Sismotettonica e geodinamica dell’area marchigiana e adriatica

The Adriatic coastal and offshore area is part of the chain-foredeep-foreland system of the Umbria-Marche Apennines. The stratigraphic succession present is therefore that typical of the Umbrian-Marche sector of the chain, with a succession of neogenic foredeep that shows a great variability of facies and a temporal discrepancy between the inner and outer portions resulting from the progressive migration of the foredeep basin towards the foreland (Ricci Lucchi, 1987; Menichetti et al., 1991). If many structural, stratigraphic and geodynamic models are available in the Apennine area, integrated studies of seismotectonics are still lacking for the offshore one. The most recent works (Kastelic et al., 2013; Mazzoli et al., 2014) focus on the strictly seismogenic characterization of the area, while very few on its structural and geodynamic interpretation (Chicco et al., 2019; Maesano et al., 2013; Santini et al., 2016; Bally et al., 1986; Argnani & Gamberi, 1995; Argnani 1998). The Adriatic foredeep of this portion of territory, reaches important thicknesses of Plio-Pleistocene sediments with large lateral variations. This structural and stratigraphic complexity, together with 1D seismic wave velocity models calibrated for the innermost areas, makes earthquake location difficult and their depth not constrained, especially if these events are located outside the seismographic network and therefore offshore. The main objective of this study was to clarify the structural, geological and seismological structure of the foredeep part of the Marche-Romagna Apennines to arrive at a seismotectonic and seismogenic interpretation of the area. For these reasons, carrying out a study on hypocentral-structure relationships, identifying the seismologically active faults and the related focal mechanisms, has required basic quality data and, above all, good crustal models suitable for the investigated area. From the analysis of a good number of seismic profiles, calibrated with the well logs data, a geological-structural map of the tectonic features was created, and a series of balanced geological sections were constructed. From the digitalisation in a threedimensional modelling software it was possible to create a 3D model of the investigated area that validated the geometries and the relationships in depth between the structures and the horizons. This three-dimensional model was then used for the implementation of a 3D seismic wave velocity model that took into account the subsurface data and the lateral and thickness variations of the slow plio-pleistocene sediments. From an evolutionary point of view, the general structure of NE-vergent anticlines and synclines bordered by thrusts, relative backtrusts and finally cut by NS strike-slip faulting, can be derived from a first phase in which, after compaction and beginning of compressive stress, anticlines are formed by buckling and a phase in which these are cut by a reverse fault on the IV outer side and by a backtrust on the inner side. These trhust related folds are then cut from the following transpressive faults. The structural structure recognized in depth is due to a thickskinned deformation style since the compressive deformation also involves the basement. However, not the whole succession is affected by the deformation in the same way. The miopliocene multilayer is in fact the most deformed one and inside it are also recognizable some minor detachments, while in the oldest part of the Umbria-Marche succession there are simpler structures but with greater displacements and wavelengths. The classical flower structures of the plio-quaternary transpressive faults are also clearly visible in section. The balanced sections made it possible to evaluate the shortening of the Apennine foredeep in a range ranging from 25.7% to 31%. The shortening values gradually increase from the northernmost to the southernmost sections compatibly with the compressive stress regime detected by the literature data. Using a velocity model calibrated on the study area that provides low speeds for the most recent sediments and, innovative softwares such as NonLinLoc and HypoDD, has allowed us to obtain more restricted and better locations in terms of calculated errors. The seismicity of the Rimini offshore shows a NW-SE distribution with depths between 10 and 15 km and focal mechanisms generally not very constrained since the seismicity of the area isn’t frequent and, at least for the time interval considered, at very low magnitude. For the Senigallia-Falconara area we obtain three groups of events with depths ranging from 5 to 20 km. The focal mechanisms are reverse, NNW-SSE oriented like the thrust that determines the asymmetric anticline of the Monte Conero, and oriented N-S strike-slip perfectly positioned where two strike-slip faults were recognized. These would be the seismologically active structures in the central onshoreoffshore sector of the investigated area. The seismicity of Monte Conero counts a distribution of NW-SE epicenters with a depth between 4 and 10 km and a slight tendency to deepen towards SE. Focal mechanisms give mostly reverse focal NNW-SSE solutions. The three main events of the summer 2013 sequence are reverse, pure or with a minimal oblique component, with NNW-SSE strike, 15 ° to 25 ° dip and an around 100 ° rake. These show an almost perfect correspondence with a NW-SE backthrust that has been recognized on the inner flanc of an important Conero offshore anticline structure. It is assumed to release major stresses associated with the related thrust.