The devastating tsunamis are created when a tectonic plate makes up-down movement. This causes the uplift movement of water that spreads across the ocean. In the deep ocean, the tsunami has an extra long wavelength and a small amplitude (wave height)
so it's barely noticeable. But when it comes to the shallow water, due to the wave shoaling, the wavelength shortens and the amplitude increases. Tsunami wave is like a wave in physics, an energy transfer through the elastic material, it has a phase, an amplitude and a velocity. Water in a wave does only
vertical shift, very little horizontal. Imagine the ocean consisting of thin water columns of water molecules, the wave starts by the lift of the first column that
spreads to the other columns because of the elastic bonds between water molecule in first and second column. In the deep water where each column has a lot of molecules, energy is spread, each molecule has just a little part of it (little shift). In the shallow water, column has fewer molecules and each molecule has to take a lot more energy (has more shift). OK, to stay short, the idea is as follows:
Use explosive depth charges to create a wave that would create the destructive interference with incoming wave. The destructive interference is
when one wave meets other wave of the same frequency that is out of phase. A rise of one wave meets the fall of the other
and they annihilate each other. Depth charge should not be like an antisubmarine mine that makes spherical blast but more like a
claymore mine , with directed blast. Location and depth of charges should be calculated. Areas that cause tsunamis are across fault lines, so it's generally known where tsunamis can emerge. There are systems (DART) that sense submarine earthquakes, measure shift and send it via satellite so it is possible to know necessary information. The Tohoku tsunami took about 10 - 20 minutes to reach the shore. There's enough time to prepare and activate charges. Charges would be set in front of cities or other important locations , like in the bay entrances. Charges could be put before, anchored to the sea bed or thrown by the military anti submarine airplanes. This method is attainable by current technology, it's adaptable and cheaper than concrete barriers.