GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 1.1 21 ANALYSIS OF THE AUGUST - OCTOBER 2016, CENTRAL ITALY, COSEISMIC SURFACE FAULTING SLIP-DISTRIBUTIONS, PARAMETERIZATION AND COMPARISON WITH GLOBAL EARTHQUAKES F. Brozzetti 1 , P. Boncio 1 , D.Cirillo 1 , F. Ferrarini 1 , R. de Nardis 1-2 , A. Testa 1 , F. Liberi 1 , G. Lavecchia 2 1 CRUST-DiSPUTer, University “G. d’Annunzio”of Chieti-Pescara, Via dei Vestini 31, 66100, Chieti, Italy 2 Italian Department for Civil Protection, Via Vitorchiano 4, 00189, Rome, Italy Introduction. We analyze in detail the coseismic surface faulting exposed along a segment of the Mt Vettore-Mt Bove fault system (VBF, central Italy), that activated during the 24 August 2016, Amatrice event (M w 6.0) and soon after re-activated during the 30 October 2016, Norcia mainshock (M w 6.5). We systematically recognized the coseismic surface ruptures of the two events, which document an example of rupturing on the same seismogenic structure in close temporal succession. Coseismic surface slip on the same fault planes, during both the 24 August and 30 October events is shown by the “double-slip free faces” well exposed along the southern segment of the VBF, named here Vettoretto-Redentore segment (VRS). Such a feature represents , the outcropping geological expression of the 24 August-30 October foreshock-mainshock pair nucleated on the VRS. Reconstructing the geometrical pattern of such a pair and comparing the spatial distribution of coseismic slip with the deep sources of the two events might help understanding the causes of this double activation and the control played by structural complexities on rupture nucleation and propagation. We provide the coseismic parameters of the 24 August VRS rupture and of the entire earthquake rupture, which are the surface rupture length (SRL), average displacement (AD) and maximum displacement (MD). Further aims of our work are: i) demonstrate the primary origin of the surveyed surface ruptures; ii) analyze the slip distribution of the 24 August and 30 October events and compare them with the long-term (Pleistocene to present) segmentation pattern and displacement, as obtained from structural geology and fault scarp profiling and, iii) discuss the coseismic parameters (SRL, AD, MD) and slip profiles with those available in the literature for global earthquakes (M w >6). Methods ad phases of work. After the 24 August earthquake, we performed a detailed fieldwork along the fault scarp of the VRS, aimed at mapping in detail (1:1000) the long-term fault trace and at surveying the associated coseismic ground ruptures. The survey was carried out at a high-resolution, with a sampling rate of at least 1 site/50m, and locally of 3 sites/1m. All the sampling sites with evidence of primary coseismic fracturing were characterized in terms of rupture type, attitude, kinematics, slip vector and displacement (throw, opening and net displacement). After the 26 October foreshock, and before the 30 October mainshock, the few data collected along the northern VBF attested primary surface faulting SE and NE of Ussita with throw in the range of 8-15 cm. From 31 October to December 2016 we surveyed in detail the 30 October ruptures, from the southern tip of the VBF to Mt Porche. The ruptures north of Mt Porche were mapped from Spring to Fall 2017. Also the survey of the 30 October ruptures was carried out with a high- resolution sampling procedure. The data collected on the entire VBF have been analyzed in order to constrain the coseismic parameters (SRL, MD, AD). The average net displacement was computed as both arithmetic and integral means, with the latter corresponding to the ratio of the area subtended by the displacement profile to the rupture length. The robustness of our estimates is suggested by the high number of data collected and processed. In fact we surveyed 1747 evidence of coseismic ruptures, 325 fault plane attitudes along theVRS, and over 4000 data along the entire VBF. We subsequently calculated both the geologic (i.e. long-term) and topographic (i.e Late Quaternary) displacement along the VRS of the VBF in order to compare obtained values and their along-strike variation with the coseismic displacement of 2016 earthquake. The geologic displacements were calculated by restoring the offset of the stratigraphic markers