GNGTS 2014 - Atti del 33° Convegno Nazionale

In particular, the Plio-Pleistocene Marine deposits represent the local geological bedrock. Mechanical and dynamic properties were attributed at each lithotechnical unit according to literature data (Bozzano et al., 2008; Caserta et al., 2012). The geological model was integrated by a geophysical dataset available fromfield survey in order to provide an engineering-geological model of the valley. In the June-October 2009, 3 ambient noise surveys were carried out in the Fosso di Vallerano Valley, using a 4Hz digital data-logger TROMINO (Micromed) set to a 128 Hz sampling rate; in the June – July 2014 an additional campaign of ambient noise records was carried out in the valley, using a Lennartz seismometer LE-3D/5s set to a 250 Hz sampling rate. The ambient noise analysis, performed according to the HVSR (H/V spectral ratio) Nakamura technique (Nakamura, 1989), pointed out a stratigraphic seismic response of the valley with a fundamental resonance frequency of 0.8-0.1 Hz. On the other hand, no amplification resulted in the surrounding volcanic hills. From June to July 2009 a free-field velocimetric array operated in STA/LTA (Short Time Average to Long Time Average) acquisition mode in the Fosso di Vallerano Valley, in order to record weak-motion events during the tail of the L’Aquila seismic sequence. The computed receiver functions (Lermo et al. , 1993) of the recorded events confirm the results obtained from the HVSRs derived by the noise measurements. Numerical modeling. In the last decades great efforts were aimed at seismic response numerical modeling since it represents a powerful tool for validation and prediction of physical phenomena related to the seismic wave propagation in problems of civil engineering and earthquake geotechnical engineering. A lot of numerical codes based on different mathematical approaches, i.e. Boundary Element Method (BEM), Finite Element Method (FEM) and Discrete Element Method (DEM), were implemented in both the time and the frequency domain. The numerical modeling actually represents the main tool to estimate the local seismic response and the seismically-induced effects, particularly in the urban area (Rovelli et al. , 1994, 1995; Bouden-Romdhane et al. , 2003; Panza et al. , 2004; Semblat and Pecker, 2009; Bonilla et al. , 2010) where the geophysical measurements are often not suitable for highlighting the local seismic response. Calibration of the seimo-stratigraphic subsoil model. A calibration process by using the seismometric records collected in the Fosso di Valerano study area and the outputs from a 1D numerical modeling performed by EERA code [Equivalent – linear Earthquake Response Analysis: Bardet et al. (2000)] was performed to refine the seismo-statigraphy of the Fosso di Vallerano valley as well as to highlight the role played by the different layers (i.e., corresponding to the distinguished lithotecnical units) on the amplification function (A(f)). Based on the available high-resolution geological model, the calibration process was performed only by varying the shear wave velocity (Vs) of each lithotechnical units. To best fit the numerical model outputs with the A(f) derived from the recorded weak motions the records obtained at the reference site were de-convoluted to the local seismic bedrock. At this aim the soil column corresponding to the site of seismometric station that was operative during the Summer 2009 was selected (Fig. 1a). The average site to reference spectral ratio (SSR) obtained from the recorded weak motions (Borcherdt, 1994; Lanzo and Silvestri 1999) were compared with the ones obtained from the 1D numerical modelling performed by assuming different Vs values for the log-stratigraphy of the analysed soil column. The Vs profile shown in Fig. 1a corresponds to the best fit between the records and the model outputs (Fig. 1b). The resulting differences between the modeled and the recordedA(f) functions are attributed to 2D effects due to both the valley shape and the heterogeneous soil fill that cannot be modeled by the 1D EERA code. Results of the 1D numerical modeling. The seismo-stratigraphic model of the subsoil was extrapolated to be applied at the entire Fosso di Vallerano valley; at this aim, 13 soil columns were selected from the reconstructed engineering-geological model that are representative for different layering conditions. The resultingA(f) functions were compared each other to highlight GNGTS 2014 S essione 2.2 295