EARTHQUAKES IN GEOLOGY
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
_Earthquakes_, 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 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.
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 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.
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 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 _Strokkur_, which had been born during the earthquake of 1789,
suddenly ceased its eruption and came to an end.
[Illustration:
_Stereo Copyright, Underwood & U._ _London and New York_
A Geyser in Action
In geysers the suggestion that the fountains of steam and hot water
originate by the contact of water with hot rock is irresistible.]
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 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.
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
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.
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 diminishing in the amount of rise and fall till
they die out.
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 _Wateree_,
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 _Wateree_,
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 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.
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.
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 _Florence
Nightingale_, 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 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.
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.
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. 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.