GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 2.2 GNGTS 2023 Site effects in a narrow and faulted valley G.P. Mazzetti 1 , S. Gilli 1 , L. Martelli 2 1 Centrogeo Survey snc, RE 2 Geology, Soil and Seismic Area, Emilia-Romagna Region Introduction Between 21 and 30 June 2013, the Lunigiana (Northern Tuscany) was struck by a seismic sequence with an estimated main shock magnitude of 5.36 and a depth of 7 km. Over the following days, three more strong shocks were recorded with estimated magnitudes of 4.30, 4.63 and 4.77, and hypocentre depths of 8.1 km, 6.1 km and 9.2 km (Rovida et al., 2022). As a result of these events, in Castelnovo ne' Monti, a town in the Reggio Emilia Apennines 35÷37 km away from the epicentres, a three-storey building, with a habitable attic and basement, built in 1993, was seriously damaged and declared not habitable. Analysis of the building revealed damage comparable to that observed in the epicentral zone. The building is located in a narrow valley and, to identify the causes of these anomalous effects, the Municipal Administration, which already has first and second level seismic microzonation (carried out with contributions OPCM 4007/2012, DGR 1302/2013), commissioned a third level in-depth study to assess possible two-dimensional local effects (with contributions OCDPC 52/2013, DGR 1919/2013). Geological setting The area under examination belongs to the western portion of the Vetto-Carpineti syncline. The subsoil of the damaged building consists of a clayey-silty detrital blanket with intercalation of silty-sandy lenses and angular gravels. The substratum consists of alternating marls, siliceous marls and marly-arenaceous levels of the Lower Miocene age referable to the Antognola and Contignaco Formations (Epiligure Succession). Geological maps indicate the presence of an “inferred” (buried) fault, trending approximately south-north, in the middle of the valley. Figure 1 shows an excerpt of the geological map supporting the third level seismic microzonation.