GNGTS 2019 - Atti del 38° Convegno Nazionale

480 GNGTS 2019 S essione 2.2 provides a case study to understand how subsurface geology influences site effects in a Plio- Quaternary intermontane basin of central Italy. Indeed, the investigated area is highly prone to ground-motion amplifications due to 1D and 2D stratigraphic conditions and to coseismic instabilities such as, above all, liquefaction and active and capable faults. Physical properties of the sedimentary infill, sharp seismic velocity contrasts, and geometric complexity in the buried bedrock topography are highly impacting the wave reverberation and local ground- motion amplification (Pagliaroli et al. , 2019). For these reasons, the surface geology has been constrained by detailed geological surveys and several geophysical investigations. During the seismic microzonation, we also mapped several zones affected by local hazard due to coseismic surface faulting along active or possible active normal faults. The results from this study might have implications for the methodological approach to the seismic microzonation for future land-use planning. The seismic microzonation Level 1 map and sections were elaborated based on detailed geological and geomorphological surveys and borehole stratigraphy and the seismic site characterization was carried out by using geophysical investigations such as HVNSR data, MASW, DH, and seismic refraction tests integrated with the previously collected in the L’Aquila area after the 6 th April 2009 earthquake (Lanzo et al. 2011; Del Monaco et al. , 2013). Geological setting. The Preturo-Sassa area is located within the Plio-Quaternary intermontane Western L’Aquila Basin, whose clastic infill deposited in the hangingwall of the extensional Upper Aterno Valley fault system (UAV) (Falcucci et al. , 2015). The oldest infill deposits, cropping out in the Scoppito area, can be referred to heterometric breccias, with clayey-silty matrix, deposited in slope and debris flow environment during the late Piacenzian- Gelasian (ver1) and overlay unconformably the Meso-Cenozoic bedrock (BDR) (Cosentino et al. , 2017). The subsequent unit all1 drapes the underlying units, and consists of silt, sand, and clayey silt, containing lignite horizons, deposited in a meandering fluvial system during the Calabrian. Coeval with all1, slope-derived deposit, consisting of grain-supported calcareous breccia with reddish silty matrix, crops out (ver2). The most widespread deposits are younger alluvial fan and fluvial sediments, which piled since Middle Pleistocene to Holocene and covered the above-mentioned deposits, with three alluvial terraced units (at1, at2, and at3). The oldest terraced alluvial unit (at1) was tectonically uplifted at the footwall of the UAV. The unit at1 covers the bedrock and is formed by sand- supported conglomerate with interlayers of tephra of Middle Pleistocene. A younger alluvial unit (at2), consists of dense sandy-silty gravel referred to the Late Pleistocene. Between at1 and at2, a large calcareous breccia (dbf) of the late Middle Pleistocene is located. The smallest outcropping unit corresponds to the alluvial terrace confined in the alluvial plain of the Aterno River (at3) (Nocentini et al. , 2017). Close to the Aterno River, the Pleistocene formations are covered by Holocene fluvial fine- grained deposit (all2). Coarse-grained (fal) and fine-grained (col) characterized slope break of the surrounding reliefs. Finally, anthropic material is widespread within villages and industrial areas (ant). Discussion. The seismic microzonation of Preturo-Sassa area shows several significant features concerning the amplification site effect and permanent coseismic deformations (active and capable faults and liquefaction). In the Plio-Quaternary intermontane basin of central Italy, the amplification site effect is critical due to the great depth of the bedrock. Differently to the standard procedure for Level 1 microzonation, a great number (more than 120) of microtremor recordings were performed. Their interpretation with the HVSR method, assuming Vs values performed with DH, CH, and SDMT tests in same geological units, permitted to evaluate an approximate bedrock depth as suggested by Albarello and Castellaro (2011). The bedrock depth obtained by geological survey information, deep boreholes reaching the bedrock, and the microtremor data, was summarized by a contour line map (Fig. 1) and by geological cross- sections (Fig. 2) evidencing a shallow basin geometry with a shape ratio (H/L<0.1) (Bard and