THE CAUSES OF EARTHQUAKES
Now that we have before us some of the examples of the changes which
historic earthquakes have brought about in the face of the country,
it is easy to see what an important effect they must have exerted
in geological history. But there are still weighty questions to be
answered about earthquakes. We have seen that an earthquake can
contort, upset, and twist the surface strata of the earth as easily
as we can crumple a sheet of cardboard. We have yet to find whether
the crumpling of the strata is always _produced_ by earthquakes, or
whether an earthquake is the culminating symptom that some agency
is at work crumpling the strata. Let us try to imagine an example
on a small scale. Suppose we take the top of a pill-box, and,
holding it in the crook between our thumb and forefinger, compress
it very tightly on all sides. What will happen? The lid of the
pill-box, being subjected to stress or strain on all sides, will
presently buckle and crack. We shall have produced an earthquake on
a small scale, and there will be an earthquake fracture--perhaps an
earthquake fissure. If the whole pill-box had been used for the
purpose of our experiment, and had been packed to the brim with
ointment or thick liquid, and if it had been squeezed in a vice
instead of in our hands, then perhaps we should have provoked still
more striking symptoms of an earthquake. The ointment might have
broken out through the lid. Perhaps even tiny jagged holes or craters
would have been formed in the lid. Thus we see how strain may produce
earthquakes. Take some more examples. Suppose a cork is very tightly
fixed in a wine bottle, and in order to get it out we employ a very
powerful lever corkscrew. The neck of the bottle, under the effect
of the too powerful pressure put on the inside surface of the glass,
will crack or break. Similarly if we screw down a microscope too
hard on a slip of glass the glass will often crack suddenly. Both
these instances recently occurred within the writer's experience,
and few readers can have escaped noticing one or other of them. The
breakage in these instances is always caused because a strain is set
up somewhere in the glass--there is more pressure at one point than
another, and the glass, unable to resist this unequal pressure, gives
way.
[Illustration:
_Stereo Copyright, Underwood & U._ _London and New York_
The Track of an Earth Wave
Showing portion of a street in San Francisco after the terrible
earthquake of 1906. The resemblance of the break in the ground to the
appearance of a stationary wave should be noted.]
What happens when it gives way? To answer this question we had better
carry our minds to the example of the pill-box lid. If the top of
the box were of very brittle material, like pottery or glass, then
after the breakage we know that most likely one of the broken pieces
would be a little higher than the other--would perhaps overlap it.
That is what we often see when examining the geological earth strata.
One stratum, instead of lying evenly with another where a crack
has occurred, rests a little above it or below it. This inequality
or unevenness geologists call a fault. Now we can easily see that
whenever, and by whatever causes, a fault is produced, there is
probably at the same time an earthquake. The fault cannot be produced
without a great and shaking disturbance. Mr. John Milne, the most
distinguished of British authorities on earthquakes, says that all
large earthquakes originate from the formation or extension of these
"faults" or great cracks in the strata.
The occurrences of these "faults" are most frequent when the process
of mountain building has been going on over the earth. In other
words, if we imagine a period in which some great continental area
of the world's surface was slowly rising above the oceans, then at
that time we should expect that there would have been great "faults"
occurring in the strata, and great earthquakes following them. To
quote his words: "If therefore we wish to know when earthquakes
were greatest and most frequent, we must turn to those periods in
geological history when mountain ranges were built, when volcanic
activity was pronounced, and when great 'faults' were made. The first
of these periods would be coincident with the creation of the Ural
Mountains, the Grampians, and other mountain ranges. This took place
in the earliest geologic times. Another period of mountain formation
was when the Himalayas and the Alps were slowly but intermittently
brought into existence. In both these periods volcanic activity was
pronounced, and beds of coal were formed. When the crust of the earth
was crumbling, mountains grew spasmodically--(they sprang up, as it
were, from out of the giant forces which we have described earlier
in this book)--'faults' gave rise to earthquakes, volcanic forces
found their vents, and conditions existed which gave rise to the
accumulation of materials to form coal."
But, the reader will naturally inquire, if "faults" gave rise to
earthquakes, and faults are the result of pressure, what produces the
pressure? And what produces the mountains? Before we answer that we
must again have recourse to examples taken from common experience.
A sheet of glass or of marble we usually regard as a thing that may
break but does not bend. But all of us have seen glass strips, if
they are long enough, bend under their own weight; and there are
even marble mantelpieces which, if examined, show that the slab of
marble has _bent_. Thus we can readily imagine that sometimes the
strata of the earth will bend by reason of the weight put upon them.
If that weight is not put on them quite evenly the strata will be
still more likely to bend; it will go from bending to buckling,
and from buckling to breaking. As soon as it breaks there will be
a "fault" formed. Those who recollect what we have said about the
enormous weight of the rocks one above the other will not have to
search far for the cause of weight sufficient to bend or buckle the
rock strata of the earth. And those who have followed carefully all
that has been written about the shifting of materials from the land
to the sea by the processes of denudation and erosion set up by rain
and wind and carried out by streams and rivers, will easily discover
where and how the shifting of great weights of the earth's surface
goes on. Little by little great weights are taken from one place
on the earth's surface to another, as we might shift the weights
in balanced scale-pans grain by grain, till at last the heavier
scale-pan goes down quickly, or it may be with a crash, and the
"fault" occurs, while the earthquake follows.
There remains another question, however, to be answered, and it is,
How were the mountains formed? Mountains are very closely connected
with earthquakes, for nearly all the regions of the earth where
the great disturbances take place are in the neighbourhood of
great mountain ranges; and many, indeed most, of the students of
earthquakes believe this to be the case, because the great weight of
the mountains, especially when near the deep sea, induces pressure
on the rocks, and consequently slipping and "faults." But the causes
of mountain formation are very difficult to show with certainty.
One such explanation, that of the continual shifting of portions
by weight of the earth, we have already given. There is another
one which may perhaps supply an additional cause. It is that just
as "faults" produce earthquakes, perhaps in some cases earthquakes
produce "faults." In the illustration we gave of the bottle-neck
being broken by continual pressure, or the slide of a microscope
suddenly breaking with a crash, because the increasing pressure of
a screw was greater than it could bear, we have considered cases
where the pressure was _slowly_ applied. But a tap with a hammer or
any sudden shock would also produce a breakage--and "faults," and
an earthquake on a small scale--and it is possible that some of the
convulsions of nature and some of their permanent effects are caused
by sudden and violent causes.
One such cause might be the violent and sudden formation of steam by
the contact of water with rock at a very high temperature. Everybody
knows what happens to the kitchen boiler after a severe frost. The
frost clogs the pipes with ice, so that the kitchen boiler becomes
dry because no water is reaching it; but it continues to grow hotter
and hotter till the iron plates or iron lining become red-hot. Then
the frost perhaps gives way and a small amount of water finds its
way into the red-hot boiler. The water is converted instantly into
steam; and, as a result, the boiler, if it is a weak one, is blown
out into the kitchen, causing grave personal inconvenience to the
cook. If the boiler is a strong one, it may merely crack. Now, apply
these considerations to the instance of the earth, its oceans, its
thin crust, and its hot rocks situated at a depth of not more than
thirty miles, and perhaps at a good deal less depth than that. What
would happen if the ocean leaked through into the strata of red-hot
or molten rocks? There would be enormous quantities of steam formed;
and if, owing to the vast pressure of strata and water above, these
quantities of steam did not instantly produce violent explosions,
yet underneath there would be imprisoned, peak, forces of tremendous
power which would only await a favourable opportunity in order to
manifest themselves. The hot steam until this favourable opportunity
arose would be absorbed by the rocks, just as hot steel can be shown
to absorb gases.
[Illustration:
_Stereo Copyright, Underwood & U._ _London and New York_
A Geyser at Rest in Yellowstone Park, U.S.A.
The encrusted deposits of mineral salts should be observed.]
We are thus face to face with the following situation, as it is
expressed by Dr. T. J. J. See, the American physicist:--The internal
temperature of the earth is extremely high with heated rocks quite
near the surface, while the crust is fractured and leaky everywhere,
and especially where the depth of the sea is greatest. The sea
covers three-fourths of the earth's surface, and earthquakes are
found to be most violent where the sea is deepest, and volcanoes
most numerous on the adjacent shores. Can we then suppose that both
earthquakes and volcanoes depend on the explosive power of steam
which has developed in the heated rocks of the earth's crust? We
have said that earthquakes and volcanoes are most common in regions
where high mountains are near deep oceans--as on the westward of
South and Central America, the Aleutian Islands, the Kurile Islands,
Japan, Sumatra, Java, and other islands of the East Indies bordering
on the deep waters of the Indian Ocean, New Zealand, and the Lesser
Antilles in the West Indies, Iceland, Italy, and Greece. These are
also the regions where, owing to the existence of high mountains, the
weight, the pressure, the tear and stress on the underlying strata
are greatest, and where consequently there is the greater chance of
strata slipping or bending or giving way. Mr. John Milne divides the
world into eleven such great "world-earthquake" districts; and he
has endeavoured to show that all the great convulsions of the earth
have their origin in one or other of these areas--where usually a
great mountain range slopes steeply down to the sea. There are eleven
such world-earthquake districts. There is the Alaskan region, where
on the shore Mount Elias rises to a height of 18,000 feet and where
the water is 7000 feet deep sixty miles from the shore--altogether
a drop of 25,000 feet from the top of the mountain to the bed of
the ocean in 200 miles. This drop, without going into measurements,
may be taken as typical of the rest, which are classified as the
Cordillerean region, the Antillean region (in the earthquake district
of which Mount Pelée at Martinique and the Soufrière in St. Vincent
are situated); the Andean district; the Japan district; the Javan
district; the Mauritius district; the Antarctic district; three
submarine districts of sunken ridges in the North-Eastern Atlantic,
the North-Western Atlantic, and the Northern Atlantic; and one great
land district, distant from the sea, which lumps together all the
mountains of the Alps, the Balkans, the Caucasus, and the Himalayas.
In thirteen years, from the time in which earthquake investigation
has become a science, 750 great earthquakes originated in these
districts. On the average, about sixty great earthquakes occur
every year, or a little more than one a week. In addition to these
world-shaking effects there are about 30,000 small earthquakes every
year, England's annual contribution to this number being about half a
dozen.
What is the meaning of a "great earthquake," and how do we define
"small" earthquakes? A small earthquake is one such as we have
described as taking place in the valley of the Mississippi, and
which, even though it may produce considerable disturbance in its
neighbourhood, is not perceptible at any great distance away. A great
earthquake is one which sends its vibrations thousands of miles. A
very large earthquake, originating in any part of the world, _may
be recorded in any other part of the globe_. Although only a few
people in Great Britain have been privileged to feel a home-made
earth tremor, every one of us is very many times a year moved by
earthquakes. We do not perceive them because the back-and-forth
motion of the ground is performed too slowly, while if there is
a movement of the ground the undulations are so very flat that
they cannot be perceived. But at several places in England and at
earthquake observatories (seismological stations) all over the
world, from Japan to Australia and from South Africa to Greenland,
instruments are set up which are sensitive enough to record these
tremors, though not always to locate them. Sometimes when Professor
Behar in Germany, or Mr. Milne from his observatory in the Isle of
Wight, telegraphs to the newspapers that signs of a great earthquake
have appeared on their instruments, the world hears no more of these
disturbances. They have occurred we are certain, but the place where
the great cataclysm which has thus shaken the whole round world took
place has been fortunately remote from inhabited portions of the
earth, and has very likely been beneath the waters of some ocean.
Earthquake waves start out from the great area where the cataclysm
took place, and begin to disturb the earth in all directions,
just as if we were to put a row of marbles on a table and were to
strike the end marble of the row. The marble farthest from it would
presently receive the shock as it travelled along the row of marbles.
Any one of our readers who has ever seen a train of luggage wagons
being shunted is familiar with the way in which the shock of a
sudden pull or push on the part of the engine travels all down the
line of wagons, and we may think of the shock of an earthquake as
travelling along and through the earth in the same way. Observation,
however, shows that these waves are propagated farthest in one
particular direction. For example, the chief movement following
the San Francisco earthquake, which originated from fault lines
running parallel to the coast of California, was much more marked in
countries lying to the east or west of California than in countries
lying towards the south. England and Japan obtained large records of
the disturbance, while in Argentina the records were extremely small.
In the case of the Jamaica earthquake, where the lines of origin ran
east and west, the phenomenon was reversed. Toronto received a large
quantity of motion, and England a very little. Another peculiarity of
this phase of earthquake motion is that it may be propagated in one
direction round the world to a greater distance than in an opposite
direction. The suggestion is that the initial impulse was delivered
in the direction towards which motion was propagated farthest. That
which happens corresponds to what we see if we dip the blade of
a spade in water and suddenly push the blade in some particular
direction. The water waves thus created travel farthest in the
direction of the impulse.
Another curious phenomenon connected with the large waves of certain
earthquakes is that they may be very marked for one thousand miles
round their origin, and may be perceived on the exactly opposite
side of the earth (though, of course, much reduced in size), but
cannot be recorded on the earthquake instruments of the regions in
between. For example, an earthquake originating near New Zealand may
be recorded in that country, but not in India, Egypt, West Asia, or
east of Europe, though in Britain it may make itself evident on the
seismometer's record. The phenomenon may be compared to a water wave
running down an expanding estuary. At the mouth of such an estuary it
may have become so flat that it is no longer recognisable. Should it,
however, run up a second estuary, we can imagine concentration taking
place, so that near the top of the second estuary it would eventually
become recordable on instruments. In these antipodean survivors we
see the final efforts of a dying earthquake. It is only occasionally
that the precursors and the followers of these large waves have
sufficient energy to reach their antipodes. They die _en route_.
From the earliest times philosophers have held that the causes of
earthquakes were associated with the contact between fire and water.
Plato, Aristotle, Strabo, and Pliny all held that water and air
penetrate into the earth through hollows, fissures, and crevices,
thus developing in the heated interior great vapour, a part of
which is expelled from volcanoes. Aristotle correctly associated
seismic sea waves with earthquakes, and even Homer assigned these
great disturbances of the sea to Poseidon's trident, which was also
the means employed for raising up islands from the sea bottom. The
withdrawal of the water from the shore after an earthquake and its
return as a great wave were familiar to Aristotle, and are implied in
his description of the sinking of Helike in 373 B.C.
Before leaving the subject of earthquakes we may quote some passages
from Mr. John Milne on the influence which these great disasters have
exercised on the emotions. Immediately after the Kingston earthquake
we read of the dazed and almost insane condition of the people. Many
were affected with an outburst of religious ecstasy, thinking the
last day had come. The negro population camped on the racecourse
and spent their time in singing hymns. Somewhat similar scenes took
place in Chili; men and women ran hither and thither, mad with terror
and devoid of reason. Amid shrieks and sobs and the wailing of a
multitude an "Ora pro nobis" or a "Pater noster" might now and then
be heard. In early civilisations underground thunderings have so far
excited the imagination that subterranean monsters or personages
have been conjured into existence, and these in many instances have
played a part in primitive religions. At the time of an earthquake in
Japan the children are told that the shaking is due to the movement
of a fish which is buried beneath their country, and in Japan we
find references to this fish in the pictorial art, pottery and
carving, literature, and everyday conversation, all of which would
be unintelligible if we did not know the story of the earthquake
fish. In other countries the subterranean creature will be a pig, a
tortoise, an elephant, or some other animal.
The most interesting myths, however, relate to underground
personages. The forty-five Grecian Titans, who were of gigantic
stature and of proportionate strength, were confined in the bowels
of the earth. According to the poets, the flames of Etna proceeded
from the breath of Enceladus, and when he turned his weary body the
whole island of Sicily was shaken to its foundations. Neptune was
not only a god of the oceans, rivers, and fountains, but with a blow
of his trident he could create earthquakes at pleasure. The worship
of Neptune was established in almost every part of the Grecian
world. The Livians, in particular, venerated him, and looked upon
him as the first and greatest of the gods. The Palici were born in
the bowels of the earth, and were worshipped with great ceremonies
by the Sicilians. In a superstitious age the altars of the Palici
were stained with the blood of human sacrifices. In Roman mythology
two very familiar deities are Pluto and Vulcan. These and a host of
other deities, the outcome of imagination, excited by displays of
seismic and volcanic activity, we meet with every day in picture
galleries, in museums, in literature, and in our daily papers.
Earthquakes have led to the abolition of oppressive taxation, the
abolition of masquerades, the closing of theatres, and even to the
alteration in fashions. A New England paper, of 1727, tells us that
"a considerable town in this province has been so far awakened by the
awful providence in the earthquake that the women have generally laid
aside their hooped petticoats."
In the next chapter we shall consider more particularly the terrible
effects of earthquakes on geological history.