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A tsunami destroys many offshore and seafront structures supported by the sea floor. However, a disaster mechanism to address these concerns has not yet been completely formulated. In this paper, we discuss the ways in which multi-phase interactions among soil, water, and gas and multi-scaling problems from soil, water, and gas particles affect these structures. Based on centrifuge model tests and SPH simulation with three phase interactions, it is shown that the destabilization of offshore structures due to a tsunami not only impacts the sea-front structures, but also causes water seepage into the seabed/seafront soil, which results in the scouring of the soil due to turbulence. Excessive pore water pressure in the soil is generated and liquefaction occurs as a result, which decreases the stiffness of the soil. The scouring and liquefaction occur both at the particle scale and, in the soil, at mass scale, thereby destroying the structures: this is a multi-scale interaction. This study attempts to explain the scouring mechanism by focusing on the tractive force and pore water pressure in the ground. During a tsunami, the traction water flow and the water overflow in the soil generates a vertically upward hydraulic gradient along with excess pore water pressure in the saturated ground and, consequently, scouring is facilitated. Scouring is also expanded when the pore air is blown out of the unsaturated ground. This study finds the scouring in the ground to be facilitated by a three-phase interaction among the soil particles, the pore water, and the pore air.