EARTHQUAKES
An _earthquake_ is a shaking of the earth. It may vary in intensity
from a shaking so feeble that it requires the use of a delicate
instrument to detect it, to a shaking violent enough to overthrow
heavy buildings, and even to make great rents or fissures in the crust.
An earthquake then is an _earth-shake_. It may be caused by anything
capable of shaking the earth; for example, as the falling of a heavy
weight on its surface. Now, a shaking so caused is only felt in the
immediate neighborhood of the place the weight strikes the earth. On
the contrary, in an earthquake, the shaking spreads in all directions
through the earth's crust, until, in the case of very violent
earthquakes, it reaches portions that may be situated many thousands
of miles away from where the shock started. This spreading of the
earthquake waves through the solid earth is not unlike the spreading
of the circular waves that are set up in a still water surface when a
stone is tossed in.
Any shaking of the earth's crust produces what may be called an
earth-shake or earthquake. The mere falling of a raindrop on the earth
produces a slight shaking. The falling of a heavy stone produces a
stronger shaking, and sets up a series of minute waves, generally
called vibrations, that spread around the place in all directions from
where the stone struck. These movements, however, while they spread
in all directions, just as they do in a surface of a lake, when a
stone is thrown into it, are of course much more quickly stopped by
the solid earth than similar movements are by the more readily movable
water.
But, while any shaking of the earth's crust constitutes an earthquake,
yet, strictly speaking, an earthquake is produced only by some force
that acts suddenly on the earth, _at a point below its surface_, and,
therefore, out of sight. This, of course, would rule out all such
shakings as are caused by bodies striking the outer surface of the
earth.
Earthquakes may occur in any part of the world, and at any time of the
day or year. They do occur, however, most frequently in certain parts
of the world, at certain seasons of the year and at certain hours of
the day.
Earthquakes are far from being unusual occurrences. In some parts of
the world, such as the island of Java, they are very common, and in
Japan, under certain circumstances, scarcely a day passes without one
or more shocks in some part of that little empire.
Professor Mallet, who has made a very extensive study of earthquakes,
published in 1850 to 1858, in the Philosophical Transactions, brief
abstracts or descriptions of all the more important earthquakes he
could find records of during the past 3,456 years. The number of
earthquakes thus recorded during this period reached 6,830. Of this
great number nearly one-half occurred during the last fifty years.
It should not be inferred from the above figures that the number of
earthquakes has really increased so greatly in the past half-century.
The explanation of the apparent increase is that greater care has
been taken recently in recording earthquakes, and that an apparatus
called a _seismometer_, or _earthquake-recorder_, has been invented
which automatically produces a record of the smallest shocks; so that
a great many have been recorded that would otherwise have passed
undetected.
It is the opinion of Le Conte that if the records of all the
earthquakes of 3,456 years had been thus made there would have been
found during the entire time of Mallet's researches to have occurred
no less than 200,000, while during the last four years of Mallet's
records, the number would have probably reached two earthquakes per
week.
Since Mallet's time, Prof. Alexis Perry published (1876) a much larger
list of earthquakes. Perry finds that from 1843 to 1872 there have
been 17,249 earthquakes, or 575 every year. Perry's list, however,
is incomplete, since it fails to record earthquakes that occurred in
mid-ocean, and in the unexplored and uncivilized parts of the world.
So it seems likely that earthquakes are so common that our earth, at
some part or other of its surface, is continually shaking or quaking.
Earthquakes are such tremendous phenomena that they were necessarily
observed by the ancients. We find more or less complete accounts of
them in various writings. Lucretius (Titus Carus Lucretius, a great
Roman poet) speaks as follows, in his De Rerum Natura (On the Nature
of Things). We use Munro's translation here:
"Now mark and learn what the law of earthquakes is. And
first of all take for granted that the earth below us as
well as above is filled in all parts with windy caverns,
and bears within its bosom many lakes and many chasms,
cliffs and craggy rocks; and you must suppose that many
rivers hidden beneath the crust of the earth roll on with
violent waves and submerged stones; for the very nature
of the case requires it to be throughout like to itself.
With such things then attached and placed below, the
earth quakes above from the shock of great falling masses,
when underneath, time has undermined vast caverns. Whole
mountains, indeed, fall in, and in an instant from the
mighty shock tremblings spread themselves far and wide from
that centre. And with good cause, since buildings beside
a road tremble throughout, when shaken by a wagon of not
such very great weight; and they rock no less, where any
sharp pebble on the road jolts up the iron tires of the
wheels on both sides. Sometimes, too, when an enormous mass
of soil through age rolls down from the land into great
and extensive pools of water, the earth rocks and sways
with the undulation of the water just as a vessel at times
cannot rest, until the liquid within has ceased to sway
about in unsteady undulations....
"The same great quaking likewise arises from this cause,
when on a sudden the wind and some enormous force of air
gathering either from without or within the earth have
flung themselves into the hollow of the earth and there
chafe at first with much uproar among the great caverns
and are carried on with a whirling motion, and when their
force, afterwards stirred and lashed into fury, bursts
abroad and at the same moment cleaves the deep earth and
opens up a great yawning chasm. This fell out in Syrian
Sidon and took place at Ægium in the Peloponnese, two towns
which an outbreak of wind of this sort and the ensuing
earthquake threw down. And many walled places besides fell
down by great commotions on land and many towns sank down
engulfed in the sea together with their burghers. And if
they do not break out, still the impetuous fury of the
air and the fierce violence of the wind spread over the
numerous passages of the earth like a shivering-fit and
thereby cause a trembling."
Of course, no one at the present time believes this ridiculous
explanation as to the cause of earthquakes.
Aristotle, a Greek philosopher, speaks thus concerning earthquakes. We
quote the translation employed by Mallet:
"Three theories on the subject have been handed down to
us by three different persons; namely, Anaxagoras of
Klazomene, before him Anaximenes the Milesian, and later
than these Democritus of Abdera.
"Anaxagoras says that the ether of nature rises upward, but
that when it falls into hollow places in the lower parts
of the earth it moves it (the earth); because the parts
above are cemented or closed up by rain, all parts being by
nature equally spongy or full of cavities, both those which
are above (where we live) and those which are below. Of
this opinion it may perhaps be unnecessary to say anything,
as being foolish, for it is absurd to suppose that things
would thus exist above and beneath, and that the parts of
bodies which have weight would not on every side be borne
to the earth, and those which are light, and fiery, rise;
especially since we see the surface of the earth to be
convex and spherical, the horizon constantly changing as
we change our place, at least as far as we know. And it is
also foolish to assert on the one hand that it remains in
the air on account of its great size, and on the other to
say that it is shaken, when struck from beneath upwards.
And besides these objections, it is to be remarked that
he has not treated of the attendant circumstances of
earthquakes, for neither every time nor place is subject to
these convulsions.
"But Democritus says, that the earth being full of water,
and receiving much also by means of rain, is moved by
this. For when the water increases in bulk, because the
cavities cannot contain it, in its struggles it causes an
earthquake. And when the earth becomes partially dried up,
the water being drawn from the full reservoirs into those
which are empty, in passing from one to the other, by its
movements it causes an earthquake also.
"Anaximenes, however, says that the earth, when parched up
and again moistened, cracks, and by the masses thus broken
off falling on it, is shaken; wherefore earthquakes occur
in drouths and again in times of rain; in drouths, because,
as we have said, it cracks, when highly dried, and then,
when moistened over again, it cracks and falls to pieces.
Were this the case, however, the earth ought to appear
in many places subsiding. Why then is it that hitherto
many places have been very subject to these convulsions
which do not present any such remarkable differences from
others? Yet such ought to be the case. And, moreover, those
who think thus must assert that earthquakes constantly
become less and less, and at last cease altogether. For
the continual condensation of the earth would cause this.
Wherefore, if this be not the fact, it is plain that this
is not the correct explanation."
Besides the above, there are numerous references to earthquakes in the
works of other writers. Thales, Seneca, and Pliny all speak of these
phenomena and appear to describe correctly the movement of the earth
in waves both in the solid land, as well as on the sea.
Coming down to less ancient writers, Mallet refers to a book by
Fromondi, published in Antwerp, in 1527, that contains much valuable
and interesting information. Among other things Fromondi declares
that in the year 369, in the reign of Valentinian, there was a great
earthquake that shook nearly the entire world and that another
earthquake of almost equal severity occurred in 1116. He also states
that in 1601 an earthquake continued for nearly forty days; that a
great earthquake in Italy, in 1538, lasted fifteen days, and that
another, in Spain, lasted for nearly three years.
This does not mean that these earthquakes actually continued to shake
the earth violently for the times mentioned. These are only the times
during which, at intervals of greater or less length, successive
shocks were felt in these localities.
Another of the less ancient writers referred to by Mallet is
Travagini, who published a book in Venice in 1683. This book contains
a description of a terrible earthquake occurring in Italy on the 6th
of April, 1667, which affected large portions of the country adjacent
to Ragusa.
Without attempting at present to discuss the various theories of
earthquakes, it will suffice to say that earthquakes can be divided,
according to their origin, into two classes: _volcanic earthquakes_,
or earthquakes that are caused by practically the same forces that
cause volcanoes, and _tectonic[3] earthquakes_, or those produced by
the slipping of a large mass of rock lying along the lines of old or
new fractures.
Earthquakes of the first class are found especially in volcanic
districts, while those of the second class are found in all parts
of the world, whether in volcanic districts or elsewhere. According
to Dana, earthquakes of the second class generally start in the
neighborhood of mountains, where old lines of fractures are especially
abundant.
As regards the direction of the shaking movements of the earth,
earthquakes can be divided into three different classes: _explosive
earthquakes_, or those in which the force acts vertically upwards;
_horizontal earthquakes_, or those in which the force moves in a more
or less horizontal direction, or parallel to the general surface
of the earth, and _rotary earthquakes_, or those in which the earth
rotates or moves in great eddies or whirls.
When the earthquake wave is started below the earth's surface, it
spreads through the crust in all directions. The direction these waves
will have on emerging, or coming out of the surface, will depend on
the distance of this point from the place the waves started. When a
place is situated directly over where the wave started, the waves will
emerge so as to move vertically upwards, so that the earth at this
point will be shaken by an explosive earthquake. As the point where
the waves pass out is situated further and further from the place
where the waves start, the waves will emerge more nearly horizontally,
the greater the distance from the source.
In explosive earthquakes, which, as just explained, occur at areas
almost immediately above the point where the disturbance starts,
the force is, generally speaking, the greatest. In earthquakes of
this character the force is sometimes sufficiently great to throw
large bodies high up into the air. In the case of the great Riobamba
earthquake of 1797, the force was not only sufficiently great to
fracture the earth in various places, but also to throw bodies lying
on the surface great distances into the air. Bodies of men were thrown
several hundred feet into the air and were afterwards found on the
other side of a broad river or high up on the side of a hill.
It is possible that Humboldt did not inquire with as much care as
he should have done into these reports. They were probably greatly
exaggerated, since it is difficult to understand how a force great as
this would have failed to detach the soil at these places, and hurl it
after the people. This much, however, can be accepted, that the upward
force was very great.
In the great Calabria earthquake of March, 1783, Dolomieu states that
the tops of the granite hills of Calabria were distinctly seen to rise
and fall. In some cases houses were suddenly raised a great distance
in the air, and were afterwards brought down again to a position of
rest, at a higher level without any damage occurring to them. In a
similar manner during the Caracas earthquake of March, 1812, the
ground was seen to rise and fall in a nearly vertical direction. But,
perhaps, one of the most terrible earthquakes of this character was
the earthquake that destroyed the greater part of Jamaica in June,
1793. During this earthquake the entire surface of the ground at Port
Royal assumed the appearance of a rolling sea. Houses were shifted
from their old sites. Many of the inhabitants who had succeeded in
escaping from the city to the neighboring country were thrown great
distances into the air. Some of these, by good fortune, fell into the
harbor, from which, in some cases, they escaped with their lives. Here
again the projectile force was probably greatly exaggerated.
Vertical movements characterized the great earthquake of Lisbon, on
November 1st, 1755, the city appearing to have been not far from the
point of origin.
The commonest type of earthquakes is the horizontal, where the waves
emerge at the surface in a direction either horizontal or parallel
to the general surface, or at least inclined to it at a very small
angle. Where the materials of the earth's crust, through which the
waves spread, are of the same kind and of the same density in all
directions, the area shaken is approximately circular, but where the
materials of the crust are more or less dense in some directions than
in others, the area of disturbance is of course oblong or elliptical.
In some cases earthquakes of the horizontal type are limited almost
entirely to a single direction. This is especially the case with
earthquakes that occur in mountainous districts. These earthquakes are
known as _linear earthquakes_, since they spread almost in a single
line.
When earthquake waves pass from one medium to another, that is, from
one kind of rock to another, the greater portion of the waves is
refracted or bent out of their straight direction as they pass into
the new medium; a part of the waves, however, are reflected. It is
these reflected waves that probably cause rotary earthquakes.
The speed with which the surface waves move outwards in all
directions, varies not only with the force of the wave, but also with
the kind of material through which they pass. This velocity may be
in the neighborhood of twenty miles per second, while in others the
velocity is as great as 140 miles per second.
Naturally, one would suppose that the most severe earthquakes are
those in which the waves move the most rapidly. On the contrary,
however, the comparatively feeble shocks are sent through the earth
with greater velocity.
In rotary earthquakes, as the name indicates, the ground is whirled
or twisted in the manner of a violent eddy, and is often left in
this twisted condition. In the great Calabria earthquake, huge
blocks of stone forming obelisks were twisted on one another in a
manner represented in Fig. 39. In this case the pedestals remained
unaffected, but the separate blocks of stone were partially turned
around, as shown. During this earthquake the earth was so twisted that
trees, which had been planted in straight lines before the earthquake,
were left standing in zigzags. During the great Charleston earthquake,
South Carolina, the chimney-tops of the houses were separated at
places where they joined the roof and were twisted around these places
without being overthrown. In some of the houses wardrobes or bureaus
were turned at right angles to their former positions, and in some
cases were even found with their faces turned towards the wall.
[Illustration: FIG. 39. HEAVY STONE OBELISKS TWISTED BY CALABRIAN
EARTHQUAKE OF 1783]
Mallet suggests that in some cases the rotary motion is more apparent
than real, being due only to a to-and-fro motion without any twisting,
the apparent turning being due to the greater freedom of motion of the
object in one direction than in another. A twisting motion, however,
has actually taken place in some earthquakes.
While separate shocks, in a given locality, may follow one another
at intervals for fairly long times, yet the principal shock or shake
that produces the greatest damage is generally of exceedingly short
duration. In the Caracas earthquake the greatest destruction was
accomplished in about one minute. There were three distinct shocks,
each of which lasted but three or four seconds. The great Calabria
earthquake, of 1783, lasted but two minutes. The earthquake of Lisbon,
in 1755, lasted five minutes, but the first, and worst, shock, was
only from five to six seconds.