I believe that this is due to how Japan is near one of the plate boundaries or mid-ocean ridges, where it's easier for a tsunami (from the ridges) to accelerate its speed and for an earthquake (from the plate boundaries) to collide or interact in a myriad of ways. I know that this wasn't so specific, as you have to find the name of the ridge or plates involved in this type of action.
because when tectonic plates crash into each other they cause an earthquake in earthquake can cause giant waves in the ocean which can become bigger as it reaches land.
