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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS002 Oral Presentation 1755
Reconsideration of the tsunami source model of the 1755 Lisbon Tsunami
Dr. Angela Santos
Civil Engineering and Environmental Engineering Tohoku University IASPEI
Shunichi Koshimura, Fumihiko Imamura
The November 1st, 1755 Lisbon Earthquake, one of the most catastrophic events to have ever occurred
in Portugal, Spain and Morocco, caused severe damage and casualties. The tsunami generated by this
earthquake is well documented in the historical accounts, being reported throughout the Atlantic Ocean,
reaching not only Portugal, Spain and Morocco, but also the Madeira and Azores Archipelagos, England,
Ireland and the Caribbean. In spite of all the available information, the overall picture of this event
remains unknown. On the other hand, there are uncertainties regarding the tsunami travel time, since
some historical descriptions have ambiguous or unclear interpretation. One example is Cape St. Vincent,
where we considered a travel time between 6 and 18 minutes. In other places we did not have access
to the original documents, so we considered the travel time mentioned in previous studies. Since there
are discrepancies in those articles, we also considered a possible range for determination of tsunami
travel time. This occurred in: Huelva, 35-60 minutes; Porto Santo, 30-60 minutes; Figueira da Foz, 35-
55 minutes. Other historical accounts are very accurate and we considered the exact value mentioned:
Cadiz, 78 minutes; Funchal, 90 minutes; Safi, 30 minutes. The Gorringe Bank is the largest tectonic
structure around this area, and with 200 km by 80 km, is enough to generate an Mw=8.7 earthquake.
Johnston (1996) proposed possible source mechanisms with lengths ranging from 100 km to 500 km. In
this study we followed Johnston (1996) proposal and reconsidered the possible tsunami source model
for this event, taking account of the uncertainties of tsunami travel time suggested above. First,
considering the range of tsunami travel time obtained from the historical accounts, we determined the
spatial extent of tsunami source area by using wave ray analysis. Secondly, referring the fault
parameters proposed by Johnston (1996), we calculated the initial sea surface displacement by using
the theory of Okada (1985). Thirdly, we conducted the tsunami propagation model based on the nonlinear
shallow water theory. The model results were validated from the point of view of consistency with
the reported tsunami travel time. The good agreement between the model and historical accounts for
the travel time was obtained in some locations, leading us to conclude that the subsurface rupture
length should be between 150 km and 300 km. Therefore, Gorringe Bank area could be a potential
source for the 1755 tsunami and further study on that area should be carried out.
Keywords: 1755 lisbon tsunami, gorringe bank, numerical model