Whereas common tsunamis reach a maximum height of 10m, megatsunamis can reach 100 to 150m, a hugh wall of water which does not break as it approaches the shoreline but passes several kilometres inland, depending on the gradient. Common tsunamis are produced by a slippage along a fault line in the sea floor; megatsunamis are much rarer events produced by huge landslides into the sea. Land collape on such massive scale is characteristic of volcanic islands, such as Hawaii in the Pacific Ocean and Cape Verde in the Atlantic Ocean. Such events occur only once every several thousand years and the most recent was around 4,000 years ago when part of the island of Reunion (east of Madagascar) slumped into the Indian Ocean producing a giant wave which struck the north north-west coast of Australia.
One volcanic island currently shows disturbing similarities to the Reunion event -- La Palma in the Canary Islands, situated off the north cost of Africa. The vertical lava dikes within the Cumbre Vieja volcano have dammed huge volumes of water. When the forces are sufficient, movement along the north-south rift zone would cause the south western section of the island to slip westwards into the ocean and generate a megatsunami which would travel across to the east coast of the USA.