GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 3.2 715 HINTS ON THE SEISMIC HAZARD OF L’AQUILA HISTORICAL CENTRE THROUGH THE GEOLOGICAL INTERPRETATION OF HIGH-RESOLUTION SEISMIC REFLECTION INVESTIGATION M. Tallini 1 , M. Spadi 1 , D. Cosentino 2 , M. Nocentini 3 , G. Cavuoto 4 , V. Di Fiore 4 1 Dipartimento di Ingegneria Civile, Edile-Architettura e Ambientale, Università degli Studi dell’Aquila, Italy 2 Dipartimento di Scienze, Università degli Studi di Roma Tre, Italy 3 Istituto di Geologia Ambientale e Geoingegneria - CNR, Roma, Italy 4 Istituto di Scienze Marine-CNR, Napoli, Italy Introduction. We present the updated geological subsoil model of L’Aquila via the study of a high-resolution seismic reflection 1000 m-long profile located in the main street crossing the historical centre (Corso section) (Fig. 1). The Corso section was geologically interpreted through the integration of the seismic refraction tomography, the cross-check of borehole stratigraphy, and detailed geological survey data (Del Monaco et al. , 2013; Nocentini et al. , 2017). The main goals were the reconstruction of the buried top surface of the Meso-Cenozoic bedrock providing the 3D model essential to estimate the seismic site amplification through numerical modelling and to evaluate the existence of possible active faults in L’Aquila historical centre. From a more speculative point of view, the geophysical investigation allowed us also to improve the Plio-Quaternary tectono-stratigraphic evolution of the Western L’Aquila Basin in which L’Aquila historical centre is located. Geological setting. Following recent studies, it has been possible to outline the geological setting of the intermontane Western L’Aquila Basin (Cosentino et al. , 2017; Nocentini et al. , 2017, 2018; Tallini et al. , 2012, 2019). The Colle Cantaro Fm. (CCF) represents the oldest Plio-Quaternary unit of the Western L’Aquila Basin. It overlays locally the faulted Meso- Cenozoic bedrock and consists of mud-supported breccias sedimented with debris flow events during the late Piacenzian-Gelasian in slope environment. The Madonna della Strada Fm. (MDS) unconformably overlays CCF and the Meso-Cenozoic bedrock environment. MDS is composed of pelite, sand and lignite horizons sedimented during the Calabrian in a meandering alluvial environment characterised by floodplain and swamps. The Meso-Cenozoic bedrock, CCF and MDS are sometimes covered by the Fosso di Genzano Synthem (FGS). FGS is composed of grain-supported gravel and sand sedimented during the Middle Pleistocene in a braided or wandering alluvial environment. Most of L’Aquila historical centre is placed on a hill composed of the Colle Macchione-L’Aquila Synthem (CMA). CMA consists of heterometric and massive calcareous breccia sedimented with debris flow and rock avalanche events during the late Middle Pleistocene in a slope environment. CMA unconformably drapes the older Plio- Quaternary formations and the Meso-Cenozoic bedrock and are interlayered with alluvial sand and gravel in the southern zone of the L’Aquila historical centre. The top of L’Aquila hill is mantled by the Collemaggio Synthem (COM), consisting of reddish clayey silt, interpreted as reworked paleosols sedimented during the early Late Pleistocene. At last, the youngest units are alluvial, colluvial, slope, and anthropic deposits of Late Pleistocene and Holocene age. The seismic facies versus the geological interpretation of the Corso section. Five seismic facies (termed BC, L, Ls, R, and S) were recognized in the Corso section basing on the characteristics of the seismic reflectors, such as geometry, seismic reflection amplitude, continuity, and qualitative reflection frequency (Fig. 1). The recognised seismic facies were additionally checked by examining the information from the borehole stratigraphy. The ubiquitous presence in the examined boreholes of the CMAcalcareous breccia integrated with the extremely chaotic seismic facies BC allowed the identification of this late Middle Pleistocene Fm. for the upper sector of the Corso section. Conversely, the Upper Pleistocene- Holocene anthropic, alluvial, and colluvial deposits, overlaying CMA and characterized by a few meters of thickness, are not distinguishable with respect to CMA, due to the resolving power of the used seismic method. The seismic facies L is composed of laterally extensive and parallel reflectors with medium