In seismotectonic studies, seismic reflection data are a powerful tool to unravel the complex deep architecture of 12 active faults. Such tectonic structures are usually mapped at surface through traditional geological surveying whilst seismic 13 reflection data may help to trace their continuation from the near-surface down to hypocentral depth. In this study, we 14 propose the application of the seismic attributes technique, commonly used in seismic reflection exploration by oil industry, 15 to seismotectonic research for the first time. The study area is a geologically complex region of Central Italy, recently struck 16 by a long-lasting seismic sequence including a Mw 6.5 main-shock. A seismic reflection data-set consisting of three vintage 17 seismic profiles, currently the only available across the epicentral zone, constitutes a singular opportunity to attempt a 18 seismic attribute analysis. This analysis resulted in peculiar seismic signatures which generally correlate with the exposed 19 surface geologic features, and also confirming the presence of other debated structures. These results are critical, because 20 provide information also on the relatively deep structural setting, mapping a prominent, high amplitude regional reflector 21 that marks the top basement, interpreted as important rheological boundary. Complex patterns of high-angle discontinuities 22 crossing the reflectors have been also identified. These dipping fabrics are interpreted as the expression of fault zones, 23 belonging to the active normal fault systems responsible for the seismicity of the region. This work demonstrates that 24 seismic attribute analysis, even if used on low-quality vintage 2D data, may contribute to improve the subsurface geological 25 interpretation of areas characterized by high seismic potential. 26 Introduction 27Studying the connections between the earthquakes and the faults to which they are associated is a primary assignment of 28 seismotectonics (Allen et al., 1965; Schwartz and Coppersmith, 1984). Clearly, this is not an easy task: it is in fact generally 29 complex to fill the gap between the exposed geology including the active "geological faults" mapped by the geologists and 30 https://doi.
Legacy seismic reflection data constitute infrastructure of tremendous value for basic research. This is especially relevant in seismically hazardous areas, as such datasets can significantly contribute to the seismotectonic characterization of the region. The quality of the data and the resulting image can be effectively improved by using modern tools, such as pre-conditioning techniques and seismic attributes. The latter are extensively used by the hydrocarbon exploration industry, but are still only poorly applied to the study of active faults. Pre-conditioning filters are effective in removing random noise, which hampers the detection of subtle geologic structures (i.e., normal faults). In this study, a workflow including pre-conditioning and extraction of seismic attributes is used to improve the quality of the CROP-04 deep seismic reflection profile. CROP-04 was acquired in the 1980s across the Southern Apennines mountain range, one of the most hazardous seismically active regions in Italy. The results show the capacity of this method to extract, from low-resolution legacy data, subtle seismic fabrics that correspond to a dense network of fault sets. These seismic signatures and the enhanced discontinuities disrupting the reflections, which were invisible in the original data, correlate well with the main regional normal faults outcropping at the surface. Moreover, the data reveal higher structural complexity, due to many secondary synthetic and antithetic structures, knowledge of which is useful in modeling of the local and regional distribution of the deformation and potentially in guiding future field mapping of active faults. This proposed approach and workflow can be extended to seismotectonic studies of other high-hazard regions worldwide, where seismic reflection data are available.
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