GNGTS 2018 - 37° Convegno Nazionale

22 GNGTS 2018 S essione 1.1 in a set of 1:25.000 scale geological sections. The topographic displacement was calculated by realizing several topographic profiles across the VRS fault scarp. Elevation data were extracted from a Digital Elevation Model (DEM) built from topographic map at scale 1:5000. The offset of the far-field topographic slope across the fault scarp was calculated and interpreted as displacement cumulated by several slip events, after the demise of the Last Glacial Maximum (LGM), that is after ~15±3 kyrs ago. Structural analysis. The 24 August to 30 October earthquakes ruptured at the surface almost entirely the VBF. The pattern of coseismic ruptures indicate a common primary (i.e., tectonic) origin testified by the following geological observations: - most of the ruptures reactivate long-term fault scarps or are arranged in alignments whose direction is independent of the orientation of the topographic surface; - the set of ruptures which originated along the VRS after both the events, develops with continuity along its geological trace also where it crosses and displaces the Sibillini thrust, with an associate long-term offset of ~300m propagating within the thrust footwall block; - coseismic slip of both the events shows locally oblique kinematics which are clearly unrelated to the local dip-direction of the slope; - some kilometer-scale ruptures, due to the 30 October event, affect flat areas or foot- slope debris characterized by very low slope-gradient, allowing to exclude significant gravitational deformation. The VRS is the southeasternmost segment of the VBF; it extends along strike for a length of 10.5 km in direction N160° and is characterized by clear and continuous fault scarp. A number of synthetic and antithetic splays affect the hanging wall on the distance of ~ 2 km from the fault trace. The segment has a hierarchic organization in second-order fault sections (FS) and third order sub-sections identified based on sharp strike-deviations, along-strike terminations and step- over zones, with spacing in the order of hundreds of meters. The VRS has been subdivided into three FS, with lengths in the order of 3-4 km, and into six sub-sections. From north to south, we recognized FSI= Quarto San Lorenzo section, FSII= Redentore section and FSIII= Vettoretto section. All FS show an average surface dip-angle of about 65°. They differ in the long-term slip vector attitudes, which highlight a slight right-lateral component on FSII and a left-lateral component on FSI and FSIII. The calculated long term average slip vector, for the entire VRS is 232/65. The geological displacement reaches a maximum value of 500 m (throw=470 m) across the FSII; the post LGM displacement, ranges from 13 to 29 m along FSI and FSII. The corresponding post-LGM (15±3 kyrs) throw-rate is in the order of 1.45±0.3 mm/y. The 24 August event ruptured almost completely the fault sections FSII and FSIII for an along-strike length (L) of 5.8 km, in an average N160° direction. The related coseismic throw (CT) shows values in the 0-27 cm range, with four relative maxima progressively decreasing southward. The highest value (27 cm, corresponding to a maximum net displacement MD=28,5 cm) is measured in the central portion of FSII; the other three peaks are observed along FSIII and mark the median parts of corresponding fault sub-sections, in which FSIII is articulated. The mean integral throw of the 24 August rupture is ~ 12 cm (average net displacement, AD = 12.7 cm). The azimuthal distribution of the fractures shows a general good agreement with the fault strike. The 30 October mainshock ruptured at the surface the VRS for 7.6 km. The ruptures mainly occurred along the main fault trace, but also along subsidiary splays outcropping within a few kilometres from the main fault trace. The CT values, for this event (i.e. subtracting the 24 August slip) varies in the 0-217 cm range, with four relative maxima nearly coincident with the 24 August ones. In the northern part of FSII, the CT rapidly increases, up to ~ 100 cm and, in its middle part, its “Acme Zone” occurs, with the absolute maximum (217 cm, corresponding to a maximum net displacement of 222 cm near Scoglio dell’Aquila). In detail, the “Acme zone” is characterized by two well distinct peaks separated by a narrow drop whose corresponding