Japan gets tons of earthquakes, about one every year, because of Japan's position. Japan sits on a subduction zone, meaning plates slide under another causing friction and earthquakes. Sometimes, Japan will get a tsunami caused by their frequent earthquakes.
Japan lays on a large earthquake fault which means they get alot of earthquakes. Some of which are very strong. Strong earthquakes in the ocean can cause tsunamis. And since Japan gets alot of earthquakes, they will get more tsunamis then normal.