THE CAUSE OF EARTHQUAKES.
Modern views respecting the cause of earthquakes—Earthquakes due
to faulting—To explosions of steam—To volcanic evisceration—To
chemical degradation—Attractive influence of the heavenly
bodies—The effect of oceanic tides—Variation in atmospheric
pressure—Fluctuation in temperature—Winds and earthquakes—Rain
and earthquakes—Conclusion.
As the results of modern inquiries respecting the cause of earthquakes,
we see many investigators chiefly attributing these phenomena to
special causes. A few attribute them to several causes. It seems to us
that they might be attributed to very many causes which often act in
a complex manner. The primary causes are telluric heats, solar heat,
and variations in gravitating influences. These may be the principal,
and sometimes the immediate, cause of an earthquake. The secondary
causes are those dependent upon the primary causes, such as expansions
and contractions of the earth’s crust, variations in temperature,
barometrical pressure, rain, wind, the attractive influences of the
sun and moon in producing tides in the ocean or the earth’s crust,
variations in the distribution of stress upon the earth’s surface
caused by processes of degradation, the alterations in the position of
isogeothermal surfaces, &c.
The part which may be played by these various causes in the production
of oscillations, pulsations, and tremors will be referred to.
_Earthquakes consequent on faulting._—In the chapter on Earth
Oscillations, the causes producing the phenomena of elevation and
depression are briefly indicated.
By the variations in stress accompanying elevations and depressions,
cracks are produced. Inasmuch as compression would crush the rocks
constituting the earth’s crust, we must conclude with Captain Dutton
that these cracks are formed by tension. By elevation, the upper rigid
crust of the earth is stretched, and fissures are produced. The sudden
formation of these fissures or faults gives rise to earthquakes, and
perhaps also to volcanic vents. That earthquake and volcanic regions
are situated on areas where there is evidence of rapid elevation is
strikingly illustrated round the shores of the Pacific.
Lasaulx considered that the earthquake of Herzogenrath was more
or less intimately connected with the great mountain fissure—the
_Feldbiss_—which crosses the coal region of the Wurm.[125] The sudden
elevation or sinking of large areas at the time of an earthquake may be
a consequence of these dislocations.
It has already been pointed out that the earthquake region of Japan
is the one where we have evidence of recent and rapid elevation. That
certain earthquakes of this region may possibly be the result of
faulting we have the evidence of our senses and of our instruments. The
sudden blows and jolts which are sometimes felt are indicative of the
sliding of one mass of rock across another.
Should the ground be simply torn asunder, this tearing would give rise
to a series of waves of distortion, vibrating in directions parallel
to the plane of the fissure. Supposing this motion to be propagated
to a number of surrounding stations, it would be recorded at each of
these as having the same direction. To those situated on a line forming
a continuation of the strike of the fissure, the vibrations would
advance so to speak _end on_, whilst to those stations lying in a line
perpendicular to the strike of the fissure, the motion would advance
_broadside on_.
Motions like these latter have been recorded in Tokio, where
earthquakes which from time observations were known to have come from
the faulted and rising region to the south have been registered as a
series of east and west motions, or vibrations transverse to this line
of propagation.
It must, however, be here mentioned that the registration of only
transverse motion may possibly be due to the extinction of normal
motion, although this is not generally regarded as probable.
It would therefore appear that certain earthquakes and faults are
closely related phenomena, the former being an immediate effect of
the latter. Faults are due to earth oscillations, and to a variety of
causes producing disturbances in the equilibrium of the earth’s crust;
the principal cause of all these phenomena being alterations in the
distribution of heat, and the attractive force of gravity.
_Earthquakes consequent on the explosion of steam._—Humboldt regarded
volcanoes and earthquakes as the results of a common cause, which he
formulated as ‘the reaction of the fiery interior of the earth upon
its rigid crust.’ Certain investigators, who have endeavoured to
reduce Humboldt’s explanation to definite limits, have suggested that
earthquakes may be due to sudden outbursts of steam beneath the crust
of the earth, and its final escape through cracks and fissures.
Admitting that steam may accumulate by separating out from the cooling
interior of our globe, its sudden explosion might be brought about by
its own expansive force, or by the movements in the bubbling mass from
which it originated.
Others, however, rather than regard the steam as being a primeval
constituent of the earth’s interior, imagine it arises from the gradual
percolation of water from the surface of the earth down to volcanic
foci, into which it is admitted against opposing pressures, by virtue
of capillary action.
Mallet, in his account of the Neapolitan earthquake, shows that the
whole of the observed phenomena can be accounted for by the admission
of steam into a fissure, which by the expansive force exerted on its
walls was rent open. Just as at the Geysers we hear the thud and
feel the trembling produced by the sudden evolution and condensation
of steam, so may steam by its sudden evolution and condensation in
the ground beneath us give rise to a series of shocks of varying
intensity, accompanied by intermediate vibratory motions—that is to
say, a motion which, as judged of by our feelings, is not unlike many
earthquakes. Often it may happen that the result of the explosion may
be the production of a fault, or at least a fissure; and thus in the
resulting movements we may have a variety of vibrations, some being
those of compression and distortion, produced by the blow of the
explosion, and others being those of distortion alone, produced by the
shearing action which may have taken place by the opening of the fault.
Sometimes one set of these vibrations may be prominent, and sometimes
the other. Thus, when we say that an earthquake has shown evidence by
the nature of its vibrations that it was produced by a fault, this
by no means precludes the possibility that an explosion of steam may
also have been connected with the production of the disturbance.
Mallet threw out the suggestion that the opening of fissures beneath
the ocean might admit water to volcanic foci. During the time that the
water was in the spheroidal state, the preliminary tremors, so common
to many earthquakes, would be produced. These would be followed by the
explosion, or series of explosions, constituting the shock or shocks of
the earthquakes.
The chief reasons for believing that the earthquakes of North-Eastern
Japan are partly due to explosive efforts are:—
1. That the greater number of disturbances, perhaps ninety per cent.,
originate beneath the sea, where we may imagine that the ground,
under the superincumbent hydrostatic pressure, is continuously being
saturated with moisture.
2. Many of the diagrams show that the prominent vibrations, of which
there are usually from one to three, in a given disturbance have the
same character as those produced by an explosive like dynamite, the
greatest and probably the most rapid motions being inwards towards the
origin.
It may here be remarked that a very large proportion of the destructive
earthquakes of the world have originated beneath the sea, as has often
been testified by the succeeding sea waves. Also, it must be observed,
that earthquake countries, like volcanic countries, are chiefly those
which have a coast line sloping at a steep angle beneath the sea—that
is to say, earthquakes are frequent along coasts bordered by deep water.
The earthquakes which occur at volcanic foci constitute another class
of disturbances which may be accredited to the explosive efforts of
steam.
_Earthquakes due to volcanic evisceration._—By the ejection of ashes
and lava from volcanic vents, there is an extensive evisceration of
the neighbouring ground. When we look at a volcano like Fujiyama,
13,000 feet in height, and at least fifty miles in circumference, and
remember that the mass of cinders and slag of which it is composed
came from beneath the area on which it rests, the point to be wondered
at is, that earthquakes, consequent on the collapse of subterranean
hollows, are not more frequent than they are. At the time of a single
eruption of a volcano, the quantity of lava ejected amounts to many
thousand millions of cubic feet. In 1783 the quantity of lava ejected
from Skaptas Joknee, in Iceland, was estimated as surpassing ‘in
magnitude the bulk of Mont Blanc.’[126] Admitting that hollow spaces
are the results of these eruptions, and that in consequence of this
evisceration the ground is rendered unstable, the instability being
increased by the additional load placed above the eviscerated area, it
would seem that from time to time earthquakes are inevitable.
Facts, however, teach us that volcanoes act as safety valves, and
that, as a rule, at or shortly after an eruption, earthquakes cease.
The relationship of earthquakes to volcanic eruptions would therefore
indicate, notwithstanding the arguments put forward to show that an
area loaded by a volcano has in consequence of the evisceration and the
load a quaquaversal dip, that evisceration does not take place beneath
volcanoes as is usually supposed, and we may conclude that it is but
few earthquakes which have an origin due to these causes.
_Earthquakes and evisceration by chemical degradation._—A powerful
agent, which tends to the formation of subterranean hollows, is
chemical degradation. The effects of this have been often measured by
quantitative analysis of the solid materials which are daily carried
away by many of our springs. In limestone districts this is very great.
Prof. Ramsay estimates that the mineral matter discharged annually by
the hot springs of Bath is equivalent in bulk to a column 140 feet in
height and 9 feet in diameter. At San Filippo, in Tuscany, the solid
matter discharged from the springs has formed a hill a mile and a
quarter long, a third of a mile broad, and 250 feet in thickness.[127]
Many other examples of subterranean chemical degradation will be found
in text-books of geology.
By this chemical action large cavernous hollows are produced. Beneath
a volcano it is probable that liquid material immediately takes the
place of that which is ejected, and that hollows are not formed as in
the case of chemical degradation. If a cavern becomes too large, it
eventually collapses.
Of the falling in of large excavations we have examples in large mines.
As a consequence, not only is a trembling produced, but also a noise,
which is so like that produced by certain earthquakes that the South
American miners have but one word, ‘bramido,’ to express both.[128]
Boussingault, who was an advocate for the theory that many earthquakes
are produced by the sinking of the ground, calls attention to the fact
that we have evidences of the subsidence of great mountains, like
the Andes, the districts around which are so well known for their
earthquakes. Capac Urcu is one of these mountains which legends tell us
has decreased in height.
The variation in the height of mountains is a subject which deserves
attention. That mountains may possibly be hollow, we have the
remarkable results attained by Captain Herschel, who found that the
attractive force of gravity in the neighbourhood of the Himalayas was
not so great as it ought to have been had these mountains been solid.
The Rev. O. Fisher gives another explanation of this phenomenon.
Palmieri considers that the terrible earthquake which devastated
Casamicciola (1881) was due to the hot springs having gradually eaten
out cavernous spaces beneath the town. The extremely local character of
this shock was certainly favourable to such a view.
The earthquake which, in 1840, caused Mount Cernans, in the Jura, to
fall, is also attributed to the solvent action of waters in undermining
its foundations. This undermining action was in great measure probably
due to a large spring, which, twenty-five years previously, had
disappeared, and which subsequently may possibly have been slowly
disintegrating the foundations of the mountain. Earthquakes of this
order would be principally confined to districts where there are rocks
which are more or less soluble, as, for instance, rock salt, gypsum,
and limestone.
_Earthquakes and the attractive influences of the heavenly bodies._—The
most important attractions exercised upon our planet are those due to
the sun and moon. To these influences we owe the tides in our ocean,
and possibly elastic tides in the earth’s crust. Some theorists would
also insist upon liquid tides in the fluid interior of our earth. The
nature of the earth’s interior is, however, a question on which there
is a diversity of opinion.
One doctrine, which, until recent years, received much support, was
that the interior of the earth was a reservoir of molten matter
contained within a thin crust. Hopkins showed that the least
possible thickness of such a crust must be from 800 or 1,000 miles,
otherwise the motions of precession and nutation would be subject to
interference.
M. Delauney objected to the views of Hopkins, on the supposition that
the fluid interior of the earth had a certain viscosity.
Sir William Thomson arrives at the conclusion that the earth on the
whole must be more rigid than a continuous solid globe of glass. Mr.
George H. Darwin’s investigations on the bodily tides of viscous or
semi-elastic spheroids tend to strengthen the arguments of Sir William
Thomson.
Some philosophers hold the view that the central portion of the earth,
although intensely hot, is solid by pressure, whilst the outer crust is
solid by cooling. Between the two there is a shell of liquid or viscous
molten matter.
Another argument is, that although the interior of the globe may be
solid, it is only retained in that condition by an immense pressure, on
the relief of which it is liquefied—it is potentially liquid.
As these views, and the arguments for and against them, are to be
found in all modern text-books of geology, we will at once proceed
to consider the effect of solar and lunar attractive influences in
producing earthquakes upon a globe which is either solid, partially
solid, or which has an interior wholly liquid.
_Effect of the attractive influences of the sun and moon._—In 1854
M. F. Zantedeschi put forward the view—that it is probable there is
a continual tendency of the earth to protuberance in the direction
of the radii vectores of the two luminaries which attract it. In
consequence of these protuberances, pendulums ought at one time to
swing more slowly than at others. Zantedeschi remarks that the periods
of earthquakes appear to confirm such a view, insomuch as they occur
more often at the syzygies, or epoch of the spring tides, than at neap
tides—an observation found in the works of Georges Baglivi (1703) and
Joseph Toaldo (1770).[129]
Prof. Perrey, of Dijon, who did so much for seismology, held the view
that the preponderance in the number of earthquakes felt at particular
seasons was possibly due to the attractive influence of the sun and
moon producing a tide in the fluid interior of the earth, which, acting
on the solid crust, produced fractures.
Rudolf Falb, whose writings have of late years attracted considerable
attention, brings forward views which may be regarded as amplifications
of those suggested by Perrey.
According to Falb, the inner portion of the earth must be regarded as
fluid. In the crust above this fluid reservoir are cracks and channels,
into which, by the attraction of the moon and sun, the fluid is drawn.
On entering these cracks cooling takes place, together with explosions
of gas and subterranean volcanic disturbances. The attractions
producing the internal tides required by Falb are chiefly dependent
upon the following factors:—
1. The nearness and distance of the sun from the earth (January 1 and
July 1).
2. The position of the moon with regard to the earth, which in every
twenty-seven days is once near and once distant.
3. The phases of the moon—whether full or new moon (syzygies), or
whether first or last quarter (quadratures).
4. The equinoxes, the position of the sun in the equator, and the
relative position of the earth.
5. The position of the moon relative to the equator.
6. The concurrence of the ‘centrifugal force’ of the earth with the
last quarter of the moon.
7. The entrance of the moon on the ecliptic—the so-called nodes.
Assuming that earthquakes are wholly consequent on these attractions,
it at once becomes possible to predict their occurrence. This Falb
does, and when his predictions have been fulfilled he has certainly
gained notoriety.
He commenced by the predictions of great storms. In 1873 he predicted
the destructive earthquake of Belluno, which earned for himself a
eulogistic poem, which he has republished in his ‘Gedanken und Studien
über Vulkanismus.’ After this, in 1874, he predicted the eruption of
Etna. He also explained why, in B.C. 4000, there should have been a
great flood, and for A.D. 6400 he predicts a repetition of such an
occurrence.
When we approach the question of the extent to which the attraction
of the sun and moon may influence the production of earthquakes, a
question which we have to answer is, whether it is likely that the
attractive power of the moon is so great that it could draw up the
crust the earth beyond its elastic limits. We know what it can do with
water. It can lift up a hemispherical shell 8,000 miles in diameter
about two or three feet higher at its crown than it lifts the earth.
Even supposing the solid crust to be lifted 100 times the apparent
rise of the tide, is it likely that a hemispherical arch 8,000 miles
in diameter when it is raised 200 feet at its crown could by any
possibility suffer fracture? If an arch is 12,000 miles in length, all
that we here ask is, whether the materials which compose the arch are
sufficiently elastic to allow themselves to be so far stretched that
the crown may be raised 200 feet. The result which we should arrive
at is apparently so obvious that actual calculation seems hardly
necessary. If we regard the earth as being solid, the question resolves
itself into the inquiry as to whether a column of rock, which is equal
in length to the diameter of the earth, or about 8,000 miles, can be
elongated 200 feet without a fracture. This is equivalent to asking
whether a piece of rock one yard in length can be stretched one seventy
thousandth of a foot. Considering that this is a quantity which is
scarcely appreciable under the most powerful of our microscopes, we
must also regard this as a question which it is hardly necessary to
enter into calculations about before giving it an answer. To vary
the method of treating such a question, may we not ask what is the
utmost limit to which it would be possible to raise up or stretch the
crust of the earth without danger of a fracture? Thus, for instance,
to what extent might a column of rock be elongated without danger
of its being broken? From what we know of the tenacity of materials
like brick and their moduli of elasticity, it would seem possible to
stretch a bar of rock 8,000 miles in length for approximately half
a mile before expecting it to break. As to whether there is a wave,
the height of which is equal to half this quantity, running round our
earth as successive portions of its surface pass beneath the attracting
influences of the sun and moon, is a phenomenon which, if it exists,
would probably long ago have met with a practical demonstration.
The deformation which a solid globe or spherical shell would
experience under the attractive influences of the sun and moon has
been investigated by Lamé, Thomson, Darwin, and other physicists and
mathematicians.
A conclusion that we are led to as one result of these valuable
investigations is, that if the interior of the earth be fluid, and
covered with a thin shell, then enormous elastic tides must be
produced. A consequent phenomenon, dependent on the existence of these
tides, would be a marked regularity in the occurrence of earthquakes.
As this marked regularity does not exist, we must conclude that
earthquakes are not due to the attractive influences of the sun and
moon acting upon the thin crust of the earth covering a fluid interior.
The periodicity of earthquakes corroborates the conclusions of Sir
William Thomson, who remarks that if the earth were not extremely rigid
the enormous elastic tides which must result would be sufficient to
lift the waters of the ocean up and down so that the oceanic tide would
be obliterated.
Assuming that the earth has the rigidity assigned to it by mathematical
and physical investigators, we nevertheless have travelling round our
earth, following the attractions of the moon and sun, a tidal stress.
This stress, imposed upon an area in a critical state, may cause it to
give way, and thus be the origin of an earthquake. Earthquakes ought
therefore to be more numerous when these stresses are the greatest.
The periods of maximum stress or greatest pull exerted by the moon and
sun will occur when these bodies are nearest to our planet—that is, in
perigee and perihelion, and again when they are acting in conjunction
or at the syzygies. That earthquakes are _slightly_ more numerous
at these particular periods than at others is a strong reason for
believing that the attractions of the moon and sun enter into the list
of causes producing these phenomena.
Had there been a strongly marked distinction in the number of
earthquakes occurring at these particular seasons as compared with
others, we might have attributed earthquakes to the existence of
elastic tides of a sensible magnitude. As the facts stand, it appears
that the maximum pulls exerted by the moon and sun are only sufficient
to cause a slight preponderance in the number of earthquakes felt at
particular seasons, and therefore that these pulls only result in
earthquakes when the distorting effort has been exerted on an area
which, by volcanic evisceration, the pressure of included gases, and
other causes, is on the verge of yielding.
_Earthquakes and the tides._—If we assume that earthquakes are in many
cases due to the overloading of an area and its consequent fracture,
such loading may occur by the rising of the tide. A belief that the
earthquakes of Japan were attributable to the tides may be found in the
diary of Richard Cocks under the date November 7, 1618, who remarks:—
‘And, as we retorned, about ten aclock, hapned a greate earthquake,
which caused many people to run out of their howses. And about the lyke
hower the night following hapned an other, this countrey being much
subject to them. And that which is comunely markd, they allwais hapen
at a hie water (or full sea); so it is thought it chauseth per reason
is much wind blowen into hollow caves under ground at a loe water, and
the sea flowing in after, and stoping the passage out, causeth these
earthquakes, to fynd passage or vent for the wind shut up.’[130]
Although we may not acquiesce in Cock’s views respecting the imprisoned
wind, it would seem that a comparison of the occurrence of earthquakes
and the state of the tide would be a legitimate research. Inasmuch
as the stresses which are brought to bear upon an area by the rising
of the tide are so very much greater than those due to barometrical
changes, it is not unlikely that a marked connection would be found.
But it must be remembered that because researches, so far as they have
gone, tend to show that earth movements are more frequent when an area
is relieved of a load, it is not unlikely that the greatest number of
earthquakes may be found to occur at low water. Prof. W. S. Chaplin
attempted to make this investigation in Japan, but not being able to
obtain the necessary information respecting the tides, was compelled to
relinquish this interesting work.
Every foot of rise in a tide is equivalent to a load being placed on
the area over which the tide takes place of sixty-two pounds to the
square foot. This load is not evenly distributed, but stops abruptly at
a coast line. Lastly, it may be observed that many coast lines are not
simultaneously subjected to stresses consequent upon this load. Japan,
for instance, may be regarded as an arch placed horizontally. The
area near the crown of this arch is loaded by the tidal wave crossing
the Pacific before the areas near the abutment, and farther there is
a horizontal pressure at the crown which, if Japan were like a raft,
would tend, as the tide advanced, to straighten its bow-like form, but
as the wave passed its abutments to increase its curvature.
Prof. G. Darwin has calculated the amount of rise and fall of a shore
line due to tidal loads (see p. 336, ‘Earth Pulsations’). The result
of these calculations apparently indicates that these loads may have a
considerable influence upon the stability of an area in a more or less
critical condition.
Mr. J. Carruthers suggests that tidal action may hold a general but
indirect relationship to volcanic and seismic action by the retardation
it causes on the earth’s rotation. By this retardation the polar axis
tends to lengthen, and tensile stresses are induced, resulting in
fracture. The fluid interior of the earth, being no longer restrained,
would move polewards, and, leaving equatorial portions unsupported,
this would gradually collapse. The primary fractures would be north and
south, while the secondary fractures would be east and west.[131]
That the rise of the tide is accompanied by a greater percolation
of water to volcanic foci, which, in consequence, assume a greater
state of activity, is a theory which was advanced many years ago. To
determine how far tides may directly be connected with earthquakes, the
necessary records have yet to be examined.
_Variations in atmospheric pressure._—When we consider the immense
load which, by a sudden rise of the barometer, is placed upon the area
over which this rise takes place, it is not difficult to imagine that
this rise may occasionally be the final cause which makes the crust of
the earth to give way. A barometric rise of an inch is equivalent to a
load of about seventy-two pounds being put upon every square foot of
area over which this rise takes place. On the other hand, a fall in the
barometric column indicates that a load has been removed, and whatever
elastic effort may be exerted by subterranean forces in endeavouring to
escape, being met by less resistance, they may burst these bonds, and
an earthquake will result. For reasons such as these the final cause
of earthquakes has often been attributed to variations in atmospheric
pressure. In Japan there are practically as many earthquakes with a
high barometer as with a low one.
The extent to which barometric fluctuations have acted as final causes
in the production of earthquakes may be judged of by a comparison of
the times of barometric variation and the times at which earthquakes
have occurred.
Three important laws of barometric variation are the following:—
1. In the world generally the average barometric pressure is highest in
winter. (Exceptions occur near Iceland and in the North Pacific.)
2. The summer and winter monthly mean barometer differs least near the
equator and over the great oceans. They differ most over the great
continents and generally with increasing latitude.
3. The greatest number of barometrical fluctuations usually take place
in winter.
Inasmuch as there are generally more earthquakes in winter than in
summer, the first of these laws would indicate that this might be due
to the greater load which acts upon the crust of the earth at that
season. The second law would indicate that the distinction between
the winter and summer earthquakes ought to be most marked in high
latitudes, which, if we refer to the table on p. 257, we observe to be
borne out by the results of observation. The countries where there are
as many earthquakes in winter as in summer are chiefly those in low
latitudes. The number of these countries from which we have records
are, however, few.
Facts opposed to the idea that earthquakes may be caused by an increase
of barometric pressure are the results of observations like those of
Schmidt and Rossi, which show that earthquakes chiefly occur with a low
barometer.
Assuming that these latter observations will be found by future
investigators to be generally true, we must conclude that the relief
of atmospheric pressure has an influence upon the occurrence of
earthquakes. Such a conclusion would partially accord with the third
barometrical law, or the fact that there are more occasions on which we
get a low barometer during the winter months.
Other writers who have examined this question are Volger, Kluge,
Andrès, and Poly. The latter investigator sought a connection between
earthquakes and revolving storms, in the centres of which there is
usually an abnormal decrease of atmospheric pressure. If an area over
which such a sudden change in pressure took place was in a critical
state, it is not difficult to see that storms such as Poly refers to
might sometimes be accompanied by earthquakes.
_Fluctuations in temperature._—Inasmuch as fluctuations in temperature
are governed by the sun, it may at once be said that there is a
connection between earthquakes and readings of the thermometer.
Certainly earthquakes occur mostly during the cold months or in
winter. Similarly, as changes in temperature are so closely connected
with barometric fluctuations, and these are said to have a direct
influence upon the yielding of the earth’s crust, seismic phenomena are
indirectly linked to fluctuations in temperature. A rise in temperature
is usually accompanied by a fall in the barometer, and this in turn may
be a condition favourable for the occurrence of an earthquake.
If we regard solar heat as an agent causing expansions or contractions
in the earth’s crust, then fluctuations in temperature become an
immediate cause of earthquakes. The probability, however, is that
solar heat has little or no connection with the final cause producing
earthquakes, although at the same time coincidences between the
occurrence of earthquakes and unusual fluctuations in temperature may
from time to time be observed.
_Winds and earthquakes._—Although it may be admitted that high winds
exert enormous pressures upon mountain ranges, and might occasionally
give rise to stresses causing rocky masses in unstable equilibrium to
give way, the coincidences which have been established between the
occurrence of storms and earthquakes can usually only be regarded as
occurrences which have synchronised by chance.
Storms are usually accompanied with a barometric depression, and the
relation of diminutions in atmospheric pressure to earthquakes has been
discussed.
_Rain and earthquakes._—It has already been shown that earthquakes
have occasionally been found to coincide with rain and rainy seasons.
Whether the saturation of the ground with moisture or the percolation
of the same to volcanic foci may be a direct effect producing
earthquakes it is difficult to say. The probability, however, is
that, rain being dependent on phenomena like changes in temperature,
barometric fluctuations, and winds, we must regard it and the
earthquakes which happen to coincide with these precipitations of
moisture as congruent effects of more general causes.
_Conclusion._—Although it would be an easy matter to discuss the
relationship of earthquakes and other phenomena, we must conclude
that the primary cause of earthquakes is endogenous to our earth, and
that exogenous phenomena, like the attraction of the sun and moon and
barometric fluctuations, play but a small part in the actual production
of these phenomena, their greatest effect being to cause a slight
preponderance in the number of earthquakes at particular seasons. They
may, therefore, sometimes be regarded as final causes. The majority of
earthquakes are due to explosive efforts at volcanic foci. The greater
number of these explosions take place beneath the sea, and are probably
due to the admission of water through fissures to the heated rocks
beneath. A smaller number of earthquakes originate at actual volcanoes.
Some earthquakes are produced by the sudden fracture of rocky strata or
the production of faults. This may be attributable to stresses brought
about by elevatory pressure. Lastly, we have earthquakes due to the
collapse of underground excavations.