GNGTS 2017 - 36° Convegno Nazionale

424 GNGTS 2017 S essione 2.2 The tremor recordings were acquired with a three-component short period sensors (Lennartz 3D-5s) specifically designed to record seismic noise. Seismic noise was recorded at least 60 minutes at each site with a sampling frequency of 50 Hz. Microtremor data were analyzed using the open source software “Geopsy” which contains a dedicated tool to obtain Horizontal to Vertical spectral ratio starting from 3 signals registration. The seismic data were processed using the following parameters: short-term averaging equals 2 s, long-term averaging equals 30 s, with low and high thresholds of 0.2 and 3.0, respectively. Before averaging, the individual spectra were smoothed using the Konno and Ohmachi (1998) method, using a bandwidth coefficient (b) value of 40. Finally, a mean horizontal-to-vertical (H/V) curve was estimated for each site (Fig. 2) following the standard criteria adopted globally (SESAME, 2004). The location of 20 site tests (Tab. 1) are shown in Fig. 1C. Results. HVSR analyses of 20 free-field measurements taken on an approximate 0.16 Km 2 area. The frequency distribution is in a relatively large range: 1.73–24 Hz. The maximum Kg values are found at stations 18 and 19, in the central south-eastern part of the island. In the south-western part the stations 1, 2 ,3, 4, 5, 6, 7, 8, 9, 10, 11 show peaks in the frequency range 14.68-24 Hz, which have medium amplitudes (about 2). This indicates a relatively very- thin sedimentary cover, implying that the Kg values are lowest at these sites (0.13-0.48). Points 13, 15, 18 which are located in the north-eastern part of the Island show very low frequencies (1.73-2.66 Hz), which have relatively high amplitudes (4-6). This indicates a thick layer of sediments and a strong impedance contrast with the bedrock; the estimated vulnerability index range 5.91 to 11.04. In the north-eastern part, points 12, 16, 17, 20 are characterized by slightly higher frequencies (8.4-11.31 Hz) and slightly better expressed peaks. The frequencies and the amplitude of the HVSR peaks determined at 20 points were used to contour the iso-frequency and the ground vulnerability index contour maps (Figs. 3A and 3B), showing the resonance frequency of the sediments. Conclusions. Since the geotechnical characteristics of sediments and their thickness are poorly known in the Magoodhoo Island, due to lack of borehole or geophysical data, we performed single microtremor measurements to determine the characteristics of site response of the shallow subsurface. The entire Magoodhoo island area was surveyed with a dense grid of free-field measurements, in a reasonable time and with relatively low costs in order to detect the potential danger of soil-structure resonance. Amap of the fundamental frequencies of the sediments shows a distribution in a wide range 1.54-21.32 Hz (Fig. 3A). There is a good correlation of different frequency ranges with the supposed lithology and thickness of the Quaternary sediments. The high-frequency range is characteristic of the south-western part of the Island, where the sediments are thin. The north- eastern part of the Island is characterized by medium-low sediment frequencies, indicating thick Plio-Quaternary sediments. The transition between higher and lower frequencies zone is clean. According to our study there is a strong difference in impedance contrast between the south-western and north-eastern area of the Island. The area located at south-western is characterized by a high resonance frequency due to the presence of very shallow beach-rock. It is evident because there aren’t structures building. On the other side, in the north-eastern part of Magoodho, the strong influence of incoherent and large thickness sediments (very low resonance frequency) and the presence of water table variation in shallow sand deposit, produce partial or total liquefaction effects with serious building damages. In particular, the weakest zone is in the central south-eastern part of the Island (Fig. 3B); larger Kg values are found in the stations 18 and 19 characterized by high amplitude at lower fundamental frequency. Therefore the extensive damage would be expected in this area of the island. Acknowledgements The authors would like to acknowledge the help and cooperation of the Bicocca University staff: Paolo Galli (Responsible Center of Milano-Bicocca University - Magoodhoo Island), Simone Montano, Andrea Di Pietro, Laura Bernasconi de Luca, Angelica Cajiao, Davide Seveso. We wish to thank Bicocca Milan University