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GEOMETRY AND STRUCTURE OF A FAULT-BOUNDED EXTENSIONAL BASIN BY INTEGRATING GEOPHYSICAL SURVEYS AND SEISMIC ANISOTROPY ACROSS THE 30 OCTOBER 2016 MW 6.5 EARTHQUAKE FAULT (CENTRAL ITALY): THE PIAN GRANDE DI CASTELLUCCIO BASINF. Villani, V. Sapia, P. Baccheschi, R. Civico, G. Di Giulio, M. Vassallo, M. Marchetti, D. PantostiIstituto Nazionale di Geofisica e Vulcanologia, Italy

Introduction. In the past twenty years, the central Apennines (Italy) were hit by three important seismic sequences affecting a region >100 km-long parallel to the chain axis. Those earthquakes caused heavy damage, economic losses and >600 deaths (Colfiorito sequence, 1997, Mw 6.0; L’Aquila sequence, 2009, Mw 6.1; Amatrice-Visso-Norcia sequence, 2016, Mw 6.1, 5.9 and 6.5). The mainshocks of those sequences were generated by the in-cascade activation of segmented normal fault-systems bounding Quaternary extensional basins (Chiaraluce et al., 2017). The Pian Grande di Castelluccio (PGC) basin is the main Quaternary depocenter of the Mt. Vettore - Mt. Bove normal fault-system (VBFS), responsible for the 30 October 2016 Mw 6.5 Norcia earthquake (Fig. 1). The widespread coseismic surface faulting following this earthquake (Civico et al., 2018; Villani et al., 2018a,b) affected also part of this basin, thus highlighting the existence of active fault splays capable of rupturing the surface and linked to the seismogenic fault at depth.

Unveiling the subsurface structure of the PGC basin is crucial to infer the long-term behavior of the VBFS. We performed a geophysical transect in the PGC b asin across the coseismic ruptures, by integrating electrical resistivity tomography (ERT), time-domain electromagnetic soundings (TDEM), and horizontal-to-vertical spectral ratios of ambient vibrations (HVSR). We also analyzed shear-wave splitting of S-phases (fast direction φ and delay time δt) from local earthquakes recorded during our surveys to better constrain the fracture field and the properties of the inferred fault zones.

Fig. 1 - Geological map of the Pian Grande di Castelluccio (geology simplified after Pierantoni et al., 2013) with the location of the geophysical surveys and main faults.

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Our goals are: 1) shallow characterization of fault zones bounding the PGC basin (and possible subsurface

faults); 2) mapping of the pre-Quaternary carbonatic top-bedrock along a key transect crossing the surface ruptures of the Norcia earthquake (profile B-B’ in Fig. 1).

Due to the complex tectonic setting and the >1 Myr-long extensional activity in the study area, we expect significant variations in the top-bedrock depth, which can be interpreted in terms of fault throws. We adopted ERT as a high-resolution geophysical technique capable of providing accurate images of fault zones. We assume that the top-bedrock is in most cases characterized by markedly contrasting properties with respect to the overlying continental infill (coarse-grained alluvial to fine fluvio-lacustrine sediments). Therefore, it can be detected as (i) a sharp electrical resistivity change between a deeper highly-resistive basement and a relatively conductive cover and/or (ii) a seismic impedance contrast between vertically stacked materials with different elastic moduli and seismic velocities (Di Giulio et al., 2016). We calibrated our geophysical data with available shallow boreholes (Ge.Mi.Na., 1963) and geological data (Pierantoni et al., 2013).

Data analysis and interpretation. The three ERT profiles define the shallow structure of three important normal fault zones down to 50-100 m depth, and in particular: the two basin-bounding faults (F1 and F3), and the fault that ruptured the surface during the Norcia earthquake (VF).

The TDEM surveys, performed using 50-m-sized square loop, provide 1-D resistivity models that help mapping the top-bedrock surface along two transects. They show important topographic changes that we relate to the presence of buried normal fault zones. The deepest part of the basin, between the inferred faults F5 and F6, is nearly 300 m. Those results are corroborated by the HVSR results. The estimated peak resonance frequency display changes in the 0.4-1.5 Hz range that closely follows the inferred trend of the top-bedrock depth. The results from shear wave splitting indicate the occurrence of two main fast axes directions, trending N150° and N30°, respectively.

We hypothesize that the PGC basin structure is characterized by several fault splays distributed in a >3 km-wide deformation zone (Fig. 2). The easternmost splays of the VBFS that ruptured the surface during the Amatrice and Norcia earthquakes (Cordone del Vettore Fault) are

Fig. 2 - A) 1-D resistivity curves, ERT sections and seismic stations used to constrain the geological section B-B’; b) interpretative geological section.

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the highest emergence of this extensional structure. Field data (Villani et al., 2018b), indicate that these splays exhibit a high dip angle at the surface (>75° on average), so we speculate they become less steep at depth, and eventually merge to the seismogenic fault (dipping nearly 45°, see Chiaraluce et al., 2017; Scognamiglio et al., 2018). The basin-bounding fault splay F1 is located at the base of the long-term cumulative fault scarp of Mt. Vettore-Mt. Redentore. The ERT T1 indicates that the fault zone is ~40-50 m wide. These results in addition with the absence of coseismic surface breakages following the 2016 Norcia earthquake along fault F1 (Civico et al., 2018) suggest that this structure is experiencing a very low degree of activity. The geological throw of fault F1 is nearly 1400 m, in accordance with some previous estimates (Calamita et al., 1992; Pizzi et al., 2002).

In the hangingwall of fault F1, the VF is a high-angle active splay (Galadini and Galli, 2003; Villani and Sapia, 2017) with a total throw of ~100 m. The surface fault scarp is ~2.3-2.8 m high, likely related to the last ~12 kyr of activity. As suggested by ERT models, the VF fault zone displays relatively low resistivity values (ρ < 100 Ωm), due to the presence of fluids within the sub-vertical and 10-30 m-wide granular damage zone. The fault F3 does not show evidence of recent faulting, however it played an important role during the Quaternary in shaping the PGC basin. In order to accommodate the sudden deepening of the top-bedrock surface between TDEM soundings td9 and td8, we suppose an additional E-dipping fault F6, in the hangingwall of fault F3.

In the PGC basin the dominant direction of fast S-wave polarization is consistent and changes accordingly with the strike of faults. Moreover, we also observe a secondary direction of the fast S-wave parallel to the maximum horizontal stress SHmax of the regional extensional stress field suggesting a combined effect of stress-induced and structure-induced anisotropy in the upper crust beneath the PGC basin. In the western side of the basin, NNE-SSW fast directions well match the local strike of main faults F2 and F3. We suppose that the anisotropy could be primarily controlled by the geometry of fracture systems and by the small-scale structures developing into the fault damage zones (Liu et al., 2015). Moving towards the eastern side of the PGC basin the fast polarization directions trend NNW-SSE and are clearly in accordance with both the maximum horizontal stress and the strike of the VBFS. This is often observed in extensional regimes as in the central Apennines, where SHmax direction coincides with the strike of the main active normal faults (Sibson et al., 2011) and, thus, the fast direction could be oriented parallel to both the maximum horizontal stress and the fault strike (Hurd and Bohnhoff, 2012; Baccheschi et al., 2016). It is possible, therefore, that here both the alignment of local fluid-filled micro-cracks and the faults can control the pattern of fast axes.

The Quaternary extensional activity in this sector of the Sibillini Mts. involved two differently oriented fault-systems, trending N130°-150° and N30°, respectively. Probably, in their early development, those faults acted together. In the first stage of the PGC basin formation, the N30°-trending faults may have played an important role, testified by the fact that the long axis of the basin is parallel to faults F2 and F3 (which are >5 km and >3.5 km-long, respectively). Subsequently, the N130°-150°-trending faults become dominant, as the result of the progressive growth and linkage of several aligned segments that now compose the ~25 km-long VBFS, which is a currently active and seismogenetic crustal-scale normal fault-system (Chiaraluce et al., 2017; Pizzi et al., 2017; Scognamiglio et al., 2018). The interplay of the two fault-systems was responsible for the complex shape of the PGC basin, and it can be inferred from the seismic anisotropy pattern and the bumpy morphology of the basin bottom.

The deepest part of the investigated portion of the PGC basin is ~300 m b.g.l. Such depth is the result of the combined action and interference of two different fault-systems, and unfortunately the available data do not allow deriving any long-term fault throw rates.

We cannot reconstruct with our data the throw rates of the individual fault splays within the PGC. With regard to the VF splay, Villani and Sapia (2017) infer a post-23 ka of 0.22 ± 0.07 mm/yr. With the results reported in this work, we estimate ~100 m total throw accrued by

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this fault. Under the crude assumption of constant long-term throw-rates, we hypothesize the activity of this splay started >0.4 Myr ago, which seems consistent with the depicted structural setting. This would also suggest the timing of shift of extensional activity in the PGC basin area from dominant N30°-trending faults to dominant N130°-150°-trending faults (i.e. the VBFS).ReferencesBaccheschi P., Pastori M., Margheriti L. and Piccinini D.; 2016: Shear wave splitting of the 2009 L’Aquila seismic

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Ge.Mi.Na (Geomineraria Nazionale); 1963: Il bacino di Castelluccio di Norcia, in: Ligniti e Torbe dell’Italia continentale, 207-210, Industria Libraria Tipografica Editrice (ILTE), Torino

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Scognamiglio L., Tinti E., Casarotti E., Pucci S., Villani F., Magnoni F., Michelini A., Dreger M. and Cocco M.; 2018: Complex fault geometry and rupture dynamics of the Mw 6.5, 2016, October 30th central Italy earthquake. J. Geophys. Res. Solid Earth, 123, 4, 2943-2964, doi: 10.1002/2018JB015603.

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