Most tsunamis are associated with subduction earthquakes along the Pacific Ring of Fire, and to a lesser extent with earthquakes along the edges of continental plates in the Indian Ocean. However, the Atlantic Ocean is also vulnerable to tsunamis.
All tsunamis are caused by massive water displacement. This is most commonly caused by subduction earthquakes, but other types of earthquakes can also cause tsunamis. There are also 2 other ways in which massive water displacement can occur in the Atlantic Ocean: landslide and meteorite impact.
The great Lisbon earthquake of 1755 was centered off the coast of Portugal, and had effects as far away as Finland. It was followed half an hour later by 3 tsunami waves, which in some places were nearly 20 feet high. Besides Lisbon itself, major tsunamis also struck North Africa, southwest England, Martinique and Barbados, and even Galway, on the west coast of Ireland. France Belgium, and the Netherlands were also affected.
The earthquake is believed to have been centered on the Azores-Gibraltar Transform Fault, an extension of the series of faults in the Strait of Gibraltar between Europe and Africa, although by itself, that fault cannot account for the great seismic moment and documented tsumani amplitude. There are few subduction zones in the Atlantic Ocean, but these faults may connect with a subduction zone in the Mediterranean Sea near Italy.
This series of faults is still not well understood. A similar earthquake, with similar results, could occur again in the Iberian Peninsula, although it is not likely to happen in the near future. Likely locations for other Atlantic Ocean earthquakes which could cause major tsunamis are the Puerto Rico Trench, which was responsible for the 1918 Puerto Rico earthquake and tsunami, and the Antilles subduction zone.
In 1929, a magnitude 7.4 earthquake struck off the coast of Newfoundland, which is now a part of Canada. This is not a subduction zone and normally the displacement would not have been enough to cause a tsunami. However, the earthquake dislodged sediment layers on the ocean floor which caused an underwater landslide. The resulting tsunami caused 28 deaths in Newfoundland, and was documented as far away as Portugal.
The most likely future cause for a landslide-triggered tsunami in the Atlantic Ocean is an eruption and collapse of the Cumbre Vieja volcano on the island of La Palma, one of the Canary Islands. The last tsunami of this type happened 300,000 years ago, when part of the island of El Hierro slid into the sea. The unstable land may give way gradually, in a series of small landslides which won’t cause any major problems. However, if the Cumbre Vieja basalt gives way all at once, the resulting tsunami will strike much of the east coast of North America with catastrophic force.
Tsunamis which are caused by landslides are unlikely to repeat themselves any time soon. It takes a very long time for sediment or other unstable surfaces to build up enough layers to displace enough water to create a tsunami. After the unstable ground gives way, it cannot cause another tsunami until it can build up once again.
No tsunami due to meteorite strike has occurred in recorded history. Of all known asteroids, only 1950 DA has any real chance of hitting the earth. That chance won’t occur until 2880, and even then it is only about 0.3%.
However, if 2880 does hit the Earth and land in the Atlantic Ocean, it is certain to cause a series of titanic tsunamis. The energy of that landing would completely vaporize the 2/3 mile-wide asteroid and a good chunk of ocean around it, all the way down to the ocean floor. The water rushing in to fill that sudden hole would create a series of waves which would build up into several massive tsunamis as much as 400 feet high. By the time those waves reach the Eastern Seaboard between 2 and 4 hours later, they could still be up to 200 feet high.
Even that wouldn’t be the end of it. With that amount of wave energy, any unstable undersea slopes could give way, which would create secondary tsunamis. Water crashing into mountains, cliffs, and other stone formations could dislodge enough rock and soil into the ocean to cause even more tsunamis. It could take months or even years for everything to settle down again.