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SEISMIC ATTRIBUTES IN SEISMOTECTONIC RESEARCH: AN APPLICATION TO THE MW=6.5 EARTHQUAKE OF 30 OCTOBER 2016 (NORCIA, CENTRAL ITALY) M. Ercoli1, E. Forte2, M. Porreca1, M.R. Barchi1, C. Pauselli1, G. Minelli1

1 Dipartimento di Fisica e Geologia, Università degli Studi di Perugia, Italy (CRUST Centro inteRUniversitario per l’analisi SismoTettonica tridimensionale con applicazioni territoriali)2 Dipartimento di Matematica e Geoscienze, Università degli Studi di Trieste, Italy

Introduction. In 2016-2017 a long and complex seismic sequence struck Central Italy. It encompasses one of the strongest events (30th October 2016, Mw = 6.5) recorded in the last decades in the Italian Peninsula. A region of about 80 km, elongated in NNW-SSE direction from Visso to Campotosto was interested by the sequence (Chiaraluce et al., 2017). Several sectors of a long NNW-SSE fault system activated, characterized by prevalent WSW-dipping, high-angle normal faults. However, the geological structure of the area is particularly complex and still debated. Despite a huge amount of data recorded by the geoscientists at the surface, geophysical and geological deep data are very limited and logistically complex to collect in this study area. For this reason, a series of 2D vintage seismic reflection lines, collected during the ’80-‘90, have been interpreted aiming at filling-out the gap of information between surface data and seismological records (Porreca et al., 2018). In the last decades, seismic reflection profiles have been extensively used in the Umbria-Marche region to constrain the geometry of seismogenic faults at depth, as well as the deep geological setting (Barchi and Mirabella, 2009). Seismic reflection data may represent a unique and unrepeatable source of information for the reconstruction of the regional geological model. However, their quality is extremely variable and, in some cases, poor, due to several factors (Mazzotti et al., 2000). Therefore, we have recently focused our efforts onto two main strategies to improve the data visualization and interpretability. The first is a dedicated data reprocessing, starting from the original shot gather dataset (Ercoli et al., 2018) in particular aimed at optimizing static corrections and velocity models with modern algorithms. Here we present some preliminary results of the second strategy, namely the Seismic Attribute Analysis. It is a powerful tool frequently used in the Oil and Gas exploration (Chopra and Marfurt, 2018), but not often adopted for seismic data in seismotectonic research, apart in few GPR applications (Ercoli et al., 2012, Ercoli et al., 2015). In this work, we focused on two seismic profiles intercepting the two Quaternary basins of Norcia and Castelluccio di Norcia. After testing several attributes, we provide the best images obtained after the application of amplitude and phase-based attributes, that considerably enhanced the overall interpretability of seismic lines.

Data and Method. The original dataset includes 97 2D seismic reflection profiles across the epicentral area. Within this dataset, we extracted two W-E lines, crossing the hypocentral area about four kilometers to the South of the Mw 6.5 hypocenter (Fig. 1).

Fig. 1 - Location map of the seismic lines across the Norcia and Castelluccio di Norcia basins (Central Italy).

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After loading the SEG-Y files labelled as NOR01 (migrated), NOR02 (stack & migrated) into OpendTect software (http://dgbes.com/), we tested several seismic attributes, and the most relevant are listed below:

- “Energy” (E): ratio between the sum of the squared sample values within a specified time window and the number of samples in the gate (Forte et al., 2012, Ercoli et al., 2015). The Energy is a measure of the reflectivity, useful to emphasize low or irregular amplitude reflections (acoustic rock properties), and to enhance lateral variations in seismic events.

- “Energy gradient” (EG): the first derivative of the Energy with respect to time (or depth). It is very effective to better display, with a relative low level of subjectivity, detailed 2D/3D semi-automatic mapping of horizons and lateral discontinuities like channels or faults.

- “Pseudo-Relief” (PR): the “Energy” attribute is first computed in a short time window, then it is followed by a phase rotation of -90° degrees though a Hilbert transform. Such an attribute is very commonly used in 2D seismic interpretation by the interpreters: the seismic section appears as “topography-like” image, allowing an easier detection of faults.

Fig. 2 - Attribute analysis on NOR1 (Norcia Basin, Nb). a) standard amplitude line; b) Energy-Gradient (EG) displaying the top Basement s.l. (blu arrows) and the enhanced signature of faults (red arrows).

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The attribute computation done on the original amplitude seismic lines (Fig. 2a and Fig. 3a), has generated new seismic images allowing an increased level of interpretability. We report here the images obtained after the application of EG and PR (Fig. 2b; Fig. 3b), showing peculiar geophysical signatures of geological units and faults.

Results and conclusions. The new seismic attribute images help to provide solid constraints for the subsurface geological reconstruction of the epicentral area. A high acoustic impedance contrast is visible in the profile NOR1, slightly West of the Norcia basin (Nb) located between 2-3 s TWT (blue arrows, Fig. 2b). A similar signature is clearly detectable also beneath the Castelluccio di Norcia basin (CNb) in line NOR02: this key-horizon, gently dipping toward East, is here deeper, located between 3-4 s TWT (blue arrows, Fig. 3b), and it is abruptly interrupted toward West. According to Porreca et al. (2018), this prominent reflector, which also seems to act as a threshold in depth for the seismicity, may correspond to the Top of the Basement s.l. (sensu Mirabella et al., 2008). After the attribute analysis, its signature is quite peculiar in term of amplitude of the reflector package, and it is also clearly recognizable in others surrounding lines. Similar characteristics and depth have been also highlighted by Mirabella et al. (2008) in the Colfiorito area, where another significant seismic sequence occurred in 1997-1998. From

Fig. 3 - Attribute analysis on NOR2 (Norcia, Nb, and Castelluccio Basin, CNb). a) standard amplitude line; b) Pseudo-Relief displaying the top Basement s.l. (blu arrows) and the main faults pattern (red arrows).

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a structural point of view, the attributes provide a great match between the known location of the seismogenic faults at surface (gray arrows), and their geophysical signature in depth (red arrows). In fact, both the “Energy Gradient” and “Pseudo Relief” attributes are particular effective in enhancing their display, like in the case of the antithetic fault of the Norcia Basin (NOR01): it appears E-dipping and steeper close to the surface, delineating, together with the enhanced West-dipping fault tracks, the deep geometry of the basin (Nb in Fig. 2b). Similar results have been obtained using the aforementioned attributes on the NOR02 profile, but in this case the fault zones that characterize the Castelluccio di Norcia basin (CNb) appear much more complex, displaying an overall “V shape”, formed by multiple, minor discontinuities that generate high fragmentation of the reflectors (Fig. 3b). Moreover, the seismic attributes strongly emphasize the reflectors continuity/discontinuity, better showing reflection patterns, aiding an easier interpretation of the geometries. In conclusion, we tested the use of some seismic attributes on 2D industrial seismic lines for seismotectonical purposes. Our results indicate that this approach gives the opportunity to extract additional information from vintage seismic lines characterized by poor overall quality. The new seismic images, in fact, may play an important role for the geoscientists to better constrain the deep geological setting and the fault patterns, even in complex and seismically active areas.Acknowledgments. We are grateful to Eni S.p.A for providing seismic sections, and to dGB for providing the academic version of OpenDtect software.

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