GNGTS 2014 - Atti del 33° Convegno Nazionale

48 GNGTS 2014 S essione 1.1 Coseismic and post-seismic slip of the 2012 Emilia (northern Italy seismic sequence inferred from joint inversion of high-precision levelling, InSAR and GPS data D. Cheloni 1 , R. Giuliani 2 , N. D’Agostino 1 , M. Mattone 2 , M. Bonano 3 , G. Fornaro 3 , R. Lanari 3 , D. Reale 3 , F. Deninno 4 , R. Maseroli 4 , G. Stefanelli 4 1 Centro Nazionale Terremoti, Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy 2 Ufficio Rischio Sismico, Dipartimento della Protezione Civile, Rome, Italy 3 Istituto per il Rilevamento Elettromagnetico dell’Ambiente, Consiglio Nazionale delle Ricerche, Naples, Italy 4 Sezione Geodetica, Istituto Geografico Militare, Florence, Italy Introduction. The 2012 Emilia earthquake sequence struck the central sector of the Ferrara arc, which represents the external fold-and-thrust system of the northern Apennines thrust belt buried below the Po plain. The Ferrara arc is composed by two major fold-and-thrust systems, the Ferrara and the Mirandola systems, where the two main seismic events (the 20 May M W 5.9 and the 29 May M W 5.8) occurred. However, seismological, geodetic and geological-based works that have been published so far have clarified neither the exact association between activated thrust segment and related mainshock, nor the fine detail of the geometry of the activated fault systems during the Emilia seismic sequence. Fault-plane solutions of the Emilia sequence (Malagnini et al. , 2012; Pondrelli et al. , 2012; Scognamiglio et al. , 2012; Cesca et al. , 2013) obtained very similar solutions for the two mainshocks, showing low dip angles for the south dipping planes. Preliminary GPS and InSAR based uniform slip models (Bignami et al. , 2012; Serpelloni et al. , 2012) also indicate that the two main events occurred on S-dipping low angle fault planes, tentatively associated to segments belonging to the ITCS050 “Poggio Rusco-Migliarino” (for the May 20 event) and a segment of the ITCS051 “Novi-Poggio Renativo” (for the May 29 event) composite seismogenic sources of the Database of Individual Seismogenic Sources (DISS Working Group, 2010). Conversely, preliminary seismotectonic interpretation (Lavecchia et al. , 2012) attributed both the May 20 and May 29 events to the Ferrara system. According to the authors the May 20 event ruptured the Ferrara basal thrust at depth between 3-7 km, while during the May 29 event the rupture jumped on an inner splay of the Ferrara system (assuming that the Mirandola thrust have been only subordinately involved in the sequence). Subsequent InSAR based studies (Pezzo et al. , 2013; Tizzani et al. , 2013) have allowed to solve also for more complicated geometries and for the variable slip distribution on the two fault segments activated during this sequence. In particular, Pezzo et al. (2013) identified two main fault planes for the Emilia seismic sequence, computing the corresponding slip distributions: the May 20 event was modelled with a variable dip geometry (with a shallower segment dipping at 40° SSW followed at depth by a 20° SSW sloping segment), corresponding to the middle Ferrara thrust, with a maximum slip of ~120 cm at 5 km depth; for the source location of the May 29 event the authors approximated the shape of the frontal Mirandola thrust by a shallower segment dipping 45° S continuing at depth in a 30° S dipping segment, with two main slipping areas (a maximum slip of ~54 cm at 6 km depth and a lower slip concentration of ~30 cm at 5 km depth on the western side of the modelled fault plane). Tizzani et al. (2013) by means of an advanced modelling based on a finite-element structural-mechanic method, proposed instead the activation of a single low angle fault segment for the May 20 event, corresponding to the inner Ferrara thrust, and the activation of three different segments of the Mirandola anticline for the May 29 event, which describe an overall listric geometry with the steeper part that dips ~65° between 5-8 km depths. Finally, by accurate aftershock location Govoni et al. (2014) defined two separate segments arranged in a left-stepping en-enchelon pattern, whose dip changes from steeper to flatter going west to east. In particular, from the cross sections across the two main shocks, they estimate the dip of the western (through the May 29 event) and eastern (through