How does it work?
Japan is a volcanic archipelago that resides at the boundary of 3 large tectonic plates: the Eurasian Plate, the Philippine Plate, and the Pacific Plate. This results in over 1,500 tremors in Japan every year. Because ocean crust is made up of denser materials than continental crust is, when the two plates meet and slowly move toward each other, the oceanic crust usually submerges. This results in the subducted oceanic crust re-melting into magma and moving up to the surface to fuel volcanoes. This subduction zone also results in more seismic activity because the plates do not always submerge smoothly. The rocky surfaces sliding past each other comes with friction, and there is often a buildup of pressure and periodic releases of energy in the form of earthquakes.
Japan has so many earthquakes because it is placed right near an active fault along the Ring of Fire, or the ring of volcanos that line the Pacific Plate. Japan’s characteristics as an island nation in the Ring of Fire makes the threat of tsunamis especially large when a severe earthquake occurs. Earthquake trigger tsunamis because when the crustal pressure is released, the solid rock is displaced and energy is transferred to the water above. This energy dissipates in a circular motion, and if it approaches land, it becomes a tsunami.
About the Physics
Over time, loading stress builds up within the plates as they continue to move while their contact boundaries do not. When this loading stress overcomes the force of friction between the two rock surfaces, slippage will occur rapidly. Sometimes this slippage occurs faster than others, but it is often in this stop and go fashion. This results in a rapid release of built-up energy and sometimes rapid crustal displacement. When this energy is released, it travels through the solid rock and causes the earth to shake.
Tsunamis in the open ocean often do not have wave heights of more than a meter; however, as they approach shore, they quickly become taller. This is because wave height is proportional to water depth for shallow-water waves (which tsunamis become once the water depth is < 1/2 of their wavelength). This occurs in order to maintain the same energy dissipation even as less water is available for this energy transfer as the wave approaches shore. Therefore, as tsunamis approach shore, the time between each wave crest decreases and their heights increase. This becomes a dangerous combination of more frequent waves with more height inundating the shoreline.