THE GREAT LISBON EARTHQUAKE OF 1755
Lisbon, the capital of Portugal, on the Tagus River, is built along
both banks for five miles, and on several small neighboring hills. It
is supplied with water by means of an aqueduct, called the Alcantara,
which brings the water from springs about nine miles to the northwest.
For portions of its length the aqueduct is placed underground, but
where it crosses the deep valley of the Alcantara it is supported,
for a distance of 2,400 feet, by a number of marble arches, which
in one place are 260 feet in height. This fact is put forward not
merely for the sake of its artistic interest, but because, strange to
relate, this part of the aqueduct remained uninjured during that great
earthquake, the greatest of modern times.
On the 1st of November, 1755, this frightful catastrophe, according to
Lyell, from whose excellent account much of the information contained
in this chapter has been obtained, struck the beautiful city almost
without any warning. Terrible sounds came suddenly from underground;
almost instantly afterward a violent shock threw down the greater
portion of the city; in less than six minutes 60,000 people were
killed.
The place from which this earthquake started must have been situated
on the bed of the ocean at some distance from the coast; for the great
wave thus raised in the Atlantic Ocean did not reach the mouth of the
Tagus River until about half an hour after the most severe shocks
were over. The arrival of this wave at the mouth of the Tagus was
announced by the sea retiring to such an extent as to leave the bar
dry. Then a huge wave, sixty feet in height, rolled in from the ocean
and completed the work of destruction that had been commenced by the
earthquake.
So great was the shock that the mountains in the neighborhood were
violently shaken and some of them split or fractured in a most
wonderful manner.
Particularly large was the loss of life in the churches whither
hundreds hastened for refuge when the shakings of the earth began,
for most of these buildings fell and buried the worshippers. Another
immense loss of life was caused by the destruction of a large marble
quay or wharf that was suddenly swallowed up by the sea. While the
buildings in the city were being overthrown by the violent shakings
of the earth, a multitude sought the quay as a flat place where they
could not be injured by the falling buildings. Suddenly, however,
this structure sank into the water and not only were all the people
drowned, but none of the bodies was ever afterwards found.
Failure to find any remnants of the pier or any of the people who
perished on it has been attributed to the formation of eddies or
whirls that were sufficiently strong to carry down vessels by suction
similar to that of the famous maelstrom off the coast of Norway. Of
course, in a time of boundless excitement like that of the Lisbon
earthquake, accounts are apt to be highly exaggerated. For example,
assertions are made in many books that the water left in the harbor
after the sinking of the quay was unfathomable. Now, in point of fact,
the depth of this place has been measured and found to be less than
100 fathoms.
When it is remembered that not one of the bodies of the people on that
quay was ever again seen, it is possible, as Lyell suggests, that a
deep fissure or chasm opened immediately on the ground on which the
quay stood, so that it, together with all on it, were dropped into the
chasm, which, closing, buried them deep in the earth.
The Lisbon earthquake was especially noted for the extent of country
affected by it. Humboldt estimated this area as being more than four
times the size of Europe. In parts of this area immense mountain
ranges, such as the Pyrenees, Alps, etc., were violently shaken. When
the size of these mountains is considered one realizes that it must
have required a mighty force to shake them. These shakings were so
severe that they produced a deep fissure in the ground in France.
Continuing towards the north the solid earth was shaken as far as
the shores of the Baltic and Norway and Sweden, generally. This, of
course, included the flat country of Northern Germany. The hot springs
of Toplitz disappeared for a time, but, breaking out afterwards,
discharged such quantities of muddy water that the surrounding country
was inundated.
The waves crossed the Atlantic, causing high tides on the island of
Antigua, Barbadoes, and Martinique, of the Lesser Antilles, where,
instead of the usual tides of two feet, tides of twenty feet high were
observed. Further to the north the waves reached the eastern shores of
North America, and shook the continent as far west as the Great Lakes,
and spread in the North Atlantic as far as Iceland.
Toward the south the waves affected parts of northwestern Africa,
where much loss of life occurred in the villages some eight leagues
distant from the city of Morocco. Here from 8,000 to 10,000 people
were killed, being swallowed up by deep fissures in the earth, which
afterwards closed on their bodies.
Severe shocks were in many cases felt on vessels at sea. In one
instance, although the vessels were at considerable distances from
where the waves started, the captains reported that the shocks were so
great that on several occasions it was believed the vessel had struck
a rock, till, on heaving the lead, they found that they were in very
deep water. In another instance, such was the shock to the vessel that
the planks on the deck had their seams opened. In still another case
several of the sailors were thrown into the air for a distance of
about one and a half feet.
It has been frequently observed that when great earthquakes happen,
curious changes take place in the level of the waters of lakes
entirely disconnected with the ocean; for example, mountain lakes,
far above the level of the sea, the water suddenly rising and then
resuming its original level. Sometimes the waters of such lakes have
suddenly disappeared, probably being drained off through a fissure
formed in the bed of the lake. In such event the lake generally
remains dry after the passage of the earthquake.
At the time of the Lisbon earthquake it was observed that the water of
Loch Lomond in Scotland first rose above its ordinary, then sank again
to its usual level. This difference of level is explained by Lyell as
follows: when the earthquake waves reached the lake, the water being
unable to take the sudden shove given to it by the earthquake waves,
dashed over that side of the basin which first received the shock.
Assuming this to be the case, since the rise of the level in the water
of Loch Lomond was two feet and four inches, it is comparatively easy
to calculate the speed of movement that the earthquake waves had,
when they reached this body of water. Calculated in this way it would
seem that the waves had a speed of about twenty miles a minute.
But what especially characterized the Lisbon earthquake were the great
waves that were produced in the ocean. Besides the huge wave that
entered the Tagus, a wave of the same height swept eastward along the
southern coast of Spain, and the northwestern coast of Africa. At
Tangier in Africa it swept the coast as a very high wave no less than
eighteen times, or, in other words, eighteen huge waves rolled in from
the ocean. At Funchal, on the Madeira Islands, this wave rose fifteen
feet above the high water mark.
Many attempts have been made to explain the manner in which the great
sea waves are started in earthquake movements. Some believe that they
are due to the sudden raising or heaving up of the water, far above
ordinary level. But, as Lyell points out, this explanation would not
be satisfactory for the waves produced in the case of the Lisbon
earthquake, since it would fail to account for the fact that both on
the coasts of Portugal as well as on the island of Madeira the high
wave was preceded by a movement of the water toward the point of
origin; that is, the waters moved away from Lisbon and the Madeira
Islands, so as to leave the water very low at those points, when
shortly afterwards a huge wave rushed in from the sea and swept over
the land.
Earthquake waves move much more rapidly through the solid rocks of
the earth's crust than through the waters of the ocean. The shock
transmitted through the solid earth from Lisbon to the Madeira Islands
took only twenty-five minutes to reach the islands, while the waves in
the ocean took about two and a half hours to cover the same distance.