A giant wave set in motion by a large earthquake under or near the ocean that causes the ocean floor to shift vertically.
These waves may reach enormous dimensions and have sufficient energy to travel across entire oceans. They proceed as ordinary gravity waves with a period between 15 and 60 minutes. Tsunamis steepen and increase in height on approaching shallow water, inundating low-lying areas. Where local submarine topography causes extreme steepening, they may break and cause great damage.