This Wikipedia page is very interesting: While the tsunami wave of the 2004 Indian Ocean earthquake was by far the deadliest ever recorded it was by far not the highest: that was recorded 1958 in Alaska and was created by a land slide inside a small fjord (LItuya Bay) causing a wave to reach high as 500 meters (1500 ft).

Underwater landslides appear to be a source of much bigger tsunamis than earthquakes. The Wikipedia page about a possible future tsunami:

In 2001, scientists predicted that a future eruption of the unstable Cumbre Vieja volcano in La Palma (an island of the Canary Islands) could cause a supergiant undersea landslide. Later research showed that the threat was less than had originally been theorized. The next volcanic eruption is expected in the second half of the 21st century, but it is not necessarily the eruption that causes an immediate landslide. In the worst case scenario, the western half of the island (weighing perhaps 500 billion tonnes) would catastrophically slide into the ocean. Such a landslide could cause a 100 m megatsunami to devastate the coast of northwest Africa, with a 10-25 m tsunami reaching the east coast of North America 7-8 hours later causing massive coastal devastation and the deaths of perhaps millions of people, threatening Miami, suburbs of New York, and parts of Boston, and all coastal cities in between.

The scientists have different opinions about the probability of this catastrophe.

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