By Prof. Aurelio Mercado-Irizarry
     Professor, Department of Marine Sciences
     Coastal Hazards Specialist, Sea Grant Program
     University of Puerto Rico
     Member of the International Tsunami Commission of the International Union of Geodesy and Geophysics

(December 29, 2004)

The 26th of December, 2004, Sumatra earthquake and accompanying tsunami have generated the justifiable concern of whether something like that can happen in the Caribbean, near Puerto Rico, and/or in the North Atlantic Ocean region as a whole. The available information classifies the earthquake as an approximately 9.0 magnitude event. Earthquakes of this magnitude are associated with seismic fault lengths on the order of 1000 kilometers in length which are associated with subduction zones, and the fact is that the subduction zone along the eastern Caribbean islands cannot accommodate such a fault length. Therefore, we can reliably assume that we in Puerto Rico have a very low probability of directly experiencing an event of such a magnitude, with events in the 7.5 magnitude range being a good candidate for a worst case scenario.

But, having said that, it is a historical fact (see, for example, http://nisee.berkeley.edu/lisbon/ ; Historical Depictions of the 1755 Lisbon Earthquake
http://en.wikipedia.org/wiki/1755_Lisbon_earthquake ;1755 Lisbon earthquake)
that an event of similar magnitude occurred in the North Atlantic basin as recently as the year 1755. Based on the damage and the tsunami it generated, its magnitude has been estimated as on the order of 9, similar to the Sumatra event. And the tsunami it generated seems to be on the same scale of magnitude as the recent Indian Ocean tsunami. As a matter of fact, the so-called Lisbon tsunami is the only tele-tsunami (implying it propagated more than 1000 kilometers) in recorded history in the North Atlantic basin. It not only affected Europe and northwestern Africa, but it crossed the Atlantic Ocean and impacted the Caribbean region about 7 - 8 hours later. A look at Jim Lander’s and Lowell S. Whiteside’s Table 1 on historical Caribbean tsunamis on this page (http://poseidon.uprm.edu, under Other Reports) describes the tsunami as follows:

“Waves of amplitude 7 m were observed at Saba, 3.6 m at Antigua and Dominica, 4.5 m at St. Martin, leaving a sloop anchored in 4.6 m of water was left laying broadside on the dry bottom, 1.5-1.8 m at Barbados, where the wave had a period of 5 minutes and the water was black as ink. This could be a local landslide tsunami or seiche triggered by the Lisbon wave. At Martinique, at some places the water was reported to have withdrawn for 1.6 km and at other places it flowed into the upper level rooms of the houses. The lowlands on most of the other French Islands were inundated. There is a report of Santiago de Cuba being nearly inundated in 1755 but the month and day were not given. This is probably from the Lisbon tsunami.”

In a paper describing a numerical simulation of the tsunami, Mader (“Modeling the 1755 Lisbon tsunami”, Science of Tsunami Hazards, Vol. 9, No. 2, 2001), also states a wave amplitude of 20 meters at Lisbon and along the African and south European coasts, and of 4 meters along the English coast. He also adds that the earthquake was felt over a million square miles and that it appeared to have at least two foci, one of which in North Africa caused ground motion that damaged cities 400 miles south of Lisbon.

There is no historical evidence of the Lisbon tsunami affecting Puerto Rico, nor the eastern seaboard of the USA. This is a topic of active research, but one can speculate that the eastern seaboard of the USA was spared because the fault orientation may have been approximately east-west, and it is known that most of the tsunami energy is directed perpendicular to the axis of the fault, this being so the longer the fault. As far the lack of impact on Puerto Rico, it may have been due to refraction effects while the wave passed over the Mid-Atlantic ridge and when it encountered the Puerto Rico Trench, the deepest part in the whole Atlantic basin (depths on the order of 8.5 kilometers). We hope to answer these questions in the near future.

Finally, at the March 2004 National Science Foundation Caribbean Tsunami Workshop, held in San Juan, PR, field evidence was presented of a field of huge boulders (some more than 100 tons in weight) scattered inland along the north and northeast facing shores of the southern Caribbean islands of Aruba, Bonaire, and Curacao. Since no hurricane is capable of raising and transporting these boulders, it has been speculated that the culprit must have been a very large pre-historical tsunami. Please see the abstract of the presentation at http://nsfctw.uprm.edu/agenda.html, under Anja Scheffers and Dieter Kelletat, Essen University (Germany) “New Evidence and Datings of Paleo-Tsunami Events in the Caribbean”. See also the following link: http://epubs.lanl.gov/tsunami/ts201.pdf, the article titled “Evidence of tsunamis on Curacao, Bonaire and Aruba, by Anja Scheffers A book with the full presentations of all of the abstracts will be forthcoming sometime in 2005.

Henceforth, although the tectonics of the North Atlantic Ocean region are different than that of the Indian Ocean, the historical fact is unquestionable in that 249 years ago the North Atlantic region underwent an event similar to the Sumatra event and paleo-tsunami evidence seems to show the same. And that the Caribbean Sea region is under the threat of history repeating itself, with catastrophic consequences due to the amount of population living in highly vulnerable coastal areas. And we should not forget the fact that we are also under the more probable threat of local tsunamis, like the ones occurring in 1867, 1918, and 1946. As a matter of fact, putting all of these dates together (1755, 1867, 1918, and 1946) nobody can argue that we should not worry about tsunamis in Puerto Rico and the Caribbean. And that, since it is likely that something will be done (after the fact) in the Indian Ocean to start some type of tsunami warning system, a far as historical facts are concerned, the people living in the Atlantic region also deserve the same concern.

So the answer to the title of this discussion is YES, it has already happened.

Next year the 250th anniversary of the Lisbon earthquake is going to be commemorated, and for further information please go to the following link:  http://www.meteo.pt/1755sismo/1755sismos.htm