GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 2.2 125 lacustrine deposits (V S ≈ 500-800 m/s), placed on the calcareous bedrock ( V S ≈ 2000 m/s). Low V S values were detected in the upper portion of the breccias (i.e. residual soils V S ≈ 250-350 m/s) and close to Aterno river (i.e. alluvial deposits V S ≈ 200 m/s). In this work extensive noise measurements were collected along the �� steep scarps that borders the southern edge of L’Aquila hill, the most damaged area of the town (Fig. 1)� �� . The aim was to �� investigate the relationship between horizontal-to-vertical amplitude spectra ratio (H/V method) with the geological and geomorphological variations present along the flanks of the L’Aquila terrace. In particular the analyzed sections cross the �� Saint Apollonia �� (i.e. 20 % to 45 % of medium slope)� �� , Porta di Bagno �� (i.e. 10 % to 30 % of medium slope) and Porta Napoli �� (i.e. 20 % to 45 % of medium slope) �� trenches, and Collemaggio scarp �� (i.e. 10 % to 30 % of medium slope)� �� . Otherwise the seismic stations were mainly located on the calcareous breccias geological and/or red soils units. Few measurements were performed on the alluvial deposits of the Aterno River. Each noise acquisition was recorded by a seismic three-components velocimeter (Le3d- 5s manufactured by Lennarts Electronic; http://www.refteck.com/ ), powered by an external battery. The time synchronism was provided by an external GPS antenna connected to the digitizer. The velocimeters were oriented to the North and in many cases buried for about 20 cm. The average time-length of each noise recording was about 45 minutes. The ambient vibration data were processed mostly by Geopsy (http://www.geopsy.org/ ) and SAC2000 code (Goldstein et al. , 2003). Single-station noise technique computes the horizontal-to-vertical spectral ratio, H/V method (Nakamura, 1989). The peak of the H/V ratio identifies the resonance frequency ( f 0 ) of the subsoil, which is related to the thickness ( H ) and to the shear wave velocity ( V S ) of the soft deposits. In general, in one dimensional (1D) assumption and in first approximation f 0 is derived from the formula V S /4H . Results and discussion. In L’Aquila downtown �� the horizontal-to-vertical spectral ratio curves show strong �� and polarized �� resonance peaks at low frequency (< 1 Hz) and locally at high frequency (> 5Hz)� �� . The �� low-frequency resonance peak f 0 (in Fig. 1 f 0 is represented by squares or barrettes, when the noise signal is polarized) varies from about 0.5 to 0.9 Hz for sites at the top of terraces and in proximity of the Aterno river valley, respectively. The strong low-frequency resonance, centered at about 0.5–0.6 Hz, is very diffuse in L’Aquila terrace, as Fig. 2 – Geolithological section (a) and noise measurements along Saint Apollonia trench (b).