SOME OF THE PHENOMENA OF EARTHQUAKES
The nature of an earthquake and the movements of its waves from
their starting place having now been briefly described, it remains
to explain some of the strange phenomena that precede, accompany, or
follow one.
Next to the violent shaking of the earth's crust, perhaps the most
wonderful and impressive thing is the great variety of sounds and
noises. These occur not only while the earth-waves are passing through
the crust at any place, but also long before the principal shocks
reach the place, as well as long after they have passed.
Earthquake sounds vary almost infinitely, both in intensity and
character. Some are like the gentle sighings of the wind, or resemble
faint mysterious whisperings; some are not unlike the confused
murmurings of a crowded room; some resemble the sounds of a busy
street. Some sounds are full and strong, like the deep bass notes
of a large organ. Others resemble the din of a great battle with
the reports of the large guns. Still others reach the intensity of
continuous peals of thunder. But we can better understand the nature
of earthquake sounds from an actual description of them in a number of
great earthquakes, and by inquiring at the same time into any of the
peculiar facts connected.
Humboldt in his great work, "Cosmos," thus describes the varied voice
of the earthquake:
"It is either rolling or rustling, or clanking, like
chains being moved, or like near thunder, or clear and
ringing, as if obsidian or some other vitrified masses were
struck in subterranean cavities."
That the sounds produced during earthquakes are carried through the
ground faster than through the air appears clear from the fact that
such sounds are sometimes heard in deep mines when they are not at all
heard on the earth's surface.
In describing the earthquake that occurred in Kamtschatka, in 1759,
Krashenikoff of St. Petersburg states that noises were heard like the
rushing of a strong underground wind, accompanied by a hissing sound,
which resembled the sizzlings heard when red hot coals are thrown in
water.
In an earthquake that occurred in Lincolnshire, England, February
6th, 1817, a noise was heard closely resembling the sounds of wagons
running away on a road. So complete and convincing was the resemblance
that several wagoners on one of the roads drew their teams to one side
so as to permit the runaway to pass safely.
Another kind of noise heard during earthquakes is a loud hollow
bellowing. Sometimes, however, the sounds are more musical in their
nature, being not unlike those produced by a very large organ pipe. At
other times they resemble the noises produced when steam is blown into
cold water.
The following account of earthquake sounds is given by Daubeny,
in his book on volcanoes. It appears that during March, 1822, the
people living on the island of Melida, opposite Ragusa, in Dalmatia,
were greatly alarmed by sounds that at first they believed due to
cannonading either at sea or on the neighboring coast. They afterwards
found that these sounds were due to something that was taking place
under the ground. The noises continued at intervals until August
23d, 1823, when a great earthquake occurred, during which one of the
highest mountains on the island was cleft or split in one place. The
underground noises continued from time to time and so frightened
the people that they were about to leave the island permanently and
emigrate to the mainland of Dalmatia. They were dissuaded from doing
so by the government, and while the noises continued at intervals it
so happened that no damage came to them. It is said, however, that
twenty years after an active volcano broke out on the island.
There are various causes that produce earthquake sounds. A very slight
rubbing or grinding together of rock surfaces may produce fairly
loud noises, the volume of the sound being increased by transmission
through the rock masses that lie in the path of the waves. An example
of such an increase in the loudness of sounds is seen in the case of
several of the large blocks of stone used for some of the piers of
Kingston Harbor, in Ireland. When these rocks are moved together by
blows of the waves they produce loud and appalling sounds, as if the
whole island were being washed away. The same rocks, however, when
left high and dry on the falling of the tide, can be caused to rub
together, when moved by the hand. Under these circumstances, they
produce but feeble sounds that can only be heard in their immediate
neighborhood.
No doubt, some find it difficult to understand how it is possible for
comparatively feeble sound-waves to be strengthened by their passage
through large masses of solids. This is important and should be made
clear. As everyone well knows, the ticking of a watch can only be
heard at a short distance when the watch is held in the hand, because
the sound-waves cannot readily pass through the body of the person
holding the watch to the earth, the materials of the body not being
sufficiently elastic. If, however, the watch be placed on the bare
surface of a large wooden table from which the tablecloth has been
removed, so that the watch can come directly in contact with the wood,
and nothing else is placed on the table but the watch, the sound-waves
are transmitted to the mass of the table and its entire surface sends
them out into the air. The ticking of the watch can then be heard
distinctly in almost any part of a large room.
Mallet states that in nearly all great earthquakes sounds are heard
before the principal shock, and in his description of the Calabrian
earthquake Hamilton says:
"All agreed that every shock seemed to come with a rumbling
noise from the westward, beginning with the horizontal and
ending with the vorticose (rotary) motion."
According to Dolomieu, during the Lisbon earthquake, the shocks were
preceded "by a loud subterranean noise like thunder, which was renewed
for every shock.... This great shock," he says, referring to one of
the great upward shocks, "occurred without the prelude of any slight
shocks, without any notice whatever as suddenly as the blowing up of
a mine.... Some, however, pretend that a muffled interior noise was
heard almost at the same moment."
The noises do not generally continue long after the earthquake shocks.
In some cases, however, a very loud noise is heard at intervals for a
considerable length of time after the principal shock. This was the
case at Quito and Ibarra, in which a great noise was heard for from
eighteen to twenty minutes after the principal shock. In a similar
manner during the earthquake of October, 1746, at Lima, and Callao,
South America, peals of underground thunder were heard at Truxillo
for fifteen minutes after the principal shock. In such cases it
seems probable that the noises were not caused by the same impulses
that caused the original shock, but by the forces that caused the
subsequent shock.
Humboldt relates that in 1784 there were noises heard at Guanajuato,
from the 9th to the 12th of February. They were not, however, followed
by an earthquake.
Humboldt also states that in an earthquake which occurred on the 30th
of April, 1812, on the banks of the Orinoco River, in South America,
a loud thundering noise was heard, without, however, any shock, but
at this time a volcano on the island of St. Vincent, in the Lesser
Antilles, although some 632 miles to the northeast, was pouring out
streams of lava. Again in the great eruption of Cotopaxi, in 1734,
underground noises were heard as if cannon were being fired. These
sounds were distinctly heard at as great a distance as Honda on the
banks of the Magdalena River. Now, bearing in mind that the crater of
Cotopaxi is situated on the high plateau of Quito, in a region full
of valleys and fissures, it would seem that for the sounds to have
been sent through the 436 miles between the mountains and the valley
of the Magdalena River, the waves must, for the greater part, have
been transmitted through the solid earth at some considerable distance
below the surface.
Mallet states that the underground noises which continued for more
than a month from the midnight of January 9th, 1784, at Guanajuato,
were not followed by any earthquake shocks, that it was if as thunder
clouds occupied the space below the surface at that part of the earth
and from these clouds there came the slow rolling sounds like short,
quick, snaps of thunder.
Major Dutton in his book entitled "Earthquakes in the Light of the
New Seismology" gives the following as the principal signs that herald
the coming earthquake in the open country.
"The first sensation is the sound. It is wholly unlike
anything we have ever heard before, unless we have already
had a similar experience. It is a strange murmur. Some
liken it to the sighing of pine-trees in the wind, or to
falling rain; others to the distant roar of the surf;
others to the far-off rumble of the railway train; others
to distant thunder. It grows louder. The earth begins to
quiver, then to shake rudely. Soon the ground begins to
heave. Then it is actually seen to be traversed by visible
waves somewhat likes waves at sea, but of less height and
moving much more swiftly. The sound becomes a roar. It is
difficult to stand, and at length it becomes impossible to
do so. The victim flings himself to the ground to avoid
being dashed to it, or he clings to a convenient sapling,
or fence-post, to avoid being overthrown. The trees are
seen to sway sometimes through large arcs, and are said,
doubtless with exaggeration, to touch the ground with
their branches, first on one side, then on the other. As
the waves rush past, the ground on the crests opens in
cracks which close again in the troughs. As they close, the
squeezed-out air blows forth sand and gravel, and sometimes
sand and water are spurted high in air. The roar becomes
appalling. Through its din are heard loud, deep, solemn
booms that seem like the voice of the Eternal One, speaking
out of the depths of the universe. Suddenly this storm
subsides, the earth comes speedily to rest and all is over."
There are many other curious phenomena besides earthquake sounds or
noises. Among some of the more interesting are the fire and smoke that
are seen to come out of fissures that have been rent in the ground.
It is possible that in many cases these flashes of fire are in reality
produced by electric discharges that momentarily light the clouds of
dust thrown up out of the fissure. But sometimes true flames are seen
escaping from the fissures. This was the case during the earthquake of
Lisbon, in 1755, when fire burst through fissures at several places,
burning with a lambent flame for some hours.
The clouds of dust that follow the rending of mountain masses by
earthquakes are probably to be traced to the fracture of the rock
masses, the dust so formed being violently thrown forth by the air
squeezed out of the fissures, when they are suddenly closed. The
violent compression of this air may raise this dust to incandescence.
Mallet asserts that in many cases the clouds of smoke observed do
not consist of true smoke like that produced when wood or vegetable
matters are incompletely burned, but is only ordinary air mixed with
sulphurous acid gas, and various other gases.
But not only fire and smoke are seen at times coming out of fissures
in the earth. A thing still more frequently thrown out is water, which
often spouts forth along with great quantities of mud, sand, and the
finely ground fragments of earthy materials generally. Among many
other instances where the emission of water from the crevices was
particularly noticeable, may be mentioned the earthquakes at Jamaica
in 1687 and 1692. Here the water, in some places, was thrown out of
the ground to considerable heights in the air.
Mallet calls attention to the fact that the waters of springs collect
in reservoirs consisting either of fissures or crevices of the rocks,
of small width but great depth, which are vertical or inclined to the
horizon, or in reservoirs that are formed of extended beds of sand or
gravel.
Now, when the earthquake waves moving horizontally over the surface
produce movements that squeeze these fissures together, the water in
the fissures is spurted out in high jets, and carries with it the
finely divided rock or sand formed by the rubbing together of the rock
surfaces. In the case of the reservoirs consisting of beds of sand
or gravel, lying between impervious layers, if, during an earthquake
motion, the land areas are suddenly lowered, the water rushing into
the cavity thus left will afterwards be shot out with considerable
force, when the land is suddenly raised again.
Where there are no direct openings in the ground the water will burst
through the crust in the shape of great vertical jets, thus forming a
circular hole, broken or fractured at its edges. Water jets of this
character were especially numerous during the earthquake of Calabria
in 1783. In a swampy plain, known as Rosarno, many of these circular
wells or openings about the size of an ordinary carriage wheel, though
in some cases much larger, were to be seen crowded together. The
appearance of the openings are represented in Fig. 40.
Some of these were filled with water, but the greater number were dry
and filled with loose sand. These latter, when examined by digging,
were shown to be funnel-shaped, as seen in Fig. 41. As seen, the
margins of the wells exhibit a series of cracks or crevices extending
radially outward from the centre. Their origin is evident. As the
water was violently expelled by the squeezing motion of the upper
and lower impervious strata, it shot upwards, thus producing the
funnel-shaped tube. At the same time the force of the eruption was
sufficiently great to produce the radial fissures or fractures at the
sides.
[Illustration: FIG. 40. CIRCULAR HOLLOW FORMED BY CALABRIAN
EARTHQUAKE]
[Illustration: FIG. 41. SECTION OF CIRCULAR HOLLOW FORMED BY
CALABRIAN EARTHQUAKE]
But greater fissures than these have been formed by earthquakes,
especially those of the class created by a slipping of the earth's
strata. In the case of an earthquake on the South Island of New
Zealand, in 1848, a fissure having an average width of eighteen
inches could be clearly seen extending in a direction parallel to
the mountain chain for a distance of sixty miles, and during a later
earthquake in the same region, in 1855, a fracture was formed that
could be clearly traced for a distance of nearly ninety miles.
In some cases these fissures or fractured parts of the crust are left
with one of their sides at a higher level than the opposite side. This
was the case of the great Japanese earthquake of October 28th, 1891.
There are three kinds of waves produced by earthquakes; namely, the
earthquake waves proper through the earth; the sound waves in the air,
and great forced waves in the sea.
The sound waves of course reach the air from the point of origin below
the earth's surface through the solid materials of the crust, and take
on the curious varieties already described in connection with the
sounds accompanying earthquakes.
We have already briefly described the manner in which the earthquake
waves travel through the materials of the earth's crust. There remain
to be discussed the great waves that are rolled up in the ocean
during an earthquake shock. These waves are, perhaps, among the most
destructive phenomena of great earthquakes. The following are only
some of the more remarkable of such waves, and have been taken from
Mallet's collection of earthquake data.
During some of the great earthquakes on the coasts of Chile and Peru,
huge waves from the ocean did great damage when they reached the land.
In the earthquake of 1590, ocean waves rushed for several leagues
inland over the coast of Chile, carrying with them ships that were
left high and dry as the wave receded. In the earthquake of 1687,
Callao was inundated by a great wave from the Pacific Ocean, and ships
were carried a full league into the country. During the earthquake of
1746, Callao was again swept away by a huge ocean wave. At later times
earthquake waves have caused great damage to several other parts of
the coast of South America.
Ocean waves of this character are formed by successive upward and
downward movements at the bottom of the ocean, following each other at
very brief intervals. Le Conte points out that the sudden upheaval of
the bed of the ocean forms a huge mound in the surface of the water
which results in a large wave that spreads rapidly in all directions.
Waves produced in this manner sometimes reach a height of fifty to
sixty feet. They are not readily observed in the deep ocean, but
as soon as they reach the shallow waters near the shore they rush
forward, forming waves from fifty to sixty feet in height and, rushing
over the land, sweep everything before them.
During the great Lisbon earthquake of 1755 a huge wave started at a
point fifty miles off the coast of Portugal. Half an hour after the
earthquake was over several waves, the largest of which was sixty
feet in height, rushed over a part of the city and greatly increased
the ruin already wrought by the earthquake. According to Le Conte the
great waves so formed moved in all directions across the Atlantic
Ocean. They were thirty feet high when they reached Cadiz, eighteen
feet in height at Madeira, and five feet on the coast of Ireland. They
even crossed the Atlantic, being observed on the coasts of the West
Indies.
A great ocean wave accompanied the Japanese earthquake in 1854. As in
the case of the Lisbon earthquake this wave started in the bed of the
ocean off the coast of Japan and only reached the island half an hour
afterwards. It was thirty feet in height, and completely swept away
the town of Simoda.
Owing to water's greater freedom of motion earthquake waves travel
greater distances through the water than they do on land.
Of course, great earthquake shocks as a rule cause a very large loss
of life. The following figures from Mallet give some idea of the
extent of this loss, which is generally a matter of a few moments.
In the Lisbon earthquake, where the worst shock lasted a few seconds,
60,000 people were killed. During other earthquakes the losses have
been as follows: 10,000 at Morocco; 40,000 in Calabria; 50,000 in
Syria, and probably 120,000 in earthquakes that occurred in Syria in
A. D. 19 and in A. D. 526.
But even these figures give only a meagre idea of the vast loss of
life that has occurred during the past. It is said that during the
reign of Justinian, earthquakes repeatedly shook the whole Roman
world. The city of Constantinople was visited by earthquake shocks
that continued at intervals for forty days. Deep chasms were opened in
the earth and huge masses were thrown into the air. Enormous sea-waves
were formed. At Antioch, during the earthquake of May 20th, A.
D. 526, 250,000 people are believed to have been killed.
On the 31st of July, A. D. 365, in the second year of
Valentinian, a dreadful earthquake shook the Roman world, and a great
wave rolled in from the Mediterranean and swept two miles inland,
carrying ships over the tops of houses. During this earthquake 50,000
people lost their lives at Alexandria.
In the earthquake of Messina in 1692, 74,000 people are said to have
been killed; and, according to other accounts, 100,000. In the year
A. D. 602, another earthquake at Antioch killed 60,000 people.
During the earthquake of Quito, in 1797, Humboldt estimates that
40,000 natives were either buried in crevices in the earth, under
the ruins of buildings, or were drowned in lakes and ponds that were
temporarily formed.
In this connection Mallet writes as follows:
"Such are the numbers to be met with in narratives, and
if we suppose that there occurs one great earthquake in
three years over the whole earth and that this involves the
entombment of only 10,000 human beings, and that such has
been the economy of our system for the last 4,000 years, we
shall have a number representing above 13,000,000 men thus
suddenly swallowed up, with countless bodies of animals of
every lower class. Sir Charles Lyell then with good reason
suggests that even in our own time we may yet find the
remains of men and of their habitations and implements thus
buried deep and embalmed, as it were, by earthquakes that
occurred in the days of Moses and the Ptolemies."
Necessarily the progress of a great earthquake wave will produce great
changes in the earth's surface features; for example, landslides,
where immense layers of clay or other material slip or slide to a
lower level and are thrown across the course of a river, causing its
waters to be dammed up and then by spreading to form great lakes.
Sometimes, after vast bodies of water have been collected in this
manner, disastrous floods result later from a sudden giving way of the
barrier, and the loss thus caused is occasionally far greater than
that directly due to the earthquake.
Permanent changes of level are frequently caused by earthquakes, as,
for example, the coast of Chile during the earthquake of November
19th, 1822, where the coast for many miles was raised from three to
four feet above its former plane.
In other cases the level of the ground is permanently lowered. This
occurred in the Bengal earthquake in 1762, when an area of some sixty
square miles suddenly sank, leaving only the tops of the higher points
above water.
In some cases of changes in the level of the ground, large areas being
raised in one place and lowered in another, rivers take new courses,
and their old courses are completely obliterated.