GNGTS 2019 - Atti del 38° Convegno Nazionale

54 GNGTS 2019 S essione 1.1 FAULT ROUGHNESS ASSESSMENT OF THE MONTE VETTORE FAULT SYSTEM AFTER THE MASSIVE 2016 SEISMIC SEQUENCE OF CENTRAL APENNINES, ITALY A. Corradetti 1 , M. Zambrano 2 , S. Tavani 3 , A. Pitts 2 , T. Seers 1 , E. Tondi 2 1 Department of Petroleum Engineering, Texas A&M University at Qatar, Qatar 2 School of Science and Technology, Geology Division, University of Camerino, Italy 3 DiSTAR, Università di Napoli Federico II, Italy Introduction. After the major seismic events that struck the Northern Apennines (Central Italy) in 2016, a considerable surface rupture exposed fresh fault surfaces in the Monte Vettore- Castelluccio di Norcia area. The seismic sequence comprised three mainshocks: i) Amatrice Mw 6.0, August 24, ii) Visso Mw 5.9, October 26, and iii) Norcia Mw 6.5, October 30, the strongest earthquake of the sequence (e.g. Chiaraluce et al. , 2017). The surface rupture caused by this seismic sequence has been well-document by the Open EMERGEO Working Group (Civico et al. , 2018; Villani et al. , 2018). These authors reported that the coseismic ruptures are generally organized in a systematic pattern of dominantly synthetic (N135°-160° striking, SW-dipping) and subordinately antithetic (N320°–345° striking, NE-dipping) components. In most cases, the ground ruptures follow the trace of mapped faults (e.g. Pierantoni et al. , 2013 and references therein), and sometimes along fault splays previously unrecognized (Civico et al. , 2018). The average coseismic throw of the entire sequence is ~0.3 m. However, more than 2 km of the ruptures were characterized by an average coseismic throw higher than 1 m, with a maximum of near 2.5 m along the so-called Cordone del Vettore fault scarp (Civico et al. , 2018; Villani et al. , 2018). The assessment of the surface ruptures is useful for supporting studies on the mechanics of earthquake faulting and adjusting seismic sources modelling for joint inversion of geophysical datasets. In particular, the study of fault topography (e.g. exposed due to coseismic surface deformation) can provide insight into the direction (current or ancient) of slip and the relative exposure time of the surface to atmospheric elements altering the pristine fault morphology at several scales. Various approaches have been reported in the literature for mapping the surface of fractures and faults in the field or laboratory involving the use of Lidar (Candela et al. , 2009; 2012), laboratory profilometers (Renard et al. , 2006; 2012; 2013), and Structure from Motion-MultiView Stereo (SfM-MVS) photogrammetry (e.g., Kim et al. , 2013, Corradetti et al. , 2017; Zambrano et al. , 2019). Among these methods, the latter is better suited for rough terrain conditions (e.g. seismically modified terrain, mountainous areas, or generally physically inaccessible outcrops) when compared to Lidar and does not require rock sampling. The method is also cost-efficient since it only requires consumer-grade photography equipment. Thanks to this versatility, SfM-MVS photogrammetry has been successfully used as an analytical tool to gather geologic data from outcrops at several scales. For instance, it has been applied to gather data from >100 meters outcrops by means of drones (e.g. Corradetti et al. , 2018; Pitts et al. , 2017), down to the scale of sub-mm fault asperities like in this work. This study presents preliminary results of a topography (i.e. roughness) assessment of the exposed coseismic fault surfaces using SfM-MVS photogrammetry. The quantitative analysis of fault surface roughness is achieved by implementing the power spectral density (PSD), which provides an objective description of the roughness based on the frequency distribution of the asperities in the Fourier domain. This approach has been successfully applied by previous authors for describing the roughness of fractures (e.g., Ogilvie et al. , 2006) and fault surfaces (Candela et al. , 2009; 2012; Renard et al. , 2013; Corradetti et al. , 2017). The Monte Vettore fault system was analysed in two localities: Forca di Presta and Colli Alti e Bassi (see map of Pierantoni et al. 2013, Fig. 1). The studied fault surfaces are well-cemented and polished, and the surface coseismic displacement is recognisable at a simple view. Methods. In this work, we present a multiphase integrated methodology for characterizing fracture surfaces. This approach combines fracture surface scanning using SfM-MVS photogrammetry, and spectral description of individual natural fault surfaces.