GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 2.2 421 Microtremor HVSR analysis to highlight the effects of potential liquefaction: experimental results at Magoodhoo Island (Faafu Atoll, Maldives, Indian Ocean) M. Punzo 1 , G. Cavuoto 1 , D. Tarallo 1 , N. Pelosi 1 , P. Scotto Di Vettimo 1 , M. Iavarone 1 , S. Mazzola 2 , P. Galli 3,4 , V. Di Fiore 1 1 Institute for Coastal Marine Environment, Italian National Research Council, Naples, Italy 2 Institute for Coastal Marine Environment, Italian National Research Council, Torretta Granitola-Trapani, Italy 3 Department of Biotechnology and Biosciences, University of Milano-Bicocca, Milan, Italy 4 MaRHE Center (Marine Research and High Education Center), Magoodhoo Island, Faafu Atoll, Maldives Introduction. The Magoodhoo island (Maldives) is a part of coral atoll which presents several hazard linked to the effects of sea tides and waves, earthquakes and tsunamis. In particular, the island coralline coarse-grained sands lies on a coral rubble base; this sandy non-cohesive could experience liquefaction and therefore local subsidence following earthquake loads, severe storm wave loads, or a combination of the two. Cyclic load may cause an excess pore pressure (due to ground water mounding) and the shallow sands may be liquefied, and the island structures will become unstable. Today the effects induced by these phenomena were still visible differently on Magoodhoo island. For this purposes we performed a study by microtremor analysis (Horizontal-to-Vertical Spectral Ratio - HVSR: Nakamura, 1989) to identify possible site effects and to investigate the characteristics of soil liquefaction. Microtremor method is commonly used to detect the rigid basement which present shear wave velocity ≥ 750 m/s (Monge et al., 1999; Guéguen et al., 2000; Régnier et al., 2000). The HVSR method, in fact, is considered by many authors as giving a good estimation of the site fundamental frequency resonance (Lachet and Bard, 1995; Goula et al., 1997; Mucciarelli, 1998; Lebrun et al., 2001) and a very effective tool to decipher the local soil conditions (Singh et al. , 2017). The passive recording of ambient vibrations may provide a mapping tool of site features where geotechnical information is usually difficult to obtain. Microtremors were recorded at 20 sites with single stations. Study area. The Maldives archipelago comprises a chain of coral reefs and reef islands situated 700 km SW of Sri Lanka (Fig. 1A). The archipelago extends 868 km from Ihavandiffulu Atoll in the north (7°10’N) toAdduAtoll (0°43’ S) just south of the equator, and is 130 km wide. Its central sector consists of a double chain of atolls rising from a submerged plateau, whose Fig. 1 - A: The Maldives archipelago south-west of India, in the central equatorial Indian Ocean, is an isolated tropical carbonate platform. The archipelago comprises about 1200 smaller atolls. B: Faafu Atoll. C: Magoodhoo Island; the red circles indicate free-field measurements for which the resonance frequency was determined.