GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 1.1 GNGTS 2024 Analysis and preliminary results of the Mw 4.9, Marradi seismic sequence (September 18th, 2023), in the northern Apennines, carried out by the BSI working group. A. Lisi, L. Arcoraci, P. Batelli, M. Berardi, B. Castello, D. Latorre, A. Marchet, M. Michele, V. Misit, A. Nardi, D. Piccinini, A. Rossi, Gruppo di lavoro del Bolletno Sismico Italiano*  * A. Batelli, B. Cantucci, C. Castellano, D. Cheloni, M.G. Ciaccio,  L. Colini, M. De Caro, A. Frepoli, L. Improta, A.M. Lombardi, A. Malagnini, L. Margherit, M.T. Mariucci, G. Mele, C. Melorio, L. Miconi, G. Modica, C. Montuori, N.M. Pagliuca, M. Pastori, S. Pinzi, L. Pizzino, A. Sciarra, T. Sgroi, S. Spadoni, R. Tardini, C. Thermes, A. Bono, I. Carluccio, E. Della Bina, M. Fares, D. Franceschi, V. Lauciani, A. Mandiello, M. Maniscalco, S. Pintore, M. Quintliani, M. Moret, R. Di Stefano, L. Scognamiglio Isttuto Nazionale di Geofsica e Vulcanologia, Roma, Italy On September 18, 2023, an earthquake with a magnitude of M L =4.8 (Mw=4.9) occurred a few kilometers SW of Marradi (FI) at a depth of about 8 kilometers. The computed TDMT soluton of the mainshock suggests a normal fault oriented NW-SE (Scognamiglio et al., 2006). The earthquake, preceded by a foreshock with a magnitude of M L =3.3 (Mw=3.4), triggered a seismic sequence characterized, in the frst two months, by approximately 700 afershocks localized by the staf on duty in the Seismic Monitoring Room of the INGV in Rome, including 6 events with a magnitude of M L ≥3.0 occurred in the frst two days. The sequence occurred in a high seismic hazard region. The two closest historical earthquakes occurred in the Mugello area about 30 km SW of Marradi (and about 25 km north of Florence): one, whose magnitude (Mw) is estmated to be about 6.0, occurred on June 13, 1542 while the other with estmated magnitude (Mw) of about 6.4 occurred on June 29, 1919. The second one is among the strongest (most signifcant) Italian earthquakes of the 20th century, and also one of the strongest known to date with its epicentre in the northern Apennines. The afected area was that of Mugello, with extensive damage both in the province of Florence and on the Romagna side of the Apennines. The analysts of the BSI (Italian Seismic Bulletn) reviewed the inital three days of the sequence, paying special atenton to the hours directly following the mainshock. The BSI work mainly consists in revising the picking of P and S phases and assigning them appropriate weights, retrieving previously unused phases, and evaluatng the maximum amplitudes necessary for calculatng the value of M L . The later is a critcal aspect of the inital phases of a seismic sequence; in fact, the occurrence of events is very close in tme, making it challenging to estmate the