Geoscience Reference
In-Depth Information
Comarca de Valen¸a, Braga, northern Portugal in February 13, 1754 (
in
Biblioteca
da Ajuda, quoted by Amador 2007).
Around 11h:00 in the morning of November 1st, the waves of a tsunami caused
by the main shock of 9h:40 arrived in Lisbon. The Tagus waters initially run-down,
dragging the boats anchored near the harbour. Then, they started to increase its level,
passed over the walls of the port and invaded downtown in 300-400 yards (“Terreiro
do Pa¸o” and streets near the river banks). According to the testimony of the captain
of an English vessel, the waters raised about 16 feet, three times, during 15 min.
Only at 7h:00 in the morning of Sunday (November 2), the tide went back to normal.
However, as a result of the tsunami, during the first 10-12 days the tides did not
have a regular course, as some times they came earlier, other times were delayed,
and took 7-8 h to reach high tide and 3-4 h to reach low tide.
The impact of the tsunami of 1755 in Lisbon is described in various testimonies
of that time, such the one following (in Emergency Plan for the Seismic Risk in
Lisbon): “(
...
) Suddenly the sea enters the harbour with a furious inundation of
water (
); Surpassing it ancient limits, it passed over several buildings and flooded
S. Paulo quarter (
...
) and flood in parts with
its flow and reflux the side of the waters that came out of its river bed and flooded
the custom-house, the Terreiro square and the Vedoria building (
...
)” (Moreira de Mendon¸a 1758)
,
“(
...
)”. According
to Baptista
et al.
(1998) downtown was flooded, being the distance of penetration
of 250 m, while the “fernandine” wall (rebuilt by King Filipe I) acted as a strong
barrier to the passing waters. The area between the ancient “Ribeira das Naus”,
the “Terreiro do Pa¸o” and the “Jardim do Tabaco” - squares in the river banks -
became totally flooded”.
Another reference indicates that “the Castle of Bugio was almost covered with
water in such a way that the soldiers shooted asking for help and had to withdraw to
the highest part of the tower”.
It is important to know the tide level by the time of the tsunami wave arrival, to
take this effect into account in the progression of the waters entering Lisbon harbour.
It is difficult to analyse this subject with precision because of the error introduced in
the extrapolation of tide times into the past (two centuries before). However, accord-
ing to the US Naval Observatory Astronomical Applications Department, from the
phases of the moon, the 1st of November 1755 was two and a half days before New
Moon, far away from high tides. So, the low tide was probably around noon and
the high tide around 17h:45 (Azevedo 2004, personal communication). On the other
hand, according to historical source, the low tide in Benavente was at noon. Based
in this information, the tsunami would have arrived to the cowl of the Tagus when
the waters were in strong low tide, which turns the rising of the rivers more difficult.
Andrade (1992) confirms that the first wave of the tsunami reached the coast in
Algarve during low tide. This information is, however, in contradiction to witnesses
in Cadiz, which claim that the tsunami reached there at high tide (Campos-Romero
1989).
The tsunami was felt not only in the Portuguese coast (the harbour of Setubal
was submerged by an enormous wave; in the Algarve, the waves reached great
heights) but also in the Southeast of Spain, North of Africa, Great Britain and The
...
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