Geoscience Reference
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(the origin time of the earthquake is a matter of some controversy, with variations
between 9h:30 and 10h:00, with a better possibility between 9h:35 and 9h:45) with
essentially three phases, and preceded by an underground snore or simultaneous to
an “underground boom that lasted the time of the vibration sounding like a far away
thunder”. The phases and respective durations vary from place to place, according to
testimonies from the entire Iberian Peninsula. In Lisbon, the first phase, with dura-
tion of about one and a half minutes, not very violent, was followed, after a period of
1 min, by a more intense movement with duration of two and a half minutes causing
serious damages. After another pause of one minute, there was a third phase, with a
duration of 3 min, more violent that before. The earthquake lasted for about 9 min.
The vibrations of the first phase were essentially vertical and of higher frequency
than the others.
Duration of motion in other locations will be discussed later, Section 3.2.2. It
is difficult to establish the direction of the movement: some say it was N-S, oth-
ers, possibly in other places, indicate E-W. However, the preferential direction of
the movement of SW-NE deserves some consideration, according to the fact that
the downtown streets with that orientation did not suffer great damage, as houses
give better support to each other in the direction of the streets axes, functioning as
aggregates.
To support this interpretation two other facts should be mentioned: the recon-
struction of the new City of Lisbon developed the streets with alignment of the
longer axis of building blocks in the N-S direction; the small damage inflicted to the
“Aqueduto das Aguas Livres”, as will be discussed in Section 5.1 may be not only
due its good construction but also due to the predominant N-S direction of waves.
Some eyewitnesses refer that houses were wagged like carriages going in high
speed on a street full of stones. Romulo de Carvalho (1987) goes a bit far in his
description stating that the movement in Lisbon starts with a “slow shake but in-
creasing intensity. The walls of the buildings start to crack, to open crevices and
soon collapse, falling on people running away through the streets.”
“The stones
of the temples vaults where Catholic mass was being prayed, the columns of the
altars, the surrounding walls, etc. fall violently on people, raising dust clouds that
suffocate the few survivors”.
Besides the damages caused by the seismic movement (partial or total collapse
of the buildings), a great fire, caused by the several fires that exploded downtown,
burned during 6 days, increasing substantially the number of deaths and the material
damages.
During the first 24 h, the earth trembled in an almost continuous shaking. The
first aftershock, rather violent but of shorter duration, was felt around 11h:00. Dur-
ing the first 8 days, more than 28 aftershocks were felt, 250 aftershocks during
the first 6 months and 500 aftershocks until September 1756 (Table 2). The main
shocks were: 8/11/1755 at 5h:30, 15/11/1755 at 5h:00, 16/11/1755 at 3h:30 (with
tsunami), 18/11/1755 early in the morning, 8/12/1755 by the end of the morning,
11/12/1755 at 4h:55 and on the 21/12/1755 at 9h:00 with two shocks of 1 min
each. On 31/03/1761 another important earthquake was felt in Lisbon, an off-shore
earthquake of the Portuguese coast, causing seiches and a visible tsunami.
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