Geophysical approach to study the potential ocean wave-induced liquefaction: an example at Magoodhoo Island (Faafu Atoll, Maldives, Indian Ocean).

This paper presents an application of geophysical techniques to determine the liquefaction potential of the surface sand layer under wave loading in an atoll island, Magoodhoo Island (Faafu Atoll). Measurements of background noise (H/V technique), seismic refraction profiles (tomography), surface wave inversion and electrical resistivity tomography were applied to study the properties of the substratum. Geotechnical analysis provided the relative soil density (Dr = 55%) and the grading curves; the comparison of these parameters with geophysical data, allowed to identify indicative features related to ground liquefaction phenomena. Microtremor measurements calculated a fundamental site resonance in the frequency range of 1.42-23.19 Hz. Velocity profiles showed that bedrock interface depth varies from approximately z = 35 m north-east of the island, to z = 25 m south-west, while in the most north-eastern external area, it was not detected. These results suggested a significant lateral variability of the site seismic response throughout the study area. Liquefaction triggering with wave heights varying between 2.0 and 3.0 m and 70-300 cycles was calculated. Also, considering an average wave period of 6 s, liquefaction was more likely after 7-30 min. The soil vulnerability index Kg was also used to evaluate the soil liquefaction hazard suggesting that the north-eastern island sector is more susceptible to liquefaction, confirmed also by the building damage status as compared to other areas of the island. The results obtained in the survey highlight that in an atoll island, where potential disasters can be caused by several natural causes, saturated loose sands lose their strength and stiffness if subjected to wave loading.