How would a tsunami form in the Atlantic Ocean?

As Japan assesses the toll of yesterday's 9.0 magnitude earthquake and tsunami, onlookers half a world away are likely wondering if a similar force of nature could strike in the Atlantic. The short answer? It's improbable, but not impossible.

Tsunamis have been a phenomenon primarily located around regions of intense geological activity, such as fault lines and submarine volcanoes. Japan is situated along the Pacific Ring of Fire, and the nation's sophisticated tsunami warning systems and earthquake-ready building codes (none of the buildings in Tokyo toppled during the Sendai quakes) are just some adaptations the nation's made for living on geologically capricious terrain.

Compared to the geological hotspot that is the Pacific, the Atlantic Ocean is relatively calm. But how would a tsunami ever develop in the Atlantic Ocean? History offers us some clues as to what this rare scenario would look like. According to Gerard Fryer of the University of Hawai'i, an Atlantic tsunami could be generated near subduction zones in the Caribbean and south of South America:

Tsunamis have hit Puerto Rico and the Virgin Islands half-a-dozen times in recorded history (most recently in 1918, when 32 people died). But by far the most famous Atlantic tsunami was generated by an earthquake nowhere near a subduction zone.

Gorringe Bank is a ridge off the coast of Portugal uplifted by the northward movement of the African Plate against the Eurasian Plate (there is convergence between the plates going on, but the closest true subduction occurs far to the east, beneath Italy). On 1 November 1755 (All Saints' Day!) a magnitude 8.6 earthquake at Gorringe Bank destroyed much of Lisbon. Minutes after the earthquake, the tsunami arrived. At least three great waves about ten meters high entered the city. The waves also raked the nearby coasts of Spain and North Africa, and did extensive damage in the Azores, Madiera, and Canary Islands. Minor damage occurred as far north as Ireland and as far west as the West Indies.

Fryer also notes that an undersea landslide could trigger tsunamis, like in the case of the 1929 South Coast Disaster in Newfoundland.

Many people are unaware that landslides can generate tsunamis too (a landslide will suck down the sea surface behind it; the hole in the sea surface will oscillate to give a series of waves). There have been several landslide-generated tsunamis in the Atlantic. The most recent was in 1929, when glacial debris dropped at the edge of the continental shelf by the St. Lawrence River collapsed down the continental slope during the Grand Banks earthquake. That tsunami killed twenty eight people along the Burin Peninsula of Newfoundland. (There is argument about the origin of the 1929 tsunami. Some seismologists think it was generated by the slide, others argue that it was excited directly by the earthquake.)

Another unlikely but potential cause of an Atlantic tsunami could be a cosmic impact. As outlandish as this sounds, there is historical precedent — geologists suspect that New York City was hit by a 66-foot-high tsunami over 2000 years ago. Soil samples indicate that a tsunami hit Manhattan around 300 BC. and although tremors could have been the culprit, traces of nanodiamonds in the Hudson River show that an asteroid hit nearby at around the same time as the tsunami.

[Via University of Hawai'i and Livescience. Photo: A house being towed after the South Coast Disaster, via Newfie_penguin's Flickr]