirical evidence shown by Vannucci et al (2021) (in their Table 2) that the average difference between ("all") intensities estimated by the EMS and the MCS scales in Italy is slightly but signi cantly negative (-0.057±0.025 degrees). The difference is strongly negative (-0.591±0.046 degrees) for the MCS intensities estimated by Graziani et al (2015) for "old town centers" struck by the 2012 Emilia sequence where the building types were more similar to those existing at the time when Sieberg (1912Sieberg ( , 1932 de ned the MCS scale (see Guidoboni, 2000 andVannucci et al, 2015 for English translations of the MCS scale from the German original of Sieberg).…”

“…The difference is clearly positive (about 0.2-0.3 degrees) only for the intensities estimated by Galli et al (2021aGalli et al ( , 2021b according to the scale proposed by Molin (2003Molin ( , 2009 for the purpose of emergency macroseismic survey for civil protection uses. The latter consists of a complete rede nition of the MCS scale with the addition of a new intensity degree (V-VI) and the addition and/or the modi cation of many percentages of damaged building for the various degrees with respect to the original MCS scale (see Galli et al, 2012a and the Supplemental material of Vannucci et al, 2021 for English translations of most relevant tables of Molin, 2009).…”

“…However, as already noted by Vannucci et al (2021), the EMS does not derive directly from the MCS but rather from the Medvedev-Sponeur-Karnik (MSK) scale (Medvedev et al, 1967, Medvedev, 1977. It is therefore possible that, as a result of various rede nitions over the decades, MSK had lost its strict connection with MCS or with the way the MCS was applied in Italy by some researchers (e.g., Ferrari and Guidoboni, 2000) so that the EMS, which is based on the MSK, might no longer be compatible with the MCS.…”

Comparison Between Two Methodologies for Assessing Historical Earthquake Parameters and Their Impact on Seismicity Rates in the Western Alps

“…Events with residuals exceeding 0.5 are related to four small magnitude events Mw< 3.3 for which predictions are extrapolated beyond magnitude used for the calibration or to a specific earthquake for which the instrumental estimate may be overestimated (1920 with magnitude 5.43 Mw). Indeed the instrumental magnitudes reported before the deployment of the World Wide Standardized Seismic Network - WWSSN (i.e., 1960WWSSN (i.e., -1964) can be overestimated (Vannucci et al, 2021).…”

Determination of the major axis direction and macroseismic epicenter of the assessed intensity map based on relocated aftershock sequences

“…Discrepancies exist in the major axis directions of the assessed and actual intensity maps, as well as in the locations of macroseismically and instrumentally derived epicenters in some cases. The assessed intensity maps not only assist decisionmakers in organizing emergency rescue efforts and formulating recovery strategies but also serve as valuable resources for further seismic studies, including the exploration of empirical relationships between macroseismic intensities and epicentral distances (Sun et al, 2014;Vannucci et al, 2021). Therefore, improving the assessment of intensity direction is a challenging issue that requires attention and resolution.…”