We must go on to discuss earthquakes next, for their cause is akin to our last subject.
The theories that have been put forward up to the present date are three, and their
authors three men, Anaxagoras of Clazomenae, and before him Anaximenes of
Miletus, and later Democritus of Abdera.
Anaxagoras says that the ether, which naturally moves upwards, is caught in hollows
below the earth and so shakes it, for though the earth is really all of it equally
porous, its surface is clogged up by rain. This implies that part of the whole sphere is
'above' and part 'below': 'above' being the part on which we live, 'below' the other.
This theory is perhaps too primitive to require refutation. It is absurd to think of up
and down otherwise than as meaning that heavy bodies move to the earth from every
quarter, and light ones, such as fire, away from it; especially as we see that, as far as
our knowledge of the earth goes, the horizon always changes with a change in our
position, which proves that the earth is convex and spherical. It is absurd, too, to
maintain that the earth rests on the air because of its size, and then to say that impact
upwards from below shakes it right through. Besides he gives no account of the
circumstances attendant on earthquakes: for not every country or every season is
subject to them.
Democritus says that the earth is full of water and that when a quantity of rain-water
is added to this an earthquake is the result. The hollows in the earth being unable to
admit the excess of water it forces its way in and so causes an earthquake. Or again,
the earth as it dries draws the water from the fuller to the emptier parts, and the
inrush of the water as it changes its place causes the earthquake.
Anaximenes says that the earth breaks up when it grows wet or dry, and earthquakes
are due to the fall of these masses as they break away. Hence earthquakes take place
in times of drought and again of heavy rain, since, as we have explained, the earth
grows dry in time of drought and breaks up, whereas the rain makes it sodden and
destroys its cohesion.
But if this were the case the earth ought to be found to be sinking in many places.
Again, why do earthquakes frequently occur in places which are not excessively
subject to drought or rain, as they ought to be on the theory? Besides, on this view,
earthquakes ought always to be getting fewer, and should come to an end entirely
some day: the notion of contraction by packing together implies this. So this is
impossible the theory must be impossible too.
We have already shown that wet and dry must both give rise to an evaporation:
earthquakes are a necessary consequence of this fact. The earth is essentially dry, but
rain fills it with moisture. Then the sun and its own fire warm it and give rise to a
quantity of wind both outside and inside it. This wind sometimes flows outwards in a
single body, sometimes inwards, and sometimes it is divided. All these are necessary
laws. Next we must find out what body has the greatest motive force. This will
certainly be the body that naturally moves farthest and is most violent. Now that
which has the most rapid motion is necessarily the most violent; for its swiftness
gives its impact the greatest force. Again, the rarest body, that which can most
readily pass through every other body, is that which naturally moves farthest. Wind
satisfies these conditions in the highest degree (fire only becomes flame and moves
rapidly when wind accompanies it): so that not water nor earth is the cause of
earthquakes but wind-that is, the inrush of the external evaporation into the earth.
Hence, since the evaporation generally follows in a continuous body in the direction
in which it first started, and either all of it flows inwards or all outwards, most
earthquakes and the greatest are accompanied by calm. It is true that some take place
when a wind is blowing, but this presents no difficulty. We sometimes find several
winds blowing simultaneously. If one of these enters the earth we get an earthquake
attended by wind. Only these earthquakes are less severe because their source and
cause is divided.
Again, most earthquakes and the severest occur at night or, if by day, about noon,
that being generally the calmest part of the day. For when the sun exerts its full
power (as it does about noon) it shuts the evaporation into the earth. Night, too, is
calmer than day. The absence of the sun makes the evaporation return into the earth
like a sort of ebb tide, corresponding to the outward flow; especially towards dawn,
for the winds, as a rule, begin to blow then, and if their source changes about like the
Euripus and flows inwards the quantity of wind in the earth is greater and a more
violent earthquake results.
The severest earthquakes take place where the sea is full of currents or the earth
spongy and cavernous: so they occur near the Hellespont and in Achaea and Sicily,
and those parts of Euboea which correspond to our description-where the sea is
supposed to flow in channels below the earth. The hot springs, too, near Aedepsus
are due to a cause of this kind. It is the confined character of these places that makes
them so liable to earthquakes. A great and therefore violent wind is developed,
which would naturally blow away from the earth: but the onrush of the sea in a great
mass thrusts it back into the earth. The countries that are spongy below the surface
are exposed to earthquakes because they have room for so much wind.
For the same reason earthquakes usually take place in spring and autumn and in
times of wet and of drought-because these are the windiest seasons. Summer with its
heat and winter with its frost cause calm: winter is too cold, summer too dry for
winds to form. In time of drought the air is full of wind; drought is just the
predominance of the dry over the moist evaporation. Again, excessive rain causes
more of the evaporation to form in the earth. Then this secretion is shut up in a
narrow compass and forced into a smaller space by the water that fills the cavities.
Thus a great wind is compressed into a smaller space and so gets the upper hand, and
then breaks out and beats against the earth and shakes it violently.
We must suppose the action of the wind in the earth to be analogous to the tremors
and throbbings caused in us by the force of the wind contained in our bodies. Thus
some earthquakes are a sort of tremor, others a sort of throbbing. Again, we must
think of an earthquake as something like the tremor that often runs through the body
after passing water as the wind returns inwards from without in one volume.
The force wind can have may be gathered not only from what happens in the air
(where one might suppose that it owed its power to produce such effects to its
volume), but also from what is observed in animal bodies. Tetanus and spasms are
motions of wind, and their force is such that the united efforts of many men do not
succeed in overcoming the movements of the patients. We must suppose, then (to
compare great things with small), that what happens in the earth is just like that. Our
theory has been verified by actual observation in many places. It has been known to
happen that an earthquake has continued until the wind that caused it burst through
the earth into the air and appeared visibly like a hurricane. This happened lately near
Heracleia in Pontus and some time past at the island Hiera, one of the group called
the Aeolian islands. Here a portion of the earth swelled up and a lump like a mound
rose with a noise: finally it burst, and a great wind came out of it and threw up live
cinders and ashes which buried the neighbouring town of Lipara and reached some
of the towns in Italy. The spot where this eruption occurred is still to be seen.
Indeed, this must be recognized as the cause of the fire that is generated in the earth:
the air is first broken up in small particles and then the wind is beaten about and so
A phenomenon in these islands affords further evidence of the fact that winds move
below the surface of the earth. When a south wind is going to blow there is a
premonitory indication: a sound is heard in the places from which the eruptions
issue. This is because the sea is being pushed on from a distance and its advance
thrusts back into the earth the wind that was issuing from it. The reason why there is
a noise and no earthquake is that the underground spaces are so extensive in
proportion to the quantity of the air that is being driven on that the wind slips away
into the void beyond.
Again, our theory is supported by the facts that the sun appears hazy and is darkened
in the absence of clouds, and that there is sometimes calm and sharp frost before
earthquakes at sunrise. The sun is necessarily obscured and darkened when the
evaporation which dissolves and rarefies the air begins to withdraw into the earth.
The calm, too, and the cold towards sunrise and dawn follow from the theory. The
calm we have already explained. There must as a rule be calm because the wind
flows back into the earth: again, it must be most marked before the more violent
earthquakes, for when the wind is not part outside earth, part inside, but moves in a
single body, its strength must be greater. The cold comes because the evaporation
which is naturally and essentially hot enters the earth. (Wind is not recognized to be
hot, because it sets the air in motion, and that is full of a quantity of cold vapour. It
is the same with the breath we blow from our mouth: close by it is warm, as it is
when we breathe out through the mouth, but there is so little of it that it is scarcely
noticed, whereas at a distance it is cold for the same reason as wind.) Well, when
this evaporation disappears into the earth the vaporous exhalation concentrates and
causes cold in any place in which this disappearance occurs.
A sign which sometimes precedes earthquakes can be explained in the same way.
Either by day or a little after sunset, in fine weather, a little, light, long-drawn cloud
is seen, like a long very straight line. This is because the wind is leaving the air and
dying down. Something analogous to this happens on the sea-shore. When the sea
breaks in great waves the marks left on the sand are very thick and crooked, but
when the sea is calm they are slight and straight (because the secretion is small). As
the sea is to the shore so the wind is to the cloudy air; so, when the wind drops, this
very straight and thin cloud is left, a sort of wave-mark in the air.
An earthquake sometimes coincides with an eclipse of the moon for the same reason.
When the earth is on the point of being interposed, but the light and heat of the sun
has not quite vanished from the air but is dying away, the wind which causes the
earthquake before the eclipse, turns off into the earth, and calm ensues. For there
often are winds before eclipses: at nightfall if the eclipse is at midnight, and at
midnight if the eclipse is at dawn. They are caused by the lessening of the warmth
from the moon when its sphere approaches the point at which the eclipse is going to
take place. So the influence which restrained and quieted the air weakens and the air
moves again and a wind rises, and does so later, the later the eclipse.
A severe earthquake does not stop at once or after a single shock, but first the shocks
go on, often for about forty days; after that, for one or even two years it gives
premonitory indications in the same place. The severity of the earthquake is
determined by the quantity of wind and the shape of the passages through which it
flows. Where it is beaten back and cannot easily find its way out the shocks are most
violent, and there it must remain in a cramped space like water that cannot escape.
Any throbbing in the body does not cease suddenly or quickly, but by degrees
according as the affection passes off. So here the agency which created the
evaporation and gave it an impulse to motion clearly does not at once exhaust the
whole of the material from which it forms the wind which we call an earthquake. So
until the rest of this is exhausted the shocks must continue, though more gently, and
they must go on until there is too little of the evaporation left to have any
perceptible effect on the earth at all.
Subterranean noises, too, are due to the wind; sometimes they portend earthquakes
but sometimes they have been heard without any earthquake following. Just as the
air gives off various sounds when it is struck, so it does when it strikes other things;
for striking involves being struck and so the two cases are the same. The sound
precedes the shock because sound is thinner and passes through things more readily
than wind. But when the wind is too weak by reason of thinness to cause an
earthquake the absence of a shock is due to its filtering through readily, though by
striking hard and hollow masses of different shapes it makes various noises, so that
the earth sometimes seems to 'bellow' as the portentmongers say.
Water has been known to burst out during an earthquake. But that does not make
water the cause of the earthquake. The wind is the efficient cause whether it drives
the water along the surface or up from below: just as winds are the causes of waves
and not waves of winds. Else we might as well say that earth was the cause; for it is
upset in an earthquake, just like water (for effusion is a form of upsetting). No, earth
and water are material causes (being patients, not agents): the true cause is the wind.
The combination of a tidal wave with an earthquake is due to the presence of
contrary winds. It occurs when the wind which is shaking the earth does not entirely
succeed in driving off the sea which another wind is bringing on, but pushes it back
and heaps it up in a great mass in one place. Given this situation it follows that when
this wind gives way the whole body of the sea, driven on by the other wind, will
burst out and overwhelm the land. This is what happened in Achaea. There a south
wind was blowing, but outside a north wind; then there was a calm and the wind
entered the earth, and then the tidal wave came on and simultaneously there was an
earthquake. This was the more violent as the sea allowed no exit to the wind that had
entered the earth, but shut it in. So in their struggle with one another the wind caused
the earthquake, and the wave by its settling down the inundation.
Earthquakes are local and often affect a small district only; whereas winds are not
local. Such phenomena are local when the evaporations at a given place are joined
by those from the next and unite; this, as we explained, is what happens when there
is drought or excessive rain locally. Now earthquakes do come about in this way but
winds do not. For earthquakes, rains, and droughts have their source and origin
inside the earth, so that the sun is not equally able to direct all the evaporations in
one direction. But on the evaporations in the air the sun has more influence so that,
when once they have been given an impulse by its motion, which is determined by
its various positions, they flow in one direction.
When the wind is present in sufficient quantity there is an earthquake. The shocks
are horizontal like a tremor; except occasionally, in a few places, where they act
vertically, upwards from below, like a throbbing. It is the vertical direction which
makes this kind of earthquake so rare. The motive force does not easily accumulate
in great quantity in the position required, since the surface of the earth secretes far
more of the evaporation than its depths. Wherever an earthquake of this kind does
occur a quantity of stones comes to the surface of the earth (as when you throw up
things in a winnowing fan), as we see from Sipylus and the Phlegraean plain and the
district in Liguria, which were devastated by this kind of earthquake.
Islands in the middle of the sea are less exposed to earthquakes than those near land.
First, the volume of the sea cools the evaporations and overpowers them by its
weight and so crushes them. Then, currents and not shocks are produced in the sea
by the action of the winds. Again, it is so extensive that evaporations do not collect
in it but issue from it, and these draw the evaporations from the earth after them.
Islands near the continent really form part of it: the intervening sea is not enough to
make any difference; but those in the open sea can only be shaken if the whole of the
sea that surrounds them is shaken too.
We have now explained earthquakes, their nature and cause, and the most important
of the circumstances attendant on their appearance.