THE INTERRUPTED CHARACTER OF EARTH MOVEMENTS: EARTHQUAKES AND SEAQUAKES
(Concluded)
=Experimental demonstration of earth movements.=—The study of the
Alaskan earthquake of 1899 showed that during this adjustment within
the earth’s shell some of the local blocks moved upward and by larger
amounts than their neighbors, and that still others were actually
depressed so that the sea flowed over them. It must be evident that
such differential vertical movements of neighboring blocks at the
earth’s surface can only take place if lateral transfers of material
are made beneath it. From under those strips of coast land which were
depressed, material must have been moved so as to fill the void which
would otherwise have formed beneath the sections that were uplifted.
If we take into consideration much larger fractions upon the surface
of our planet, we are taught by the great seaquakes which are now
registered upon earthquake instruments at distant stations that large
_downward_ movements are to-day in progress beneath the sea much more
than sufficient to compensate all extensions of the earth’s surface
within those districts where the land is rising in mountains. From
under the offshore deeps of the ocean to beneath the growing mountains
upon the shore, a transfer of earth material must be assumed to take
place when disturbances are registered.
Within the time interval that separates the sudden adjustments of
the surface which are manifested in earthquakes, the condition of
strain which brings them about is steadily accumulating, due, as we
generally assume, to earth contraction through loss of its heat. It
seems probable that the resistance to an immediate adjustment is found
in the rigidity of the shell because of the compression to which it is
subjected. To illustrate: a row of blocks well fitted to each other
may be held firmly as a bridge between the jaws of a vice, because so
soon as each block starts to fall a large resistance from friction upon
its surface is called into existence, a force which increases with the
degree of compression.
It is thus possible upon this assumption crudely to demonstrate the
adjustment of earth blocks by the simple device represented in plate
4 A. The construction of this experimental tank is so simple that
little explanation is necessary. Wooden blocks of different heights
are supported in water within a tank having a glass front, and are
kept in a strained condition at other than their natural positions of
flotation by the compression of a simple vice at the top. Held firmly
in this position, they may thus represent the neighboring blocks within
the earth’s outer shell which are supported upon relatively yielding
materials beneath, and prevented from at once adjusting themselves
to their natural positions through the compression to which they are
subjected. Held as they now are, the water near the ends of the tank
is forced up beneath the blocks to higher than its natural level, and
thus tends to flow from both ends toward the center. Such a movement
would permit the end blocks to drop and force the middle ones to rise.
The end blocks are, let us say, the sections of Alaskan coast line
which sunk during the earthquake, as the center blocks are the sections
which rose the full measure of 47 feet. Upon a larger scale the end
blocks may equally well be considered as the floor of the great deeps
off the Alaskan coast, whose sinking at the time of the earthquake was
the cause of the great sea wave. Upon this assumption the center blocks
would represent the Alaskan coast regarded as a whole, which underwent
a general uplift.
Though we may not, in our experiment, vary the tendency to adjustment
by any contractional changes in either the water or the blocks, we may
reduce the compression of the vice, which leads to the same general
result. As the compression of the vice is slowly relaxed, a point is
at last reached at which friction upon the block surfaces is no longer
sufficient to prevent an adjustment taking place, and this now suddenly
occurs with the result shown in plate 4 B. In the case of the earth
blocks, this sudden adjustment is accompanied by mass movements of the
ground separated by faults, and these movements produce successional
vibrations that are particularly large near the block margins, and
other frictional vibrations of such small measure as to be generally
appreciated by sounds only. The jolt of the adjustments has thrown some
blocks beyond their natural position of rest, and these sink and rise
subsequently in order to readjust themselves with lighter vibrations,
which may be repeated and continued for some time. In the case of the
earth these later adjustments are the so-called _aftershocks_ which
usually continue throughout a considerable period following every great
earthquake. Gradually they fall off in intensity and frequency until
they can no longer be felt, and are thereafter continued for a time as
rumblings only.
┌───────────────────────────────────────────────────────────────────────┐
│ PLATE 4. │
│ │
│ [Illustration: │
│ │
│ _A._ Experimental tank to illustrate the earth movements which are │
│ manifested in earthquakes. The sections of the earth’s shell are here │
│ represented before adjustment has taken place.] │
│ │
│ [Illustration: _B._ The same apparatus after a sudden adjustment.] │
│ │
│ [Illustration: _C._ Model to illustrate a block displacement in rocks │
│ which are intersected by master joints.] │
└───────────────────────────────────────────────────────────────────────┘
=Derangement of water flow by earth movement.=—The water which
supported the blocks in our experiment has represented the more mobile
portion of the earth’s substance beneath its outer zone of fracture.
The surface water layers in the tank may, however, be considered
in a different way, since their behavior is remarkably like that
of the water within and upon the earth’s surface during an earth
adjustment. At the instant when adjustment takes place in the tank,
water frequently spurts upward from the cracks between the sinking end
blocks; and if in place of one of the higher center blocks we insert
one whose top is below the level of the water in the tank, a “lake”
will be formed above it. When the adjustment occurs, this lake is
immediately drained by outflow of the water at its bottom along one of
the cracks between the blocks (Fig. 76).
[Illustration:
FIG. 76.—Diagrams to illustrate the draining of lakes during
earthquakes.]
Such derangements of water flow as have been illustrated by the
experiment are among the commonest of the phenomena which accompany
earthquakes. Lakes and swamp lands have during earthquakes been
suddenly drained, fountains of water have been seen to shoot up
from the surface and have played for some minutes or hours before
their sudden disappearance in a sucking down of the water with later
readjustment. During the great earthquake of the lower Mississippi
valley in 1811, known as the New Madrid earthquake, the earlier Lake
Eulalie was completely drained, and upon the now exposed bed there
appeared parallel fissures on which were ranged funnel-like openings
down which the water had been sucked. In other sections of the affected
region the water shot up in sheets along fissures to the tops of high
trees. Areas where such spurting up of the water has been observed have
in most cases been shown to correspond to areas of depression, and such
areas have sometimes been left flooded with water. During the Indian
earthquake of 1819 an area of some 200 square miles suddenly sank and
was transformed into a lake.
[Illustration:
FIG. 77.—Diagram to illustrate-the derangements of flow of water at
the time of an earthquake; water issuing at the surface over downthrown
rocks, and being sucked down in upthrown blocks.]
[Illustration:
FIG. 78.—Mud cones aligned upon a fissure opened at Moraza, Servia,
during the earthquake of April 4, 1904 (after Michailovitch).]
=Sand or mud cones and craterlets.=—From a very moderate depth below
the surface to that of several miles, all pore spaces and all larger
openings within the rock are completely filled with water, the “trunk
lines” of whose circulation is by way of the joints or along the
bedding planes of the rocks. The principal reservoirs, so to speak, of
this water inclosed within the rock are the porous sand formations.
When, now, during an earthquake a block of the earth’s shell is
suddenly sunk and as suddenly arrested in its downward movement, the
effect is to compress the porous layers and so force the contained
water upward along the joints to the surface, carrying with it large
quantities of the sand (Fig. 77).
[Illustration:
FIG. 79.—One of the many craterlets formed near Charleston, South
Carolina, during the earthquake of August 31, 1886. The opening is
twenty feet across, and the leaves about it are encased in sand as were
those upon the branches of the overhanging trees to a height of some
twenty feet (after Dutton).]
[Illustration:
FIG. 80.—Cross section of a craterlet to show the trumpet-like form of
the sand column.]
Ejected at the surface this water appears in fountains usually arranged
in line over joints, or even in continuous sheets, and the sand
collecting about the jets builds up lines of _sand_ or _mud cones_
sometimes described as “mud volcanoes” (Fig. 78). The amount of sand
thus poured out is sometimes so great that blankets of quicksand are
spread over large sections of the country. Most frequently, however,
the sand is not built above the general level of the surface, but forms
a series of _craterlets_ which are largely shaped as the water is
sucked down at the time of the readjustment with which the play of such
earthquake fountains is terminated (Fig. 79). Subsequent excavations
made about such craterlets have shown them to have the form of a
trumpet, and that in the sand which so largely fills them there are
generally found scales of mica and such light bodies as would be picked
out from the heterogeneous materials of the sand layers and carried
upward in the rush of water to the surface (Fig. 80).
=The earth’s zones of heavy earthquake.=—Since earthquakes give notice
of a change of level of the ground, the special danger zones from this
source are the growing mountain systems which are usually found near
the borders of the sea. Such lines of mountains are to-day rising where
for long periods in the past were the basins of deposition of former
seas. They thus represent the zones upon the earth’s surface which
are the most unstable—which in the recent period have undergone the
greatest changes of level.
[Illustration:
FIG. 81.—Map of the island of Ischia to show how the shocks of recent
earthquakes have been concentrated at the crossing point of two
fractures (after Mercalli and Johnston-Lavis).]
By far the most unstable belt upon the earth’s surface is the rim
surrounding the Pacific Ocean, within which margin it has been
estimated that about 54 per cent of the recorded shocks of earthquake
have occurred. Next in importance for seismic instability is the
zone which borders both the Mediterranean Sea and the Caribbean—the
American Mediterranean—and is extended across central Asia through
the Himalayas into Malaysia. Both zones approximate to great circles
upon the earth’s surface and intersect each other at an angle of about
67°. It has been estimated that about 95 per cent of the recorded
continental earthquakes have emanated from these belts.
[Illustration:
FIG. 82.—A line of earth fracture indicated in the plan of the relief,
which may at any time become the seat of movement and resultant shock.]
=The special lines of heavy shock.=—Within any earthquake district
the shocks are not felt with equal severity at all places, but there
are, on the contrary, definite lines which the disturbance seems to
search out for special damage. From their relations to the relief of
the land these lines would appear to be lines of fracture upon the
boundaries of those sections of the crust that play individual rôles in
the block adjustment which takes place. More or less masked as these
lines are beneath the rounded curves of the landscape, they are given
an altogether unenviable prominence with each succeeding earthquake. At
such times we may think of the earth’s surface as specially sensitized
for laying bare its hidden structure, as is the sensitized plate under
the magical influence of the X rays.
When, at the time of an earthquake, blocks are adjusted with reference
to their neighbors, the movements of oscillation are greatest in
those marginal portions of direct contact. Corners of blocks—the
intersecting points of the important faults—should for the same
reason be shaken with a double violence, and this assumption appears
to be confirmed by observation. Upon the island of Ischia, off the
Bay of Naples, the shocks from recent earthquakes have been strangely
concentrated near the town of Casamicciola, which was last destroyed
in 1883. This unfortunate city lies at the crossing point of important
fractures whose course upon the island is marked by numerous springs
and _suffioni_ (Fig. 81).
=Seismotectonic lines.=—The lines of important earth fractures, as
will be more clearly shown in the sequel (p. 227), are often indicated
with some clearness by straight lines in the plan of the surface
relief (Fig. 82). Lines of this nature are easily made out upon
the map of the West Indies, and if we represent upon it by circles
of different diameters the combined intensities of the recorded
earthquakes in the various cities, it appears that the heavily shaken
localities are ranged upon lines stamped out in the relief, with
the most severely damaged places at their intersections (Fig. 83).
These lines of exceptional instability are known as _seismotectonic
lines_—earthquake structure lines.
[Illustration: FIG. 83.—Seismotectonic lines of the West Indies.]
=The heavy shocks above loose foundations.=—It is characteristic of
faults that they soon bury themselves from sight under loose materials,
and are thus made difficult of inspection. The escarpment which is the
direct consequence of a vertical displacement upon a fault tends to
migrate from the place of its formation, rounding the surface as it
does so and burying the fault line beneath its deposits (Fig. 43, p.
60).
This is not, however, the sole reason why loose foundations should be
places of special danger at the time of earth shocks, for the reason
that earthquake waves are sent out in all directions from the surfaces
of displacement through the medium of the underlying rock. These
waves travel within the firm rock for considerable distances with
only a gradual dissipation of their energy, but with their entry into
the loose surface deposits their energy is quickly used up in local
vibrations of large amplitude, and hence destructive to buildings.
[Illustration:
FIG. 84.—Device to illustrate the different effects upon the
transmission and the character of shocks which are produced by firm
rock and by loose materials.]
The essential difference between firm rock and such loose materials as
are found upon a river bottom or in the “made land” about our cities
may be illustrated by the simple device which is represented in Fig.
84. Two similar metal pans are suspended from a firm support by bands
of steel and “elastic” braid of similar size and shape, and carry each
a small block of wood standing upon its end. Similar light blows are
now administered directly to the pans with the effect of upsetting that
block which is supported by the loose braid because of the large range
or amplitude of movement that is imparted to the pan. The “elastic”
braid, because of these large vibrations of which it is susceptible,
may represent the loose materials when an earthquake wave passes into
them. In the case of the steel support, the energy of the blow, instead
of being dissipated in local swingings of the pan, is to a large extent
transmitted through the elastic metal to materials beyond. The steel
thus resembles in its high elasticity the firmer rock basement, which
receives and transmits the earthquake shocks, but except when ruptured
in a fault is subject to vibrations of small amplitude only.
=Construction in earthquake regions.=—Wherever earthquakes have
been felt, they are certain to occur again; and wherever mountains
are growing or changes of level are in progress, there no record of
past earthquakes is required in order to forecast the future seismic
history. Although the future earthquakes may be predicted, the time of
their coming is, fortunately or unfortunately, still hidden from us. If
one’s lot is to be cast in an earthquake country, the only sane course
to pursue is to build with due regard to future contingencies.
The danger, from destructive fires may to-day be largely met by methods
of construction which levy an additional burden of cost. Though the
danger from seismic disturbances can hardly be met as fully as that
from fire, yet it is true that buildings may be so constructed as to
withstand all save those heaviest shocks in the immediate vicinity of
the lines of large displacement. Here, also, a considerable additional
expense is involved in the method of construction, in the case of
residences particularly.
From what has been said, it is obvious that much of the danger from
earthquakes can be met by a choice of site away from lines of important
fracture and from areas of relatively loose foundation. The choice of
building materials is next of importance. Those buildings which succumb
to earthquakes are in most cases racked or shaken apart, and thus they
become a prey to their own inherent properties of inertia. Each part of
a structure may be regarded as a weight which is balanced upon a stiff
rod and pivoted upon the ground. When shocks arrive, each part tends
to be thrown into vibration after the manner of an inverted pendulum.
In proportion, therefore, as the weights are large and rest upon long
supports, the danger of overthrow and of tearing apart is increased.
In general, structures are best constructed of light materials whose
weight is concentrated near the ground. Masonry structures, and
especially high ones, are, therefore, the least suited for resisting
earthquakes, of which the late complete destruction of the city of
Messina is a grewsome reminder. Despite repeated warnings in the past,
the buildings of that stricken city were generally constructed of heavy
rubble, which in addition had been poorly cemented (Fig. 49, p. 67).
Such structures are usually first ruptured at the edges and corners,
since here the vibrations which tend to tear the building asunder are
resisted by no supports and are reënforced from neighboring walls.
[Illustration:
FIG. 85.—House wrecked in San Francisco earthquake of 1906 because the
floors and partitions were not securely fastened to the walls (after R.
L. Humphrey).]
An advantage of the first importance is evidently secured if the rods
of the pendulum, of which the building is conceived to be composed,
have sufficient elasticity to be considerably distorted without
rupture and to again recover their original position. This is the
supreme advantage of structural steel for all large buildings, which
is coupled, however, with the disadvantage that the riveted fastenings
are apt to be quickly sheered off under the vibrations. Large and high
buildings, when sufficiently elastic, have fortunately the property of
destroying the earth waves by interference before they have traveled
above the lower stories.
For large structures in which wood cannot be used, strongly reënforced
concrete is well adapted, for it has in general the same advantages
as steel with somewhat reduced elasticity, but with a more effective
binding together of the parts. This requirement of thorough bracing
and tying together of the several parts of a building causes it to
vibrate, not as many pendulums, but as one body. If met, it removes
largely the danger from racking strains, and for small structures
particularly it is the requirement which is most easily complied with.
For such buildings it is therefore necessary that the framework should
be built in a close network with every joint firmly braced and with
all parts securely tied together. Especial attention should be given
to the fastenings of floor and partition ends. The house shown in Fig.
85 could not have been subjected to heavy shocks, for though the walls
are thrown down, the floors and partitions have been left near their
original positions.
[Illustration:
FIG. 86.—Building wrecked at San Mateo, California, during the late
earthquake. The heavy roof and upper floor, acting as a unit, have
battered down the upper walls (after J. C. Branner).]
This tendency of the walls, floors, partitions, and roof to act as
individual units in the vibration, is one that must be reckoned with
and be met by specially effective bracing and tying at the junctions.
Otherwise these larger parts of the structure may act like battering
rams to throw over the walls or portions of them (Fig. 86).
READING REFERENCES FOR CHAPTERS VII AND VIII
General works:—
JOHN MILNE. Seismology. London, 1898, pp. 320.
C. E. DUTTON. Earthquakes in the Light of the New Seismology. Putnam,
New York, 1904, pp. 314.
A. SIEBERG. Handbuch der Erdbebenkunde. Braunschweig, 1904, pp. 362.
COUNT F. DE MONTESSUS DE BALLORE. Les Tremblements de Terre,
Géographie Séismologique. Paris, 1906, pp. 475; La Science
Séismologique. Paris, 1907, pp. 579.
WILLIAM HERBERT HOBBS. Earthquakes, an Introduction to Seismic
Geology. Appleton, New York, 1907, pp. 336.
C. G. KNOTT. The Physics of Earthquake Phenomena. Clarendon Press,
Oxford, 1908, pp. 283.
E. RUDOLPH. Ueber Submarine Erdbeben und Eruptionen, Beiträge zur
Geophysik, vol. 1, 1887, pp. 133-365; vol. 2, 1895, pp. 537-666; vol.
3, 1898, pp. 273-536.
Descriptive reports of some important earthquakes:—
C. E. DUTTON. The Charleston Earthquake of August 31, 1886, 9th Ann.
Rept. U. S. Geol. Surv., 1889, pp. 203-528.
B. KOTÔ. On the Cause of the Great Earthquake in Central Japan, 1891,
Jour. Coll. Sci. Imp. Univ., Tokyo, Japan, vol. 5, 1893, pp. 295-353,
pls. 28-35.
JOHN MILNE and W. K. BURTON. The Great Earthquake of Central Japan.
1891, pp. 10, pls. 30.
R. D. OLDHAM. Report on the Great Earthquake of 12th June, 1897, Mem.
Geol. Surv. India. Vol. 29, 1899, pp. 379, pls. 42.
A. C. LAWSON, and others. The California Earthquake of April 18, 1906,
Report of the State Earthquake Investigation Commission, three quarto
vols. (Carnegie Institution of Washington); many plates and figures.
_Italian Photographic Society_, Messina and Reggio before and after
the Earthquake of December 28, 1908 (an interesting collection of
pictures). Florence, 1909.
R. S. TARR and L. MARTIN. Recent Changes of Level in the Yakutat Bay
Region, Alaska, Bull. Geol. Soc. Am., vol. 17, 1906, pp. 29-64, pls.
12-23.
WILLIAM HERBERT HOBBS. The Earthquake of 1872 in the Owens Valley,
California, Beiträge zur Geophysik, vol. 10, 1910, pp. 352-385, pls,
10-23.
Faults in connection with earthquakes:—
WILLIAM H. HOBBS. On Some Principles of Seismic Geology, Beiträge zur
Geophysik, vol. 8, 1907, Chapters iv-v.
Expansion or contraction of the earth’s surface during earthquakes:—
WILLIAM H. HOBBS. A Study of the Damage to Bridges during Earthquakes,
Jour. Geol., vol. 16, 1908, pp. 636-653; The Evolution and the Outlook
of Seismic Geology, Proc. Am. Phil. Soc., vol. 48, 1909, pp. 27-29.
Earthquake construction:—
JOHN MILNE. Construction in Earthquake Countries, Trans. Seis. Soc.,
Japan, vol. 14, 1889-1890, pp. 1-246.
F. DE MONTESSUS DE BALLORE. L’art de bâtir dans les pays à
tremblements de terre (34th Congress of French Architects),
L’Architecture, 193 Année, 1906, pp. 1-31.
GILBERT, HUMPHREY, SEWELL, and SOULÉ. The San Francisco Earthquake and
Fire of April 18, 1906, and their Effects on Structures and Structural
Materials, Bull. 324, U. S. Geol. Surv., 1907, pp. 1-170, pls. 1-57.
WILLIAM H. HOBBS. Construction in Earthquake Countries, The
Engineering Magazine, vol. 37, 1909, pp. 1-19.
LEWIS ALDEN ESTES. Earthquake-proof Construction, a discussion of the
effects of earthquakes on building construction with special reference
to structures of reënforced concrete, published by Trussed Concrete
Steel Company. Detroit, 1911, pp. 46.