## 1 Introduction
### Study Area  
The study areas are located near the village of Castelluccio di Norcia (42\({}^{\circ}\)49\({}^{\prime}\)42.51\({}^{\prime}\) N 13\({}^{\circ}\)12\({}^{\prime}\)28.00\({}^{\prime\prime}\) E) in the southern part of the Sibillini Mts. chain, a deeply studied portion of the Umbria-Marche thrust and fold belt in Central Italy [17; 18; 19]. The stratigraphy of this region includes a Jurassic-Middle Miocene, shelf to pelagic, carbonate succession, followed upward by Late Miocene siliciclastic turbidites (Figure 1). The contractional structures consist of a system of NW-SE to N-S-striking asymmetric box-folds characterized by overturned eastern limbs and bounded by east-verging low-angle thrust faults, Late Miocene-Early Pliocene in age [18; 20; 21]. Towards E and SE, the outer basal thrust of the chain which, from N-S direction, turns southwards to NNE-SSW direction, causes the superposition of the Sibillini thrust sheet on the Messinian Laga basin turbidites. Since the Early Pleistocene, extensional fault systems displaced the contractional structures [22; 23; 24] with associated offset, along the single faults, locally exceeding 1500 m. Despite the fault system showing a certain geometrical complexity, on the whole, the Quaternary normal faults appear arranged in a quite a regular, NNW-SSE-striking regional alignment across the Umbria-Marche and Laga domains (Figure 1a). During the Quaternary, the activity along the major extensional faults, which dip to W-SW, led to the formation of intra-mountain basins, filled with alluvial and lacustrine deposits [25; 26; 27]. The easternmost extensional fault, the VBF, is a highly segmented, well-known active structure with a 6.5 \(\leq\) Mw \(\leq\) 6.8 maximum expected magnitude [28] (Figure 1a). It shows an along-strike length of \(\sim\)30 km, a long-term maximum displacement of \(\sim\)1500 m [21; 25; 29], and a maximum Late Quaternary slip rate of 1.3 mm/y [15]. The Piano Grande basin was developed in the VBF hanging wall during the Middle-Late Pleistocene (Figure 1b). Eluvial-colluvial deposits overlay unconformably the carbonate bedrock along the borders of the basin whereas the major depression is mainly filled by alluvial and lacustrine sediments [30; 31]. Well before the 2016 Central Italy seismic sequence, the present activity of VBF had been suggested on the base of the observed displacement of Quaternary deposits and Late Pleistocene--Holocene landforms [21; 23; 25]; subsequently its seismogenic potential was highlighted by paleoseismological evidence [28]. The diffuse surface faulting observed along the VBF after the 2016 sequence (Figure 1b) and the good correlation between the activated fault at the surface and the hypocentral distribution of the seismicity unquestionably confirmed the seismogenic role of this fault. All the three main segments in which the VBF is divided were activated during the 2016 seismic sequence; they are referred to, from south to north, as the Vettoretto-Redentore, Bove-Porche and Cupi-Ussita segments [15] (Figure 1a). Several synthetic and antithetic spalys branch from the three segments at distance up to 4 km west of the main fault trace. Two of the major splays of the Vettoretto-Redentore segment, located along the eastern border of the Piano Grande di Castellocio basin, and named hereinafter the Colli Alti e Bassi Fault and Prate Pala fault (CAB and PTP, respectively, Figure 1b) were affected by significant coseismic reactivation and have been studied in detail in this work.