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the history caused rhythmic perturbations in the water at such large distances in a
certain predominant direction of propagation (to the north). It is important to refer
that only very large magnitude events, such as the Alaska 1964 Mw9.2, have caused
phenomena of the same type denoting important energy at very short frequencies
(
0.02 Hz).
Besides the damages caused in the area of Lisbon, which will be object of a
detailed study, the earthquake was strongly felt in the south of the country. Faro was
totally destroyed with a high number of death people. The same happened in other
villages in the Algarve, mainly in the western part, like Lagos, Portimao, etc. The
south of Spain also suffered damages but not so catastrophic.
To the North of Lisbon, the intensity was quickly attenuated; Alenquer, Torres
Vedras and Obidos were the most damaged places. Coimbra did not feel great
damage
2
. However, in Corunha (Spain), 800 km away from the epicentre, some
high chimneys felt down, although the population did not feel the movement. In
Barcelona, 1,200 km away from the epicentral region, there are records of oscillation
of lamps hanging from the ceilings of the churches. This information is extremely
important in order to characterize the frequency of the vibrations at such large dis-
tances. A simple calculation shows that the seismic movement at the ground level at
these sites may have reached a few centimetres for frequencies of 0.1-0.2 Hz.
The earthquake was slightly felt in the Azores Islands and curiously one of the
few earthquakes felt in the islands of Flores and Corvo. This information confirms
the idea that the 1755 earthquake had a radius of perceptibility of the movement of
about 2,500 km.
Let's see other quotations of phenomena that should be referred:
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-
“On the ground, cracks were opened from where sulphuric gases came out, some
closed almost immediately, others remain”,
-
“Light effects were seen like rays coming out of the ground”.
The phenomena of leakage of sulphuric gases, accounted for in different loca-
tions, are difficult to associate to the opening of cracks caused by the passage of
seismic waves on locations where geothermal springs (“Hot Baths of Sao Paulo”
in Lisbon) already existed due to the large distances separating them from the seis-
mic source. However, we understand that the passage of waves may somehow have
instabilized a few areas already prone to these effects.
3.1 Isoseismals of the Earthquake
The way that the earthquake was felt in all the Iberian Peninsula and north of Africa
has been the theme of many works published during the XXth century, among which
2
In a document recently discovered, Calvario (1755-1764) confirms most of the information in
Table 2 and adds a group of earthquakes felt in Coimbra such as 6/03/1756-6h; 11/03/1756-22-23h;
25/03/1756-2h).
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