THE CAUSES OF EARTHQUAKES
Earthquakes occurred long before man appeared on earth. It is natural,
therefore, that our early ancestors, experiencing these unwelcome
phenomena, vaguely endeavored to explain their causes. These early
attempts at explanation have in many cases been of an exceedingly
fanciful character.
The ancient Mongolians and Hindoos declared that earthquakes are due
to our earth resting on a huge frog and that they occur whenever the
frog scratches its head.
In Japan, where earthquakes are very common, the ignorant people even
at a much later day declared that there exists in the depth of the sea
an immense fish which, when angry, dashes its head violently against
the coast of the island, thus making the earth tremble. This is,
doubtless, the biggest fish-story extant.
Another folk-lore explanation in Japan attributes the cause of the
tremblings of the earth to a subterranean monster whose head lies in
the north of the island of Hondo, while his tail lies between the two
principal cities. The shaking of his tail causes earthquakes.
Fantastic and foolish as these explanations are, it is worthy of
note that the first of the Japanese explanations shows no little
observation on the part of the people, since it locates the
starting-points of earthquakes as being not on the land, but on the
bottom of the sea. In point of fact, nearly all the great earthquakes
in Japan seem to start somewhere between the coasts of the islands
on the sea-bottom that leads down to a very deep part of the Pacific
known as the Tuscarora Deep.
Many years ago nearly everyone believed that earthquakes were caused
solely by the forces that produce volcanic eruptions; that all
earthquakes, whether in the neighborhood of active volcanoes, or at
great distances therefrom, were to be regarded solely as volcanic in
their origin.
It is now recognized that the most severe and far-reaching earthquakes
have no immediate connection with volcanic explosions, but are due to
the sudden slippings of the earth's strata over lines of faults; or,
in other words, earthquakes are most frequently of the tectonic type.
At the present time there is unfortunately much difference in
opinion as to the exact cause of earthquakes. By this is not meant
the immediate cause, but the ultimate cause. As to the immediate
cause, practically all are agreed that quakes of volcanic origin are
to be traced to the same forces that produce volcanic eruptions,
while quakes of tectonic origin are due directly to the slipping of
the strata along the faults. But when inquiry is instituted as to
the nature of the forces that cause the volcanic eruptions, or that
produce such an alteration of the strata as permits them afterwards to
slip and thus jar the earth, there is much difference of opinion.
As can be seen from a few quotations of well-known authorities, only
two kinds of earthquakes exist; namely, volcanic earthquakes and
tectonic earthquakes.
Dana, for example, while acknowledging that small earthquakes may be
caused by the sudden falling of large rock masses into cavities in the
crust of the earth, says:
"But true earthquakes come, for the most part at least,
from one or the other of the following sources of
disturbance.
"1. Vapors suddenly produced, causing ruptures and friction.
"2. Sudden movements or slips along old or new fractures.
"Earthquakes due to the former of these methods are common
about volcanoes, and at the Hawaiian islands shakings that
are destructive over the island of Hawaii at the moment of
some of the more violent eruptions, do not often affect the
island of Oahu, a depth of 500 fathoms of water, the least
depth between the two islands, being sufficient to stop off
the vibrations....
"Earthquakes of the second mode of origin may occur in all
regions, volcanic or not. They have their origin mostly in
the vicinity of mountain regions, where old fractures most
abound. The vibrations may begin in a slip of a few inches,
in fact; but where there has been a succession of slips,
up and up from 10,000 feet or more, as in the Appalachian,
earthquakes of inconceivable volcanic activity must have
resulted."
Dana points out that volcanoes stand on lines of fractures in the
openings of which their existence began and that, during geological
time, slips up or down these fractures have occurred, producing
earthquakes and possibly starting eruptions.
Prestwich, a well-known English geologist, speaks very decidedly
concerning the causes of earthquakes:
"For my own part, I am disposed to share the belief
expressed by Dana that the tension or pressure, by which
the great oscillations or plications of the earth's crust
have been produced, have not entirely ceased; and that
this is generally the most probable cause of earthquakes.
The uplifting of the great continental tracts and mountain
ranges must have always left the interior of the crust in
a state of unstable equilibrium, and any slight slide or
settling along an old fracture, or in highly disturbed and
distorted strata, would be attended by an earthquake shock.
"In volcanic areas the removal of the large volumes of
molten rock from the interior to the surface must produce
settlements and strains which might also result in some of
these minor earthquakes to which volcanic districts are
so subject. Where we have the two conditions combined, as
they are in the Andes in South America, these earthquake
phenomena are, as we should expect, developed on the
grandest and widest scale."
Geikie, the Scotch geologist, says:
"Various conceivable causes may, at different times
and under different conditions, communicate a shock to
the subterranean regions. Such as the sudden flashing
into steam of water in the spherodial state, the sudden
condensation of steam, the explosion of a volcanic outpour,
the falling in of the roof of a subterranean cavity, or the
sudden snap of deep-seated rocks subjected to prolonged and
intense stress."
Sir Charles Lyell, the great English geologist, holds the following
views concerning the origin of earthquakes. He speaks as follows in
his "Principles of Geology":
"1. The primary cause of the volcanoes and the earthquakes
are to a great extent the same, and connected with the
development of heat and chemical action at various depths
in the interior of the globe.
"2. Volcanic heat has been supposed by many to be the
result of the high temperature which belonged to the
whole planet when it was in a state of igneous fusion,
a temperature which they suppose to have been always
diminishing and still to continue to diminish by radiation
into space....
"The powerful agency of steam or aqueous vapor in volcanic
eruptions leads us to compare its power of propelling lava
to the surface with that which it exerts in driving up
water in the pipe of an Icelandic geyser. Various gases
also, rendered liquid by pressure at great depths, may
aid in causing volcanic outbursts and in fissuring and
convulsing the rocks during earthquakes."
Major Clarence Edward Dutton, U. S. A., an acknowledged authority on
seismology, speaks as follows:
"Thus far, then, we have two causes of earthquakes which
are apparently well sustained: (1) the downthrows, which
have often been observed to be accompanied by earthquakes,
and (2) volcanic action. But neither of them have been
shown to be connected with more than a comparatively small
number. Much the greater part of the earthquakes still
require explanation, and the indications are manifold that
some of them are produced by some cause yet to be stated."
He acknowledges, however, this unknown cause may be traceable to
volcanic agency. To quote him in full:
"It remains now to refer to the possibility that many
quakes whose origin is unknown, or extremely doubtful, may,
after all, be volcanic. This must be fully admitted, and,
indeed, it is in many cases highly probable. Evidences
that volcanic action has taken place in the depths of the
earth without visible, permanent results on the surface
abound in ancient rock exposures. Formations of great
geological age, once deeply buried and brought to daylight
by secular denudations, show that lavas have penetrated
surrounding rock-masses in many astonishing ways. Sometimes
they have intruded between strata, lifting or floating up
the overlying beds without any indication of escaping to
the surface. Sometimes the lava breaks across a series of
strata and finds its way into the partings between higher
beds. Or it forces its way into a fissure to form a dike
which may never reach the surface. In one place a long arm
or sheet of lava has in a most surprising and inexplicable
manner thrust itself into the enveloping rock-mass, and in
the older or metamorphic rocks these offshoots or apophyses
cross each other in great numbers and form a tangled
network of intrusive dikes. In other places the intruded
lava has formed immense lenticular (lense shaped) masses
(laccolites), which have domed up the overlying strata
into mountain masses. These intrusions, almost infinitely
varied in form and condition, are often, in fact usually,
inexplicable as mechanical problems, but their reality is
vouched for by the evidence of our senses. What concerns
us here is the great energy which they suggest and their
adequacy to generate in the rocks those sudden, elastic
displacements which are the real initiatory impulses of an
earthquake. They assure us that a great deal of volcanic
action has transpired in past ages far under ground, which
makes no other sign at the surface than those vibrations
which we call an earthquake."
Koto, the celebrated Japanese student of earthquakes, and a member
of the Earthquake Investigation Committee appointed by the Japanese
Government for studying the great Mino-Owaro earthquake, in Japan,
1891, is properly regarded as an authority on earthquakes. Living, as
he does, in a country where earthquakes and volcanic eruptions are of
almost daily occurrence, he has had abundant opportunity for studying
these phenomena, especially in connection with the Seismological
Institute of Japan. He speaks as follows:
"To make clear once for all my own standpoint, I may say
plainly that the chain of volcanoes and the system of
mountains of the non-volcanic earthquake, appear to me
to have very intimate and fundamental relations with the
so-called tectonic line."
Mallet regards earthquakes that can be directly traceable to volcanic
origin as unsuccessful efforts on the part of nature to establish
volcanoes. He speaks concerning this matter as follows:
"An earthquake in a non-volcanic region may, in fact, be
viewed as an uncompleted effort to establish a volcano.
The forces of explosion and impulse are the same in both;
they differ only in degree of energy, or in the varying
sorts and degrees of resistance opposed to them. There is
more than a mere vaguely admitted connection between them,
as heretofore commonly acknowledged--one so vague that the
earthquake has been often stated to be the cause of the
volcano (Johnson, 'Phy. Atlas,' Geology, page 21), and more
commonly the volcano the cause of the earthquake, neither
view being the expression of the truth of nature. They are
not in the relation to each other of cause and effect, but
are both unequal manifestations of a common force under
different conditions."
Before closing this chapter on the causes of earthquakes it may be
well to state briefly the explanations that have been suggested by
those who hold that the earth is solid and cold throughout its entire
mass, except that in the neighborhood of volcanic districts there
are limited areas situated only a comparatively few miles below the
surface where the rocks are highly heated.
Professor Mallet suggested that the source of heat for these local
areas of melted rocks was to be found in the enormous mechanical force
that is developed by the crushing of the strata in the earth's crust.
The principal objection to Mallet's theory is to be found in the
fact that, for this heat to be available for the melting of rocks,
it must be produced rapidly, and not spread out over long periods of
time. Moreover, there would appear to be no other way to account for
the production of the great force required to effect the crushing of
the earth's strata save on the assumption of a highly heated interior
still cooling and contracting.
In his "Aspects of the Earth" Shaler has suggested an hypothesis
that may be regarded to a certain extent as explaining how heat,
slowly generated, might be blanketed, or prevented from escaping and
so possibly reaching a temperature sufficiently high to melt the
materials in portions of the interior not far below the surface of the
earth.
"We thus see that in the water imprisoned in the deposits
of the early geological ages and brought to a high
temperature by the blanketing action of the more recently
deposited beds, we have a sufficient cause for the great
generation of steam at high temperatures, and this is the
sole essential phenomenon of volcanic eruptions. We see
also by this hypothesis why volcanoes do not occur at
points remote from the sea, and why they cease to be in
action soon after the sea leaves their neighborhood....
"The foregoing considerations make it tolerably clear that
volcanoes are fed from deposits of water contained in
ancient rocks which have become greatly heated through the
blanketing effects of the strata which have been laid down
upon them. The gas which is the only invariable element
of volcanic eruptions is steam; moreover, it is the steam
of sea-water, as is proven by analysis of the ejections.
It breaks its way to the surface only on those parts of
the earth which are near to where the deposition of strata
is lifting the temperature of water contained in rocks by
preventing, in fact, the escape of the earth's heat."
Another very common theory is that of chemical action, or the heat
produced by the oxidation of various substances inside the earth,
such, for example, as iron pyrites, a compound of iron and sulphur.
When Sir Humphrey Davy discovered metallic sodium and it was found
that this material, when thrown on water, possessed the power of
liberating intense heat, the discovery was welcomed by geologists
as affording a possible explanation of the cause of volcanoes and
earthquakes.
It may be said generally concerning chemical action as the source
of the earth's interior heat, that the chief objection against it
is the fact that such heat is liberated too slowly to result in the
production of a very high temperature. This objection does not exist
in the case of such substances as metallic sodium, since here the
heat is rapidly developed and is sufficient in amount to fuse the
substances produced. But in the lava produced in such great quantities
as it is in volcanic districts there must be liberated at the same
time large quantities of gaseous hydrogen. Now, although hydrogen
is, as we have already seen, sometimes given off with the gases that
escape from volcanic craters, yet the quantity which escapes is so
small that this theory of volcanic activity has been practically
abandoned.
Quite recently, however, among the various chemical substances that
are produced under the extremely high temperatures of the electric
furnace have been found, or formed, a number of curious substances
such as _calcium carbide_, _calcium silicide_, _barium silicide_,
etc., that possess the property of becoming highly heated on coming in
contact with water.
Now it is an interesting fact that the hydrogen and other gases
which are given off by the action of water on these substances are
absorbed in large quantities by the materials themselves, so that the
objection of the absence of hydrogen and similar gases in the craters
of the volcanoes would not be quite as objectionable as in the case of
such substances.
Of course, it is impossible to say whether such substances as calcium
carbide, etc., actually exist inside the earth's crust, yet, as
has been pointed out, the principal condition necessary for their
formation, i. e., a high temperature, existed at times long after
the earth, assuming the correctness of the nebular hypothesis, was
separated from the nebulous sun.
There still remains to be discussed the most curious of all possible
causes that have been suggested for the presence of the local heated
areas at comparatively short distances below the earth's crust;
namely, radio-activity.
In 1896, Henri Becquerel, a Frenchman, while investigating the power
of the X-rays, when passing through certain substances, to produce
phosphorescence, or causing the substances to shine in the dark,
made the extraordinary discovery that some of the salts of uranium
possess the power of emitting a peculiar radiation closely resembling
the X-rays, that is able to pass through substances opaque to
ordinary light as well as to affect photographic plates. But the most
extraordinary part of this discovery was that the salts of uranium
apparently possess the power of giving out this radiation continuously
without being exposed to the sun's rays.
This peculiar property was called _radio-activity_, and was shortly
afterwards found to be present in many other substances besides
uranium, and notably so in two newly discovered elements known as
polonium and radium.
Now it has been suggested that if there existed somewhere beneath the
earth's crust in these locally heated areas, large quantities of
radio-active substances, these regions would at last become highly
heated, and in this way likely to produce volcanoes and earthquakes.
It would not, however, seem that this is probably their true cause.
From what has just been said it is clear how exceedingly difficult it
has become to explain the source of the earth's interior heat when
the fact of the earth's original highly heated condition is denied.
We are, therefore, disposed with Russell to believe, as stated in the
first part of this volume, that the ultimate cause of both volcanoes
and earthquakes is to be found in the gradual cooling of an originally
highly heated globe, and that the greater part of the interior is
still in a highly heated condition, hot enough to be melted but yet
in a solid condition by reason of the great pressure to which it is
subjected.