The reason that Japan is so tectonically active is because, the entire island sits on about four to five different plates. When the plates shift it might start an earthquake or even bring a piece of land mass from the ocean floor. An earthquake could kills hurdreds if not thousands of people at a time depending on the damage that it does. If a new piece of land mass is brought to the surface it could be a new island.
Why is Japan so tectonically active?