However, the bathymetry (physical underwater conditions at the coastline) can complicate things, as the 8.3 magnitude earthquake in the Aleutian islands of March 9, 1957 produced a 16 meter (52.5 feet) tsunami on the North Coast of Oahu. There is an amplification factor, sometimes, caused by the bathymetry, when the wave hits. Then there is the confusion between the actual amplitude of the wave relative to sea level, and the so-called run-up, which can be higher, caused by the momentum of the wave.
The Great Kamchatka Earthquake of October 17, 1737, supposedly produced a tsunami greater than 50 meters on the North Kurile Islands, but this was still generally in the basic neighborhood. The Santorini eruption of 1638 BC produced a tsunami also estimated at 50 meters and the 1490 BC eruption might have caused the end of the Minoan civilization in Greece, but there were no long-distance catastrophes. 50,000 died from the tsunami that hit Taiwan on May 22, 1782 and 36,417 from the Krakatoa explosion on August 27, 1883. The bottom line is that a mega-event in or near the ocean can cause great damage in the near field, but generally does not exceed 10 meters thousands of kilometers away. Of course, a height of ten meters, or 33 feet, is a heck of a large wave, and considerable damages do occur. Also, there is something about volcano flank failures and landsides that seem to magnify the effect, at least in the vicinity of the event.
There is no scientific definition for a mega tsunami, but anything over 40 meters (130 feet) should qualify, and Charles Mader has suggested 100 meters. Gary McMurtry of the University of Hawaii has told me that a 1,600 foot (nearly 500 meters) wave appeared on the Big Island of Hawaii 110,000 years ago, probably caused by a submarine landslide from the undersea flank of Mauna Loa Volcano. This remains a controversial incident.