GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 1.1 29 of the seismic moment M 0 . Assuming that after the last strong shock on 25 January most of the fault displacement associated with the seismic sequence was released, and given that the 25 January MDPs (Fig. 3d) virtually provide the final (cumulative) damage scenario, we supposed that the relevant M w could represent with a good approximation the total M 0 released at the end of the sequence. By applying the formula of Hanks and Kanamori (1979), we first calculated the moments (Tab. 3) of each events from the M w values obtained by the Boxer code, and then we subtracted from the M 0 for a given day those of the previous shocks. For instance, from the 15 January M 0 , we subtracted the M 0 calculated for the 14; again, from the M 0 of 16 January, we subtracted the M 0 of 15 and 14 January, etc. Finally, we obtained by the same formula, the “corrected” values of M w . This procedure minimizes, at least in part, the magnitude overestimation due to cumulative damage during a sequence; while there is not difference in the case of 15 January, the corrected Ms values for 16 and 25 result increasingly smaller than the ones calculated by the Boxer code. Table 2 - Macroseismic parameters computed for the four main earthquakes and comparison with the instrumental data by ISC reported in the CPTI15 [Rovida et al. , 2016]; Δ Epicentre indicates the distance between macroseismic and instrumental locations. Date        Epicentre I 0 M W M W Δ Epicentre Lat N Long E (this study) (instrumental) (km) 14 January 1968 37.801 13.000 7.0 5.04 ± 0.35 5.15 ± 0.27 5.7 15 January 1968 37.753 12.970 9.0 5.97 ± 0.18 5.67 ± 0.23 6.0 16 January 1968 37.756 12.981 9.5 6.17 ± 0.27 5.27 ± 0.21 11.5 25 January 1968 37.756 12.995 10 6.34 ± 0.22 5.15 ± 0.21 8.8 Table 3 - Daily total seismic moment M0 obtained from the macroseismic M w through the Hanks and Kanamori (1979) formula: M w =2/3 log M 0 –10.7. The M w corrected values represent the final estimates proposed in this study. Date M W Log M 0 M 0 M 0 M w M w (cumulative) (from Boxer) (dyne-cm) (by subtraction) corrected Instumental 14 January 1968 5.04 18.260 1.8197E+18 5.04 5,01 15 January 1968 5.97 19.655 4.5186E+19 4.33659E+19 5.96 5,89 16 January 1968 6.17 19.955 9.0157E+19 4.31518E+19 5.96 5,93 25 January 1968 6.34 20.210 1.6218E+20 2.50186E+19 5.80 5.95 We validated these results with the same procedure applied in a reverse way to the instrumental M w values of all 1968 events listed in CPTI15. By adding the moments of the shocks occurring in the same day, we obtained a cumulative M 0 , which was then converted into the M w . These “cumulative” M w instrumentally derived are very similar (inside the error) to the “corrected” M w derived from macroseismic data (Tab. 3), with an evident effect of saturation since 16 January. References Bosi C., Cavallo R. and Manfredini M.; 1968: Il terremoto della Valle del Belice del gennaio 1968. in “Rassegna dei Lavori Pubblici”, n.2, febbraio 1968, Roma. Cannata D., Costantino M., D’Amore A., Gregorio G., Irti M., Pasta A., Priolo D., Stura V. and Miglietti G.; 1968: Missione di studio nelle zone terremotate della Sicilia Occidentale . In “L’industria delle costruzioni”. Rivista tecnica dell’ANCE”, settembre-ottobre 1968, Roma. De Panfilis M. and Marcelli L.; 1968: Il periodo sismico della Sicilia occidentale iniziato il 14 Gennaio 1968 . Ann. Geofis., 21, 79 pp. Gasperini P., Vannucci G., Tripone D. and Boschi E.; 2010: The location and sizing of historical earthquakes using the attenuation of macroseismic intensity with distance . Bull. Seism. Soc. Am., 100, 2035-2066.