GNGTS 2016 - Atti del 35° Convegno Nazionale

338 GNGTS 2016 S essione 2.2 Structural features. Since the signature of structural features on a landscape is essentially morphological, they can be detected exploiting the 3D capability of stereoscopic aerial photographs. Elements such as (i) alignments of escarpments and counter-escarpments systems, (ii) sudden variations in the bedding attitude, (iii) alignment of slope changes along ridges or along slopes are commonly used in the aerial photo-interpretation. In the study area, normal faults generally follow a linear trend, are associated with triangular facets, and can delimitate intramontane basins, whereas thrust systems are more curvilinear, are associated with fold systems and in places with upside-down bedding. Lithological features. The map in Fig. 1 shows two main lithological groups, one referring to the terrigenous and pelagic sediments (i.e. the local geological substratum, composed of sandstone and siltstone of the Laga Formation and minor limestone and marls), and one referring to the Quaternary continental deposits. In the map, the substratum is not colored, and shows a diffuse presence of bedding traces. On the contrary, the continental deposits are classified in the map as follows: recent alluvial deposits (cyan), alluvial fans (light blue), terraced alluvial deposits (green), and landslides (yellow). Recent alluvial deposits appear as flat areas situated in the present-day river plains. Terraced alluvial deposits appear as nearly flat areas (in places slightly concave or showing a slope of less than 2-3°) suspended at different elevations compared to the present-day river plains. In the area, villages such as Amatrice and Retrosi were built up on most of the top of the highest terraced deposits. In this paper, we use the term “landslide” to encompass only slides and slide-flows, and not flows, falls and topples. In the map, deep-seated landslides are represented with the crown area separate from the deposit area, as opposed to shallow landslides. Integration of field geology and photo-geology interpretation. In order to integrate and validate the photo-geological interpretation, field recognitions were addressed to selected areas characterized by interferences of geomorphological-structural features revealed by the photo- geological interpretation. For these areas, we focused on relationships between geometry and extent of cover deposits, bedding of the substratum, and areal arrangement and distribution of the main fault systems. Some examples are presented below. In Amatrice bedding planes of the substratum, moderate to elevate dipping toward SW and NE, are cut by N-S trending high angle normal faults. The latter have a clear morphological expression featured by fault plane scarps and triangular facets, which are critical elements for evaluating morpho-structurally induced amplification effects. Comparison of these elements with the information provided by the photo-geological map, revealed a disagreement between the two data sets. In particular, a set of evident linear elements showing N-S trending were interpreted as bedding traces, whereas the field recognition verified that such elements belong to a family of N-S trending high angle normal faults. In the Retrosi village, south east of Amatrice, the integrated approach allows, at local scale, to distinguish the inner edge and conglomerates of a fluvial terrace (the Retrosi Unit), onlapping the substratum, from the outer edge and conglomerates of a SW-prograding alluvial fan (the Sommati-Amatrice Unit), downlapping the substratum. The reconstructed morpho-stratigraphic setting would be likely used for defining different seismically homogeneous microzones (SM working group, 2015). Large scale geological field recognition activities confirmed all the main morpho-structural elements shown in the photo-geological map. In the small Torrita-Scai basin, west of Amatrice, Pleistocene alluvial-lacustrine sand and silt cover with disconformity the substratum with average thickness of 30 m, and form a relatively flat area bounded by hilly relieves. Local morphological irregularities detected by photo- and field geology may suggest thickening, corresponding to small alluvial fans, or thinning, at the base of terraces escarpments, of the covering sediments. The irregular geometry of the Pleistocene cover can be reflected into areas with different expected amplification effects (basin effects): the higher the amplification, the thinner the cover. For the Retrosi village, a large scale geological field recognition have confirmed the photo-geological interpretation. Moreover, a