Japan is prone to earthquakes because it is within 125 miles of large tectonic plates that frequently clash together. These plates move closer together each year, further increasing the risk of earthquakes. One famous example of Japan's earthquakes is the 2011 8.9 magnitude quake, which resulted in a large tsunami.
The tectonic plates close to Japan range between 6 and 30 miles thick, and they move between 3.1 and 3.5 inches per year. Each time one moves, it either slips above or beneath the other, which causes friction. Due to the size of each plate, this friction is strong enough to cause a build in pressure that eventually becomes too much for the lock between each plate, resulting in an earthquake that disrupts the Earth's crust.
In 2011, an 8.9 magnitude quake hit Japan's coastal line. It was 80 miles offshore and 15 miles beneath the sea's surface, and it produced a break in the Earth's crust that was over 180 miles long. The energy released was more than the United States uses in a year, and later aftershocks exceeded 6 and 7 on the Richter scale. As the displacement of the Earth's crust thrusted the seabed upwards, a large tsunami hit Japan's coast, with waves that were hundreds of miles long and moving at 400 mph.