Subject: Tsunamis in the Atlantic Ocean

why are there no earthquake generated tsunamis located in the atlantic ocean?

    Most tsunamis are generated by shallow earthquakes in subduction zones, since those are the commonest earthquakes which distort the seafloor. The only subduction zones around the Atlantic are the Puerto Rico Trench and the Antilles subduction zone around the eastern Caribbean and the South Sandwich Trench south of South America. These subduction zones are both smaller and much less active the subduction zones that circle the Pacific, so the Atlantic has many fewer tsunamis. That doesn't mean that it doesn't have any! 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. Gorringe Bank remains a severe tsunami threat for Portugal, and the Portuguese are now installing seafloor pressure gauges there to get advance warning.
    By the way, I am pleased that you asked about "earthquake-generated" tsunamis. 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.)

Gerard Fryer
Hawaii Institute of Geophysics and Planetology
University of Hawaii, Honolulu HI 96822

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