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mICrotremor hVsr AnAlysIs to hIGhlIGht the effeCts of potentIAl lIquefACtIon: experImentAl results At mAGoodhoo IslAnd (fAAfu Atoll, mAldIVes, IndIAn oCeAn) M. Punzo1, G. Cavuoto1, D. Tarallo1, N. Pelosi1, P. Scotto Di Vettimo1, M. Iavarone1, S. Mazzola2,P. Galli3,4, V. Di Fiore11 Institute for Coastal Marine Environment, Italian National Research Council, Naples, Italy2 Institute for Coastal Marine Environment, Italian National Research Council, Torretta Granitola-Trapani, Italy3 Department of Biotechnology and Biosciences, University of Milano-Bicocca, Milan, Italy4 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) to Addu Atoll (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.

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depth varies from over 500 m in the north to 300-400 m in the south. The archipelago is made up of 22 atolls of circular or elongated shape, the sizes varying from some kilometers to many tens of kilometers, and contains more than 1000 islands which cover a surface of 298 km2. Every atoll is formed by a marginal rim surrounding a lagoon commonly less than 50-60 m deep, though some reach depths of more than 80 m. The atoll lagoons are characterized mainly by sandy sediments; a large amount of the sand (more than 50%) derives from mechanical erosion and from bioerosion of the reef. A minor amount is due to benthonic organisms living in the sand with fewer contributions from planktonic foraminifers. Magoodhoo, is a remote small island on the Faafu Atoll (Fig. 1B), at a distance of 134 km from Malé, Republic of Maldives capital, with a population of 683 inhabitants distributed on a surface of less than 0.36 km2.

Field measurements. The basic goal of single-station ambient vibration measurements is the detection of seismic impedance contrasts in the subsoil, that are responsible for seismic resonance phenomena (e.g., Kramer, 1996).

The technique consists of recording and analyzing the seismic ambient noise to retrieve information about subsoil stratigraphy from seismic tremor. In particular, we use HVSR technique, which consists of studying the ratio between the Vertical and Horizontal spectral components of the tremor recorded at a single station. The method, provides a direct estimate of the resonance frequencies of subsoil. This well known capability allows to translate the H/V curves into stratigraphy (Castellaro et al., 2005). The theoretical bases of this technique are simple only in a 2 layer Fig. 2 - Results of HVSRs using microtremor measurements.

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1-D model in which parameters are constant within each layer. In the latter case, the depth h of the seismic discontinuity is immediately derived from the free-surface resonance equation h=V/(4f0), where V is the seismic wave velocity of the upper layer and f0 is the resonance frequency which appears as a peak in the H/V curve. Another important parameter is the vulnerability index (Kg) which can be determined using the empirical relation Kg=(Af0)

2/f0, in which Af0 is the amplitude at the fundamental frequency, which determines the damage level due to local site effects (Nakamura, 1996). It may be considered for predicting the potential for soil liquefaction. This value, in fact, is clearly site-specific and value might be useful in demarcating weak regions, which are susceptible to liquefaction (Huang and Tseng, 2002; Beroya et al., 2009).

Tab. 1 - HVSR stations: Coordinates, peaks frequency and vulnerability index.

NAME COORDINATE RESONANCEFREQUENCY VULNERABILITYINDEX

STATION1 E72.5747455 17.74 0.48 N3.0442160

STATION2 E72.5749796 18.07 0.47 N3.0442796

STATION3 E72.5751135 17.42 0.27 N3.0438757

STATION4 E72.5746621 19.84 0.28 N3.0437169

STATION5 E72.5744101 21.32 0.18 N3.0437184

STATION6 E72.5741537 24 0.13 N3.0436779

STATION7 E72.5736108 14.68 0.13 N3.0435533

STATION8 E72.5735079 21.95 0.18 N3.0435640

STATION9 E72.5739085 22.23 0.22 N3.0437146

STATION10 E72.5737335 23.95 0.23 N3.0437007

STATION11 E72.5743638 19.48 0.29 N3.0440285

STATION12 E72.5745511 11.98 0.54 N3.0440540

STATION13 E72.5757599 2.27 6.83 N3.0448697

STATION15 E72.5758687 1.73 5.91 N3.0447355

STATION16 E72.5757227 8.4 4.80 N3.0446317

STATION17 E72.5755179 11.31 3.15 N3.0443865

STATION18 E72.5753558 2.66 11.04 N3.0445186

STATION19 E72.5752942 1.54 26.68 N3.0440625

STATION20 E72.965673 9.07 1.23 N3.0803232

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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 Km2

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.

A map 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

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for support the realization of this research activities. We also wish to thank all Maldivian Authorities and Magoodhoo People for patience keep during the field research activities.

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