"A tsunami is a series of ocean waves of extremely long wave length and long period generated in a body of water by an impulsive disturbance that displaces the water.

Tsunamis are primarily associated with earthquakes in oceanic and coastal regions.

Landslides, volcanic eruptions, nuclear explosions, and even impacts of objects from outer space (such as meteorites, asteroids, and comets) can also generate tsunamis. "

Citation source: http://www.publicaffairs.noaa.gov/grounders/tsunamis.html

Mt. Fuji Off Kanagawa painted by Hokusai Katsushika