Saw a real interesting program on the Discovery Channel this morning. It was focused on mega-Tsunamis. They only happen every few thousand years or so, but they make Tsunamis look ho-hum.
A Tsunami wave may be 100 feet base-peak-base, with a crest of 30 to 60 feet. A mega-Tsunami may have a wave 100 miles base-peak-base. The wave may have a 500 foot crest. Where a Tusnami will crest and crash down on the mainland, the mega-Tsunami hits the coastline with a wall of water that may not crest until miles inland.
Mega-Tsunamis are created by large landslides into the ocean. Volcanicly created islands are the main culprits. As they mature and decay, they sometimes collapse into the ocean creating a sharp impact that will send a surge thousands of miles across the ocean. While the surge may not look like much as it rushes across the ocean, as the water turns shallow, the surge builds forming a wall up to thousands of miles long. This comes crashing in on the opposing facing edge of the ocean in question.
One such volcanic island near the canary islands is under scrutiny right now. The geography of the island and it's observable decay make it a prime candidate for such a collapse.
Should the right circumstances occur, a collapse of part of this island would cause the catastropic destruction of up to twelve miles from the coast from Florida to Maine.
This phenominon was observed first hand in Alaska, where a fisherman and his son were trapped in a sound as a large landslide created a 500' wall of water crashing into their boat. The wave removed trees 500' up the opposing embankment.
There was too much information to tell all here, but it was very interesting. And the point was clearly made, that earthquake induced Tsunamis were much less dangerous than these land collopse impacts had the potential of being.
One wave impact scenario they showed had the Empire State building standing next to an incoming wave. It was about 2/3rds as tall as the wave.