GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 2.2 403 where, for each site, f 1 is the above mentioned resonance frequency, Vs is the shear wave velocity of the Red silt (around 325 m/s as obtained from down hole investigations, Del Monaco et al. , 2013) and H cal is the thickness of the Red silt. For each site, we note a good agreement between H cal and the red soil thickness found in the borehole (H bor ). So f 1 is caused by the seismic impedance contrast due to the superposition of the Red silt onto L’Aquila breccia which has a Vs around 800 m/s. Moreover, to understand in-depth if the seismostratigraphy had a role on the buildings damage due to the two mentioned earthquakes, we compare the areal distribution of f 1 with that of buildings damage (very heavy damage and building collapse) (Figs. 1 and 2). We note that the 80% of the heavy damaged buildings are placed in sites characterized by f 1 in the range of 3-10 Hz (Fig. 3). But, considering the April 6, 2009 earthquake, the majority of the buildings of L’Aquila historical downtown have a resonance frequency also in the range of 3-10 Hz (F. Di Fabio, pers. com.). The sites of L’Aquila historical downtown, where the subsoil resonance Tab. 1 – Comparison between the Red silt thickness obtained by borehole investigation (H bor ) and the Red silt thickness calculated with the equation f 1 =Vs/4H (H cal ). The site location is reported in Fig. 1. site f 1 (Hz) H bor (m) H cal (m) H bor -H cal (m) TE 5.8 13.2 14.1 -0.9 DU 5.7 12.4 14.4 -2.0 BE 7.7 11.8 10.5 1.3 CA 6.6 13.0 12.3 0.7 BI 14.3 5.0 5.7 -0.7 SB 6.8 13.5 11.9 1.6 CE 4.6 18.7 17.7 1.0 CS 6.5 12.0 12.5 -0.5 DA 6.3 14.0 13.0 1.0 CF 11.6 6.5 7.0 -0.5 RO 4.8 17.0 16.9 0.1 Fig. 2 – Comparison of the subsoil resonance frequency (f1) and the damaged buildings due to the earthquake of February 2, 1703, Mw 6.7 (A) and April 6, 2009, Mw 6.1 (B). Blue and magenta colors refer to severe buildings damage or collapse, respectively.