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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