EFFECTS PRODUCED UPON BUILDINGS (continued).
Types of buildings used in earthquake countries—In Japan, in Italy,
in South America, in Caraccas—Typical houses for earthquake
countries—Destruction due to the nature of underlying rocks—The
swing of mountains—Want of support on the face of hills—Earthquake
shadows—Destruction due to the interference of waves—Earthquake
bridges—Examples of earthquake effects—Protection of
buildings—General conclusions.
_Types of buildings used in earthquake countries._—In Japan there
are excellent opportunities of studying various types of buildings.
The Japanese types, of course, form the majority of the buildings.
The ordinary Japanese house consists of a light framework of 4 or
5 inch scantling, built together without struts or ties, all the
timbers crossing each other at right angles. The spaces are filled
in with wattle-work of bamboo, and this is plastered over with mud.
This construction stands on the top of a row of boulders or of square
stones, driven into the surface soil to a distance varying from a few
inches to a foot. The whole arrangement is so light that it is not an
uncommon thing to see a large house rolled along from one position to
another on wooden rollers. In buildings such as these after a series of
small earthquake shocks, we could hardly expect to find more fractures
than in a wicker basket.
The larger buildings, such as temples and pagodas, are also built of
timber. These are built up of such a multitude of pieces and framed
together in such an intricate manner that they also are capable of
yielding in all directions. The European buildings are, of course,
made of brick and stone with mortar joints. Some of these, as the
buildings of the Ginza in Tokio, are not designed for great strength.
On the other hand, others have thick and massive walls and are equal in
strength to those we find in Europe.
The third type of buildings are those which are built in blocks; and
these blocks being bound together with iron rods traversing the walls
in various directions are especially designed to withstand earthquakes.
A system somewhat similar to this has been patented in America, and
examples of these so-called earthquake-proof buildings are to be found
in San Francisco.
Speaking of Japanese buildings, Mr. R. H. Brunton, who has devoted
especial attention to them says that,[24] ‘to imagine that slight
buildings, such as are seen here (i.e. in Japan), are the best
calculated to withstand an earthquake shock is an error of the most
palpable kind.’ After describing the construction of a Japanese house
in pretty much the same terms as we have used, he says ‘that with its
unnecessarily heavy roof and weak framework it is a structure of all
others the worst adapted to withstand a heavy shock.’ He tells us,
further, that these views are sustained by the truest principles of
mechanics. In order to render buildings to some extent proof against
earthquakes, some of the heavy roofs in Tokio have been so constructed
that they are capable of sliding on the walls. Mr. Brunton mentions
a design for a house, the upper part of which is to rest on balls,
which roll on inverted cups fixed on the lower part of the building,
which is to be firmly embedded in the earth. A similar design was,
at the suggestion of Mallet, used to support the tables carrying the
apparatus of some of the lighthouses erected in Japan by Mr. Brunton.
The very existence of these designs seems to indicate that the ordinary
European house, however solidly and strongly it may be built, is not
sufficient to meet the conditions imposed upon it. What is required,
is something that will give way—an approximation to the timber frame
of a Japanese house, so strongly condemned by Mr. Brunton and others.
The crucial test of the value of the Japanese structure, as compared
with the modern buildings of brick and stone, is undoubtedly to be
found by an appeal to the buildings themselves. So far as my own
experience has gone, I must say that I have never seen any signs in
the Japanese timber buildings which could be attributed to the effects
of earthquakes, and His Excellency Yamao Yozo, Vice Minister of Public
Works, who has made the study of the buildings of Japan a speciality,
told me that none of the temples and palaces, although many of them
are several centuries old, and although they have been shaken by small
earthquakes and also by many severe ones, show any signs of having
suffered. The greatest damage wrought by large earthquakes appears to
have resulted from the influx of large waves or from fires. In every
case where an earthquake has been accompanied by great destruction,
by consulting the books describing the same, it can be seen, from the
illustrations in these books portraying conflagrations, that this
destruction was chiefly due to fire. When we remember that nearly
all Japanese houses are constructed of materials that are readily
inflammable, it is not hard to imagine how destruction of this kind
has come about. To a Japanese, living as he does in a house which has
been compared to a tinder-box, fire is one of his greatest enemies,
and in a city like Tokio it is not at all uncommon to see during the
winter months many fires which sweep away from 100 to 500 houses. In
one winter I was a spectator of three fires, each of which was said to
have destroyed upwards of 10,000 houses.
Although it would appear that the smaller earthquakes of Japan produce
no visible effect upon the native buildings, it is nevertheless
probable that small effects may have been produced, the observation
of which is rendered difficult by the nature of the structure. If we
look at buildings of foreign construction, by which are meant buildings
of brick and stone, the picture before us is quite different, and
everywhere the effects of earthquakes are palpable even to the most
casual observer. Of these effects numerous examples have already been
given. Not only are these buildings damaged by the cracking of walls
and the overturning of chimneys, but they also appear to be affected
internally. For instance, in the timbers of the roof of the museum
attached to the Imperial College of Engineering in Tokio, there are a
number of diagonal faces acting as struts or ties intended to prevent
more or less horizontal movements taking place. Those which are rigidly
joined together with bolts and angle irons have apparently suffered
from their rigidity, being twisted and bent into various forms. The
buildings in Tokio, which are strongly put together, being especially
designed to withstand earthquakes, appear to have suffered but little.
I know only one example which at the time of the severe shock of 1880
had several of its chimneys damaged.
The ordinary houses in Italy, though built of stone and mortar, are
but poorly put together, and, as Mallet has remarked, are in no way
adapted to withstand the frightful shakings to which they are subjected
from time to time.
In the large towns, like Naples, Rome, and Florence, where happily
earthquakes are of rare occurrence, although the building may be
better than that found in the country, the height of the houses and
the narrowness of the streets are sufficient to create a shudder, when
we think of the possibility of the occurrence of a moderately severe
earthquake.
In South America, although many buildings are built with brick
and stone, the ordinary houses, and even the larger edifices, are
specially built to withstand earthquakes. In Mr. James Douglas’s
account of a ‘Journey Along the West Coast of South America,’ we read
the following[25]: ‘The characteristic building material of Guayaquil
is bamboo, which grows to many inches in thickness, and which, when
cut partially through longitudinally at distances of an inch or so,
and once quite through, can be opened out into fine elastic boards of
serviceable width. Houses, and even churches, of a certain primitive
beauty are built of such reeds, so bound together with cords that few
nails enter into the construction, and which, therefore, yield so
readily to the contortions of the earth during an earthquake as to be
comparatively safe.’
Here we have a house, which, so far as earthquakes are concerned, is
an exaggerated example of the principles which are followed in the
construction of an ordinary Japanese dwelling.
Another plan adopted in South America can be gathered from the same
author’s writings upon Lima, about which he says, ‘To build high houses
would be to erect structures for the first earthquake to make sport of,
and, therefore, in order to obtain space, safety, and comfort, the
houses of the wealthy surround court after court, filled with flowers,
and cooled with fountains, connected one with another with wide
passages which give a vista from garden to garden.’
History would indicate that houses of this type have been arrived at as
the results of experience, for it is said that when the inhabitants of
South America first saw the Spaniards building tall houses, they told
them they were building their own sepulchres.[26]
In Jamaica, we find that even as early as 1692 experience had taught
the Spaniards to construct low houses, which withstood shakings better
than the tall ones.[27]
In Caraccas, which has been called the city of earthquakes, it is
said that the earthquakes cause an average yearly damage amounting to
the equivalent of a _per capita_ tax of four dollars. To reduce this
impost to a minimum much attention is paid to construction. ‘Projecting
basement corners (giving the house a slightly pyramidal appearance)
have been found better than absolutely perpendicular walls; mortised
corner-stones and roof beams have saved many lives when the central
walls have split from top to bottom; vaults and key-stone arches, no
matter how massive, are more perilous than common wooden lintels, and
there are not many isolated buildings in the city. In many streets
broad iron girders, riveted to the wall, about a foot above the house
door, run from house to house along the front of an entire square.
Turret-like brick chimneys, with iron top ornaments, would expose the
architect to the vengeance of an excited mob; the roofs are flat,
or flat terraced; the chimney flues terminate near the eaves in a
perforated lid.’[28]
_Typical houses for earthquake countries._—From what has now been said
about the different buildings found in earthquake countries, it will be
seen that if we wish to put up a building able to withstand a severe
shaking, we have before us structures of two types. One of these types
may be compared with a steel box, which, even were it rolled down a
high mountain, would suffer but little damage; and the other, with a
wicker basket, which would equally withstand so severe a test. Both
of these types may be, to some extent, protected by placing them upon
a loose foundation, so that but little momentum enters them at their
base. One suggestion is to place a building upon iron balls. Another
method would be to place them upon two sets of rollers, one set resting
upon the other set at right angles. The Japanese, we have seen, place
their houses on round stones. The solid type of building is expensive,
and can only be approached partially, whilst the latter is cheap, and
can be approached closely. In the case of a solid building it would be
a more difficult matter to support it upon a movable foundation than in
the case of a light framework. Such a building is usually firmly fixed
on the ground, and consequently at the time of an earthquake, as has
already been shown by experiment, must be subjected to stresses which
are very great. In consequence also of the greater weight of the solid
structure, more momentum will enter it at its base than in the case of
the light structure. Also, we must remember that the rigidity favours
the transmission of momentum, and with rigid walls we are likely to
have ornaments, coping-stones, and the comparatively freer portions
forming the upper part of a building displaced; whilst, with flexible
walls absorbing momentum in the friction of their various parts, such
disturbances would not be so likely. Mr. T. Ronaldson, referring to
this, says, that in 1868, at San Francisco, the ornamental stone work
in stone and cement buildings was thrown from its position, whilst
similar ornaments in neighbouring brick buildings stood.
To reduce the top weight of a building, hollow bricks might be
employed. To render a building more homogeneous and elastic, the
thickness of bricks might be reduced. Inasmuch as the elasticity
of brick and timber are so different, the two ought to be employed
separately. For internal decorations plaster mouldings might be
replaced by _papier mâché_ and _carton-pierre_, the elastic yielding of
which is comparatively great.[29] Houses, whether of brick and stone,
or of timber, ought to be broad and low, and the streets three or four
times as wide as the houses. The flatter the roofs the better.
One of the safest houses for an earthquake country would probably be a
one-storied strongly framed timber house, with a light flattish roof
made of shingles or sheet-iron, the whole resting on a quantity of
small cast-iron balls carried on flat plates bedded in the foundations.
The chimneys might be made of sheet-iron carried through holes free of
the roof. The ornamentation ought to be of light materials.
At the time of severe earthquakes many persons seek refuge from their
houses by leaving them. In this case accidents frequently happen from
the falling of bricks and tiles. Others rush to the doorways and
stand beneath the lintels. Persons with whom the author has conversed
have suggested that strongly constructed tables and bedsteads in
their rooms would give protection. To see persons darting beneath
tables and bedsteads would undoubtedly give rise to humiliating and
ludicrous exhibitions. This latter idea is not without a value, and
most certainly, if applied in houses of the type described, would be
valuable.
The great danger of fire may partially be obviated by: the use of
‘earthquake lamps,’ which are so constructed that before they overturn
they are extinguished. It is said that in South America some of the
inhabitants are ready at any moment to seek refuge in the streets,
and they have coats prepared, stocked with provisions and; other
necessaries, which, if occasion demands, will enable them to spend the
night in the open air. These coats, called ‘earthquake coats,’ might
also, with properly constructed houses, be rendered unnecessary.
_Destruction due to the nature of the underlying rocks._—That the
nature of the ground on which a building stands is intimately related
with the severity of the blow it receives is a fact which has often
been demonstrated.
One cause of destruction is due to placing a building on foundations
which are capable of receiving the full effects of a shock, and
transmitting it to the buildings standing on them.
For instance, the reason why a soft bed might possibly make a good
foundation, is, as has been pointed out by Messrs. Perry and Ayrton,
because the time of transmission of momentum is increased; in fact, the
soft bed is very like a piece of wood interposed between a nail and
the blows of a hammer—it lengthens the duration of impact. For this
reason we are told that a quaking bog will make a good foundation. When
a shock enters loose materials its waves will be more crowded, and it
is possible that a line of buildings may rest on more than one wave
during a shock. There are many examples on record of the stability of
buildings which rested on beds of particular material at the time of
destructive earthquakes. As the observations which have been made
by various writers on this subject appear to point in a contrary
direction, I give the following examples:—
In the great Jamaica earthquake of 1692, the portions of Port
Royal which remained standing were situated on a compact limestone
foundation; whilst those on sand and gravel were destroyed (‘Geological
Observer,’ p. 426). Again, on p. 148 of the same work, we read,
‘According to the observations made at Lisbon, in 1737, by Mr. Sharpe,
the destroying effects of this earthquake were confined to the tertiary
strata, and were most violent on the blue clay, on which the lower part
of the city is constructed. Not a building on the secondary limestone
or on the basalt was injured.’
In the great earthquakes of Messina, those portions of the town
situated on alluvium, near the sea, were destroyed, whilst the high
parts of the town, on granite, did not suffer so much. Similar
observations were made in Calabria, when districts consisting of
gravel, sand, and clay became, by the shaking, almost unrecognisable,
whilst the surrounding hills of slate and granite were but little
altered. At San Francisco, in 1868, the chief destruction was in the
alluvium and made ground.
At Talcahuano, in 1835, the only houses which escaped were the
buildings standing on rocky ground; all those resting on sandy soil
were destroyed.
From the results of observations like these, it would seem the harder
rocks form better foundations than the softer ones. The explanation of
this, in many cases, appears to lie in the fact that the soft strata
were in a state of unstable equilibrium, and by shaking, they were
caused to settle. Observations like the following, however, point out
another reason why soft strata may sometimes afford a bad foundation.
‘Humboldt observed that the Cordilleras, composed of gneiss and
mica-slate, and the country immediately at their foot, were more shaken
than the plains.’[30]
‘Some writers have asserted that the wave-like movements (of the
Calabrian earthquake in 1783) which were propagated through recent
strata from west to east, became very violent when they reached the
point of junction with the granite, as if a reaction was produced when
the undulatory movement of the soft strata was suddenly arrested by the
more solid rocks.’
Dolomieu when speaking of this earthquake says, the usual effect ‘was
to disconnect from the sides of the Apennines all those masses (of sand
and clay) which either had not sufficient bases for their bulk, or
which were supported only by lateral adherence.’
These intensified actions taking place at and near to lines of
junction between dissimilar strata is probably due to the phenomena of
reflection and refraction.
When referring to the question as to whether buildings situated on
loose materials suffered more or less than those on solid rocks,
Mallet, in his description of the Neapolitan earthquake of 1857,
remarks: ‘We have in this earthquake, towns such as Saponara and
Viggiano, situated upon solid limestone, totally prostrated; and we
have others such as Montemarro, to a great extent based upon loose
clays, totally levelled. We have examples of almost complete immunity
in places on plains of deep clay as that of Viscolione, and in places
on solid limestone, like Castelluccio, or perched on mountain tops like
Petina.’[31]
After reading the above, we see that the probable reason why, in
several cases, beds of soft materials have not made good foundations,
consists in the fact that they have either been of small extent or else
have been observed only in the neighbourhood of lines which divided
them from other formations, which lines are always those of great
disturbances.
At the end of his description of the Neapolitan earthquake of 1857,
Mallet says that more buildings were destroyed on the rock than on the
loose clay. This, however, he remarks, is hardly a fact from which we
can draw any valuable deductions, because it so happened that more
buildings were constructed on the hills than on the loose ground.[1]
Professor D. S. Martin, writing on the earthquake of New England in
1874, remarks that in Long Island the shock was felt where there was
gneiss between the drift. Around portions to the east the observations
were few and far between. He also remarks that generally the shocks
were felt more strongly and frequently on rocky than on soft ground.[32]
From these examples, it would appear that the hard ground, which
usually means the hills, forms a better foundation than the softer
ground, which is usually to be found in the valleys and plains. Other
examples, however, point to a different conclusion. For instance, a
civil engineer, writing about the New Zealand earthquake of 1855, when
all the brick buildings in Wellington were overthrown, says that ‘it
was most violent on the sides of the hills at those places, and least
so in the centre of the alluvial plains.’[33]
In this example it must be noticed that the soft alluvium here referred
to was of large extent, and not loose material resting on the flanks of
rocks, from which it was likely to be shaken down, as in most of the
previous examples.
The results of my own observations on this subject point as much in one
direction as in the other. In Tokio, from instrumental observations
upon the slopes and tops of hills, the disturbance appears to be
very much less than it is in the plains. Thus, at my house, situated
on the slope of a hill about 100 feet in height, for the earthquake
of March 11, 1882, I obtained a maximum amplitude of motion of from
three to four millimètres only, whilst Professor Ewing, with a similar
instrument, situated on the level ground at about a mile distant,
found a motion of fully seven millimètres. This calculation has been
confirmed by observations on other earthquakes. Thus, for instance, in
the destructive earthquake of 1855, when a large portion of Tokio was
devastated, it was a fact, remarked by many, that the disturbance was
most severe on the low ground and in the valleys, whilst on the hills
the shock had been comparatively weak. As another illustration, I may
mention that within three-quarters of a mile from my house in Tokio
there is a prince’s residence which has so great a reputation for the
severity of the shakings it receives that its marketable value has been
considerably depreciated, and it is now untenanted.
In Hakodadi, which is a town situated very similarly to Gibraltar,
partly built on the slope of a high rocky mountain and partly on a
level plain, from which the mountain rises, the rule is similar to
that for Tokio, namely, that the low, flat ground is shaken more
severely than the high ground. At Yokohama, sixteen miles south-west
from Tokio, the rule is reversed, as was very clearly demonstrated
by the earthquake of February 1880, when almost every house upon the
high ground lost its chimney, whilst on the low ground there was
scarcely any damage done; the only places on the low ground which
suffered were those near to the base of the hills. The evidence as to
the relative value of hard ground as compared with soft ground, for
the foundation of a building, is very conflicting. Sometimes the hard
ground has proved the better foundation and sometimes the softer, and
the superiority of one over the other depends, no doubt, upon a variety
of local circumstances.
These latter observations open up the inquiry as to the extent to which
the intensity of an earthquake may be modified by the topography of the
disturbed area.
_The swing of mountains._—If an earthquake wave is passing through
ground the surface of which is level, so long as this ground is
homogeneous, as the wave travels further and further we should expect
its energy to become less and less, until, finally, it would insensibly
die out. If, however, we have standing upon this plain a mountain,
judging from Mallet’s remarks, this mountain would be set in a state
of vibration much in the same way as a house is set in vibration,
and it would tend to oscillate backward and forward with a period of
vibration dependent upon the nature of its materials, size, and form.
The upper portion of this mountain would, in consequence, swing through
a greater arc than the lower portion, and buildings situated on the top
of it would swing to and fro through a greater arc than those which
were situated near its foot. This explanation why buildings situated
on the top of a mountain should suffer more than those situated on
a plain, is one which was offered by Mallet when writing of the
Neapolitan earthquake. He tells us that towns on hills are ‘rocked as
on the top of masts,’ and if we accept this explanation it would, in
fact, be one reason why the houses situated on the Bluff at Yokohama
suffered more than those situated in the settlement. This explanation
is given on account of the great authority it claims as a consequence
of its source. It is not clear how the statement can be supported,
as different portions of the mountain receive momentum in opposite
directions at the same time.
_Want of support on the faces of hills._—When a wave of elastic
compression is propagated through a medium, we see that the energy of
motion is being continually transmitted from particle to particle of
that medium. A particle, in moving forwards, meets with an elastic
resistance of the particles towards which it moves, but, overcoming
these resistances, it causes these latter particles to move, and in
turn to transmit the energy to others further on. So long as the medium
in which this transfer of energy is continuous, each particle has a
limit to its extent of motion, dependent on the nature of the medium.
When, however, the medium, which we will suppose to be the earth, is
not continuous, but suddenly terminates with a cliff or scarp, the
particles adjacent to this cliff or scarp, having no resistance offered
to their forward motion, are shot forward, and, consequently, the
ground here is subjected to more extensive vibrations than at those
places where it was continuous. This may be illustrated by a row of
marbles lying in a horizontal groove; a single marble rolled against
one end of this row will give a concussion which will run through the
chain, like the bumping of an engine against a row of railway cars, and
as a result, the marble at the opposite end of the row, being without
support, will fly off. Tyndall illustrates the same thing with his
well known row of boys, each one standing with his arms stretched out
and his hands resting upon the shoulders of the boy before him. A push
being given to the boy at the back, the effect is to transmit a push
to the first boy, who, being unsupported, flies forward.
In the case of some earthquakes, most disastrous results have occurred
which seem only to admit of an explanation such as this. A remarkable
instance of this kind occurred when the great earthquake of 1857 ‘swept
along the Alps from Geneva to the east-north-east, and its crest
reached the edge of the deep glen between Zermatt and Visp. Then the
upper part of the wave-movement, a thousand or two thousand feet in
depth from the surface, came to an end; the forward pulsation acted
like the breaker of the sea, and heavy falls of rock encumbered the
western side of the valley.’
_Earthquake shadows._—If a mountain stands upon a plain through which
an elastic wave is passing, which is almost horizontal, the mountain
is, so to speak, in the _shadow_ of such a wave. If we only consider
the normal motion of this wave, we see that the only motion which the
mountain can obtain will be a wave of elastic distortion produced
by a shearing force along the plain of the base. Should, however,
the wave approach the mountain from below, and emerge into it at a
certain angle, only the portion of the mountain on the side from
which the wave advanced could remain in shadow, whilst the portion
on the opposite side would be thrown into a state of compression and
extension. Portions in shadow, however, would be subject to waves
of elastic distortion. In a manner similar to this we may imagine
that certain portions of the bluff, so far as the advancing wave was
concerned, were in shadow, and thus saved from the immediate influence
of the direct shock. A hypothetical case of such a shadow is shown in
the accompanying section, illustrating the contour of the ground at
Yokohama. The situation which might be in the shadow of one shock,
however, it is quite possible might not be in that of another. We
must also remember that a place in shadow for a direct shock might be
affected by reflected waves, and also by the transverse vibrations of
the direct shock. These effects are over and above the effects produced
by the waves of elastic distortion just referred to. It might be asked
whether whole countries, like England, which are but seldom shaken, are
in shadow.
[Illustration: FIG. 27.—Hypothetical section at Yokohama.]
_Destruction due to the interference of waves._—Referring to the
section of the ground at Yokohama (Fig. 27), it will be seen that
both the settlement and the bluff stand upon beds of gravel capping
horizontal beds of grey tuff. The gravel of that portion of the
settlement on the seaboard originally formed the line of a shingle
beach. That portion of the settlement back from the sea stands upon
ground which was originally marshy. In the central portions of the
settlement this bed of gravel is very thick, perhaps 100 feet or so,
but as you near the edge of the bluff it probably becomes thinner,
until it finally dies out upon the flanks of the scarps.
On the top of the bluff, the beds of gravel will, in every probability,
be generally thinner than they are upon the lower level. The beds of
tuff, which is a soft grey-coloured clay-like rock, produced by the
solidification of volcanic mud, appear, when walking on the seaboard,
to be horizontally stratified. If there is a dip inland, it is in all
probability very slight. Here and there the beds slightly faulted.
Taken as a whole we may consider these beds as being tolerably
homogeneous, and an earthquake in passing through them would meet
with but little reflection or refraction. At the junction of these
beds with the overlying gravels, both reflection and refraction would
comparatively be very great.
On entering the gravel, as the wave would be passing into a less
elastic medium, the direction of the wave would be bent towards the
perpendicular to the line of junction, and the angle of emergence at
the surface would consequently be augmented. At the surface certain
reflection would also take place, but the chief reflections would be
those at the junction of the tuff and the alluvium.
Under the settlement it is probable that all the reflections which
took place would be single. Thus wave fronts like A_{1} advancing in
a direction parallel to the line _a__{1}; would be reflected in a
direction _a__{2} and give rise to a series of reflected waves A_{2}.
These are shown by thicker lines. Similarly all the neighbouring waves
to the right and left of A_{1} would give rise to a series of reflected
waves. If the lines drawn representing wave fronts are districts of
compression, then, where two of the lines cross each other, there would
be double energy in producing compression. Similarly, districts of
rarefaction might accord, and, again, compression of one wave might
meet with the rarefaction of another and a neutralisation of effect
take place. A diagram illustrating concurrence and interference of this
description is given in Le Conte’s ‘Elements of Geology,’ p. 115. The
interference which has been spoken of, however, is not the greatest
which would occur. The greatest would probably be beneath the bluff and
the scarps which run down to join the level ground below. This would be
the case because it is a probability that there might not only be cases
of interference of single reflected waves, but also of waves which
had been not only twice but perhaps thrice reflected. For example, a
wave like B_{1} (which is parallel to A_{1} of the first supposition),
advancing in a direction parallel to _b__{1} might be reflected along
the line _b__{2} giving rise to waves like B_{2}, which in turn might
be reflected along _b__{3} giving rise to waves like B_{3}. The number
of districts where there would be concurrence and interference would,
in consequence of the number of times waves might be reflected, be
augmented. Here the violence of the shock would, at certain points, be
considerably increased, but as a general result energy must be lost,
so that even if some of the reflected waves found their way into the
portion we have regarded as being in shadow, their intensity would not
be so great as if they had entered it directly.
The shaking down of loose materials from the sides of hills may be
partially explained on the assumption of an increased disturbance due
to interference.
_Earthquake bridges._—In certain parts of South America there appear
to exist tracts of ground which are practically exempt from earthquake
shocks, whilst the whole country around is sometimes violently shaken.
It would seem as if the shock passes beneath such a district as water
passes beneath a bridge, and for this reason these districts have been
christened ‘bridges.’
This phenomenon appears to depend upon the nature of the underlying
soil. When an elastic wave passes from one bed of rock to another of a
different character, a certain portion of the wave is reflected, while
the remainder of it is transmitted and refracted, and ‘bridges’ we
may conceive of as occurring where the phenomenon of total reflection
occurs.
In the instances given of soft materials having proved good
foundations, it was assumed that they had chiefly acted as absorbers
of momentum. They have also acted as reflecting surfaces, and where no
effects have been felt by those residing on them, this may have been
the result of total reflection, and the soft beds thus have played the
part of bridges.
Fuchs gives an example taken from the records of the Syrian earthquake
of 1837, where not only neighbouring villages suffered differently, but
even neighbouring houses. In one case a house was entirely destroyed,
whilst in the next house nothing was felt.
In Japan, at a place called Choshi, about 55 miles east of the capital,
earthquakes are but seldom felt, although the surrounding districts may
be severely shaken.
From descriptions of this place it would appear that there is a large
basaltic boss rising in the midst of alluvial strata. The immunity from
earthquakes in this district has probably given rise to the myth of
the Kanam rock, which is a stone supposed to rest upon the head of a
monstrous catfish (Namadzu), which by its writhings causes the shakings
so often felt in this part of the world.[34]
Prof. D. S. Martin, writing on the earthquake of New England in 1874,
says that it was felt at four points; it was felt in the heart of
Brooklyn all within a circle of half a mile across; ‘and this fact
would suggest that a ridge of rock perhaps approaches the surface at
that point, though none is known to appear.’[35]
The subject of special districts, which are more or less protected from
severe shakings, will be again referred to, and it will be seen that
after a seismic survey has been made even of a country like Japan,
where there are on the average at least two earthquakes per day, it
is possible to choose a place to build in as free from earthquakes as
Great Britain.
_General examples of earthquake effects._—The following examples of
earthquake effects are drawn from Mallet’s account of the Neapolitan
earthquake of 1857.
At a town called Polla there was great destruction. Judging from
the fissures in the parts that remained standing it seemed that the
emergence of the shock had been more vertical in the upper part of the
town than in the lower, proving that whatever had been the angle below,
the hill had itself vibrated, which, being horizontal, had modified the
angle of the fissures.
Diano suffered but little, partly because it was well built, and partly
on account of its situation, which was such that before the shock
reached it the disturbance had to pass from beds of clay into nearly
vertically placed beds of limestone. Also a great portion of the shock
was cut off by the Vallone del Raccio to the north and north-west of
the town. Here the effects of the partial extinction of the wave on the
‘free outlaying stratum’ were visible in the masses of projected rock.
Castellucio did not suffer because its well buttressed knoll was end
on to the direction of shock, and on account of a barrier of vertical
breccia beds protecting it upon the east.
Pertosa stands on a mound. The destruction was least in the southern
part of the town. From the relation of the beds of breccia on which the
town stands, and the direction of the wave path, it is evident that the
southern part of the town received the force of the shock through a
greater thickness of the breccia beds than the other parts did.
Petina, standing on a level limestone spur jutting out from a mountain
slope, suffered nothing, whilst Anletta five miles to the south-west,
and Pertosa six miles distant, were in great part prostrated. (1) The
terrace did not vibrate, and (2) between Petina and Anletta there is
almost 6,000 feet of piled up limestone, so that any shock emergent at
a steep angle had to pass up transversely through these beds.
_Protection of buildings._—In addition to giving proper construction
to our buildings, choosing proper foundations and positions for them,
something might possibly be done to ward off the destructive effects of
an earthquake. We read that the Temple of Diana at Ephesus was built on
the edge of a marsh, in order to ward off the effect of earthquakes.
Pliny tells us that the Capitol of Rome was saved by the Catacombs,
and Elisée Reclus[36] says that the Romans and Hellenes found out that
caverns, wells, and quarries retarded the disturbance of the earth,
and protected edifices in their neighbourhood. The tower of Capua was
saved by its numerous wells. Vivenzis asserts that in building the
Capitol the Romans sunk wells to weaken the effects of terrestrial
oscillations. Humboldt relates the same of the inhabitants of San
Domingo.
Quito is said to receive protection from the numerous cañons in the
neighbourhood, whilst Lactacunga, fifteen miles distant, has often been
destroyed.
Similarly, it is extremely probable that many portions of Tokio have
from time to time been protected more or less from the severe shocks of
earthquakes by the numerous moats and deep canals which intersect it.
Although we are not prepared to say how far artificial openings of this
description are effectual in warding off the shocks of earthquakes,
from theoretical considerations, and from the fact that their use has
been discovered by persons who, in all probability, were without the
means of making theoretical deductions, the suggestions which they
offer are worthy of attention.
_General conclusions._—The following are a few of the more important
results which may be drawn from the preceding chapter:—
1. In choosing a site for a house find out by the experience of others
or experimental investigation the localities which are least disturbed.
In some cases this will be upon the hills, in others in the valleys and
on the plains.
2. A wide open plain is less likely to be disturbed than a position on
a hill.
3. Avoid loose materials resting on harder strata.
4. If the shakings are definite in direction, place the blank walls
parallel to such directions, and the walls with many openings in them
at right angles to such directions.
5. Avoid the edges of scarps or bluffs, both above and below.
6. So arrange the openings in a wall, that for horizontal stresses the
wall shall be of equal strength for all sections at right angles.
7. Place lintels over flat arches of brick or stone.
8. To withstand destructive shocks either rigidly follow one or other
of the two systems of constructing an earthquake-proof building. The
light building on loose foundations is the cheaper and probably the
better.
9. Let all portions of a building have their natural periods of
vibration nearly equal.
10. If it is a necessity that one portion of a building should have a
very different period of vibration to the remainder, as for instance a
brick chimney in a wooden house, it would seem advisable either to let
these two portions be sufficiently free to have an independent motion,
or else they must be bound together with great strength.
11. Avoid heavy topped roofs and chimneys. If the foundations were free
the roof might be heavy.
12. In brick or stone work use good cement.
13. Let archways curve into their abutments.
14. Let roofs have a low pitch, and the tiles, especially those upon
the ridges, be well secured.