A novel metabarrier is proposed for seismic Rayleigh wave attenuation, designed as a periodic array of soil-embedded cylindrical water tanks acting as resonant units [1]. A theoretical framework is formulated, treating the dynamics of the water tank by a well-established 3D linear, pressure-based model for fluid-structure interaction under earthquake and idealizing the soil as homogeneous and isotropic medium, in agreement with similar studies on seismic metamaterials. The dispersion diagram obtained from Floquet–Bloch dispersion analysis exhibits relevant band gaps in the low frequency range of seismic Rayleigh waves and in the higher frequency range of Rayleigh waves caused by other ground vibration sources as, e.g., railway or road traffic. Frequency-domain analyses of a soil domain with a finite array of water tanks validate the band gaps and show considerable attenuation. An appealing feature of the proposed metabarrier is that the water-tank resonant units can be tuned by varying the water level; in this manner, opening frequencies and sizes of the band gaps can be changed, with remarkable advantages over alternative seismic metamaterials that, in general, are not designed to be tunable. Comparisons with an alternative concept of water-tank metabarrier with cylindrical water tanks deployed above the soil surface [2] are discussed. All calculations are implemented in COMSOL Multiphysics.