There are only two known catastrophic tsunamis in the Atlantic Ocean, one in Puerto Rico in 1918 that killed 116 people, and one in Lisbon, Portugal in 1755 that killed 60,000 people. The greatest modern risk is the possibility that an earthquake could trigger the collapse of one of the Canary Islands (off the coast of Morocco) which would produce a tsunami wave that would have a peak wave height of 156 meters (high enough to drown every structure in downtown Denver) and would have tsumani waves there were still 13 meters high by the time that they reached Boston.
300 000 years ago . . . a part of the island El Hierro slid into the sea, triggering a mega-tsunami which carried rocks as high as a house for many hundreds of metres into the interior of the east coast of what is today the USA. The danger of a similar island collapse is seen by scientists particularly at the island of La Palma in the Canaries. Here, following a volcanic eruption in 1949 almost half of the mountain range of 20 km moved westwards towards the sea, leaving a large tear in the volcanic basalt. In the event of a fresh eruption, a huge part of the volcano could loosen itself due to differences in the types of rock and diverse water deposits within the now active volcano. As a result, the densely populated east coast of America would be massively threatened.
From the point of view of personal safety, the main warning sign of a tsumani, which may appear even if no earthquake is felt, is that the sea recedes rapidly. If you see this, immediately move as quickly as you can away from the sea or ocean to the highest point possible. Many people who die in tsunamis could have saved themselves if they ran for high ground instead of trying to collect stranded fish, tried to surf the wave, or just stayed where they were when it happened.