<h2 class="nobreak" id="CHAPTER_XIII">CHAPTER XIII</h2>
<h3>EARTHQUAKES IN GEOLOGY</h3></div>
<p>It is more than likely that earthquakes in the geological
past were very much more violent, widespread,
and frequent than they are now; and they may
have had a more potent effect in overturning the rocks
of the earth. Even now their effects and the circumstances
which accompany them are tremendous and terrifying.
When the great earthquake comes, says Major
Edward Dutton in his book on <i>Earthquakes</i>, it comes
quickly and is quickly gone. Its duration is generally
a matter of seconds rather than of minutes, though instances
have been known in which it lasted from three
to four minutes. Perhaps forty-five seconds would be
a fair average. The first sensation is a confused murmuring
sound of a strange and even weird character.
Almost simultaneously loose objects begin to tremble
and clatter. Sometimes almost in an instant, sometimes
more gradually, but always quickly, the sound
becomes a roar, the clattering becomes a crashing. The
rapid quiver grows into a rude violent shaking of increasing
amplitude. Everything beneath seems beaten
with rapid blows of measureless power; loose objects
<span class="pagenum" id="Page_138">-138-</span>
begin to fly about; those that are lightly hung break
from their fastenings. The shaking increases in violence.
The floor begins to heave and rock like a boat
on the waves. Plaster ceilings fall, the walls crack, the
chimneys go crashing down, everything moves, heaves,
tosses. Huge waves seem to rush under the foundations
as if driven by a gale. The swing now becomes
longer and still more powerful. The walls crack open.
A sudden lurch throws out the front wall into the street,
or tears off or shakes down in rubble the whole corner
of the building. Then comes a longer swaying motion,
not unlike a ship at sea, but more rapid; not alone from
side to side, but forward and backward as well, and both
motions combined with a wriggle which it seems impossible
for anything to withstand. It is this compound,
figure-8 motion which is so destructive, rending asunder
the strongest structures as if they were built of clay. It
is the culmination of the quake. It settles into a more
regular and less violent swing; then suddenly abates and
ceases.</p>
<p>Out in the open country the signs and portents are of
a different character. The first intimation is a strange
sound. Some have likened 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. 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—something like waves at sea, but of less height
and moving much more swiftly. The sound becomes a
<span class="pagenum" id="Page_139">-139-</span>
roar. It is difficult to stand, and at length becomes impossible
to do so. People fling themselves to the ground
to avoid being dashed against it. The trees are seen to
sway violently, sometimes so much that they touch the
ground with their branches.... As the waves rush past
the ground opens in cracks and closes again. As the
cracks close the squeezed-out air blows out 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 some higher Power speaking out of the depths
of the universe. Suddenly the storm subsides, the earth
comes speedily to rest, and all is over.</p>
<p>And yet, says Major Dutton, this description suggests
but a single instance, or a few instances, of what earthquakes
are like. In some the full vigour of the shock
comes with explosive suddenness. People find themselves
suddenly thrown to the earth, the ground swept from
under their feet. Sometimes the rolling waves of earth
are absent, and the movement is a rude quiver, rapidly
vibrating in every direction—twisting, contorting, wrenching
the ground as if in a determined effort to shake it
into dust. Sometimes the most pronounced motion is up
and down, as if the earth beneath were being hammered
with giant strokes. Sometimes the growth, the climax,
the dying out of the earthquake movement are repeated
before the first shocks have ceased, or a few minutes afterwards,
or even with an interval of several hours. The
last-named case is, however, uncommon, though after the
first shocks of a great earthquake there are minor shocks
<span class="pagenum" id="Page_140">-140-</span>
and tremblings for days, weeks, or months afterwards.
Some of these are of considerable force, though they do
not inflict the devastation of the principal earthquake.
The greatest earthquakes are not always those which
wreak the largest amount of destruction. Evidently an
earthquake, the centre of which is situated near a great
city, is more appalling in its effects than one which takes
place in some desert place like the steppes of Siberia.
Recent earthquakes in Italy and near San Francisco were
regarded as great earthquakes because they took place in
thickly populated neighbourhoods. In cities, writes Professor
W. H. Hobbs, to the rumbling of the earthquake
is quickly added the crash of falling masonry, and to this
succeeds an uncanny grey darkness as the air becomes
filled with the dust from broken bricks, mortar, and
plaster.... In places the ground opens and swallows
the objects which lie upon it. Ponds are sucked down
and disappear, and great fountains of water gush out
and flood the country. During the New Madrid earthquakes
of 1811 and 1812 water was shot upwards in
vertical sheets and carried to the tops of the highest
trees. Near Lake Baikal, during the earthquake on
January 26th, 1862, the surface of the steppe, over two
hundred square miles, was suddenly dropped; fountains
opened at many parts within the sunken area, and water
shot up to heights of twenty feet. The water gushed
also in great volume from the open wells, and where these
were tightly covered by wooden caps, their lids were shot
up into the air like corks from champagne bottles. On
the night of September 5th, 1896, during heavy earthquake
shocks in Iceland, a new warm spring suddenly
opened to the accompaniment of loud roaring and
whistling, and threw water, steam, and fragments of rock
to a height estimated at six hundred feet. The force of
the new geyser was, however, soon spent, and ten days
later it ceased to flow. Nearly all the Icelandic geysers
suffered changes during this earthquake, and the famous
<i>Strokkur</i>, which had been born during the earthquake
of 1789, suddenly ceased its eruption and came to
an end.</p>
<div class="figcenter"> <ANTIMG class="w100" src="images/fpage140.png" width-obs="452" height-obs="658" alt="" /> <div class="txtlf"><i>Stereo Copyright, Underwood & U.</i></div>
<div class="txtrt"><i>London and New York</i></div>
<div class="figcaption" style="clear: both; padding-top:1em;">
<p class="tdc"><span class="smcap">A Geyser in Action</span></p>
<p>In geysers the suggestion that the fountains of steam and hot water
originate by the contact of water with hot rock is irresistible.</p>
</div>
</div>
<p><span class="pagenum" id="Page_141">-141-</span></p>
<p>In steep-walled mountain valleys earthquakes nearly
always cause landslips, and these may completely block
the course of a river. The lake formed in this manner
during the great earthquake of January 25th, 1348, in
Carinthia destroyed no fewer than seventeen villages, and
to-day, nearly six centuries afterwards, the area is a great
marsh. After the earthquake near Lake Baikal in Siberia,
of which we have spoken and in which the ground sank,
the sunken area was soon after invaded by the waters of
the lake. Sometimes when the earthquake takes place
near the mouth of a great river, the channels of the
streams are changed. After the Californian earthquake
of 1857 the current of the River Kern was turned upstream;
and the San Gabriel River left its bed to follow
a new course offered to it by an earthquake fissure. After
the Japanese earthquake of 1891 a former lake was cut in
half by one of the earthquake displacements, and one half
of the lake was left high and dry. Near Flagstaff, Arizona,
there is an old earthquake crack along which the waters
of several rivers which intersect it all disappear down the
<span class="pagenum" id="Page_142">-142-</span>
crevice. The most remarkable revelation of the process
of lake draining during earthquake shocks was furnished,
however, according to Professor Hobbs, by the former
Lake Eulalie near New Madrid. After the shocks of
1812 the lake completely disappeared. On the lake
bottom thus exposed there was revealed a series of fissures
down the funnel-shaped openings which the waters had
disappeared.</p>
<p>It will be seen from many of the foregoing instances
that whatever are the principal causes of earthquakes,
they must have played a great part in the shaping of
events in the geological past; and the only limitation
which we can place in the importance of the part they
played will depend on whether we regard the earthquake
as having been caused by a movement of the
underlying strata, or whether we believe that the same
cause which produces earthquakes may produce alterations
in the lie of the strata themselves. In the next chapter
we shall describe some of the effects produced on land
by earthquakes. But impressive as some of these effects
are, it is by no means certain that the greatest earthquakes
take place on land at all. They may take place
at sea, deep underneath the ocean. Our opportunities for
observing such quakes, however, are much smaller than
are afforded by land earthquakes. The instruments
which have been devised for observing earth tremors will
measure the smallest of such disturbances, and will
record earthquakes the centres of which are thousands of
miles away. These delicate instruments often record
distant earthquakes, the exact locality of which is never
<span class="pagenum" id="Page_143">-143-</span>
determined. No doubt, some of these distant tremors
originate in the ocean bed; but the seaquake can only be
localised when the water is put into a state of vibration
sufficiently energetic to rock the ship and its loose objects
and thus affect the senses. Vast waves are sometimes
rolled in on the shores of continents, and are undoubtedly
caused by some great disturbance beneath the ocean.</p>
<p>Such waves have been known through a long period
of history in the Eastern Mediterranean, where they have
ravaged the shores of Syria and Asia Minor; and it is
sometimes supposed that the great deluge on which the
Ark of Noah floated was accompanied by a mighty sea-wave,
rolled in upon the lands of Chaldea from the Persian
Gulf. Off the Pacific coast of South America these waves
arise most often and most mightily. They have been
especially formidable in the angle where the coast of
Peru meets that of Chili, and the harbours of Pisco,
Arica, Tacna, Iquique, and Pisagua have been repeatedly
subjected to these destructive invasions. Usually they
are foreshadowed by a violent earthquake, and the inhabitants,
taking warning, fly to the hills. The sea-wave
does not, however, always follow the earthquake,
but it appears often enough to arouse serious fear that
it may come whenever the ground is strongly shaken.
The first sign of the coming disaster is the withdrawal
of the sea from the shore, leaving bare the bed of the
harbour. A few minutes later the sea returns in a
high, irresistible wave, which overflows the adjoining
lands. Again it withdraws and again returns, and
these oscillations may last for many hours, slowly
<span class="pagenum" id="Page_144">-144-</span>
diminishing in the amount of rise and fall till they
die out.</p>
<p>The most memorable seaquake of the Chilian coast
was that of August 13th, 1868, when the coast of South
America was shaken from Ecuador to Valdivia. In
the town of Arica most of the buildings were thrown
down. A few minutes later the sea began to retire
slowly from the shore, so that ships anchored in seven
fathoms of water were left high and dry. Then the
sea returned like a great wall of water, which caught
up the ships in the roadstead and swept them inland
like chips of wood. Among them was the United States
war vessel <i>Wateree</i>, which was carried inland nearly
half a mile and was left, little injured, on dry land
when again the wave receded. The wave of this catastrophe
was felt far away from Chili. It was perceived
on the coasts of Australasia, Japan, Kamchatka, Alaska,
and California. In the harbour of Hakodate, in Japan,
a series of waves was registered on the tide-gauge. The
ordinary tide in that port is only about two and a half
to three feet. On this occasion the water rose and fell
a height of ten feet in twenty minutes. It had taken
the first wave twenty-five hours to travel the distance
of 7600 miles from South America. On May 9th, 1877,
another seaquake almost as great as this was felt in
many of the same places. This was on the occasion
of the Iquique earthquake. At Arica the stranded hulk
of the <i>Wateree</i>, which had remained high and dry for
nine years, was picked up and swept farther inland.
Like its predecessor, the wave was felt all over the
<span class="pagenum" id="Page_145">-145-</span>
Pacific. At Samoa the height of the waves varied from
six to twelve feet; in New Zealand from three to twenty
feet; in Japan from five to ten feet.</p>
<p>When a wave reaches shallow water it piles itself up
to a height, as any one knows who has watched the
waves coming in on the sea-shore, so that the height
of a wave measured on the tide-gauge of a seaport is
a good deal greater than that of the height of the wave
when it is far out on the ocean. In fact, the mid-ocean
height of the wave is likely to be inches while the in-shore
wave is measured in feet. An illustration of this
can be seen on the coast of Cornwall, where sometimes,
on quite a calm day the sea that looks so still breaks
on the shore in big rollers. We cannot tell exactly how
high an earthquake wave may be in mid-ocean, but we
know it cannot usually be very great, though it travels
at great speeds—sometimes as much as five miles a
minute, or three hundred miles an hour.</p>
<p>Thus we should not expect that ships far out at sea
would often notice seaquakes unless the quake took place
very near them. There are, however, some instances.
Captain Gales, of the ship <i>Florence Nightingale</i>, reported
that on January 25th, 1859, while near St. Paul's Rocks,
not far from the Equator, "we felt a strong shock of an
earthquake. It began with a rumbling sound like distant
thunder and lasted about forty seconds. I was quite well
acquainted with earthquakes, as I had experienced a good
many on the west coast of America, but never had I felt
so severe a one. Glass and dishes rattled so vigorously
that I was surprised to find them uninjured. A good
<span class="pagenum" id="Page_146">-146-</span>
many objects fell down, and it was as if the ship were
grounding on a reef." Another report from a locality
not far from this speaks of a strange submarine noise not
unlike distant thunder, or still more like the distant
firing of heavy guns. At the same time there was a
vibration of the ship as though the anchor had been
let go.</p>
<p>The foregoing are representative of the large majority
of the reports of seaquakes. The ship quivers, vibrates;
loose objects clatter and tumble. There is a strange
thunderous noise in the sea. The first impression is as
if the ship were grinding upon the bottom, and there is
an instinctive rush of the crew to the deck to see if the
ship is not on a reef. In some instances there are some
forcible disturbances. M. Vulet d'Aourst speaks of a
seaquake so severe that "the Admiral feared the complete
destruction of the corvette." Heavy objects,
including cannon and their carriages, were thrown upon
the deck. The ship itself seemed to be hurled upwards.</p>
<p>One of the explanations offered of a phenomenon such
as the last described is that the vessel has been near a
submarine volcanic eruption of great power. The places
where some or most of the seaquakes have been observed
have been charted, and certain districts of the ocean have
been found to produce more of these disturbances than
others. Among the first to be thus determined were two,
located in the Atlantic Ocean, very near the Equator
and nearly midway between Cape Palmas on the southeastern
coast of Liberia and Cape St. Roque, Brazil.
<span class="pagenum" id="Page_147">-147-</span>
One of them is the St. Paul's Rocks district, of which
mention has already been made. Another district from
which seaquakes have been reported with exceptional
frequency is the North Atlantic in the neighbourhood
of the Azores. Between these islands and the coast of
Portugal it may be remembered that the great quake
originated which, on November 1st, 1775, destroyed Lisbon.
The West Indian Deep, that profound basin of the
Atlantic lying north of the Lesser Antilles and east of
the Bahamas, where the Atlantic has its greatest depths
and where its bottom has its greatest inequalities, is
another district from which an unusual number of seaquakes
have been reported. The usual explanation of
their origin is that in these neighbourhoods, owing to the
great pressure of water above them, there are continual
slips and fractures of the sea bottom, like landslips on
land, and that into the great cavities thus produced the
water rushes, and thus sets up disturbances which show
themselves on the surface like waves, very much in the
same way that the water rushing through the escape of
a bath produces small disturbances on the surface of the
water in the bath. To satisfy the requirements of such
a wave as rolled in upon the South American coast at
Arica in 1868 would require the sudden drop of many
hundred square miles of sea bottom—perhaps of several
thousand square miles.</p>
<hr class="chap x-ebookmaker-drop" />
<p><span class="pagenum" id="Page_148">-148-</span></p>
<div style="break-after:column;"></div><br />
1 of 2
2 of 2
Previous
Next
Update Required
To play the media you will need to either update your browser to a recent version or update your Flash plugin.