THE INTERRUPTED CHARACTER OF EARTH MOVEMENTS: EARTHQUAKES AND SEAQUAKES
=Nature of earthquake shocks.=—Man’s belief in the stability of
Mother Earth—the _terra firma_—is so inbred in his nature that even
a light shock of earthquake brings a rude awakening. The terror which
it inspires is no doubt largely to be explained by this disillusionment
from the most fundamental of his beliefs. Were he better advised, the
long periods of quiet which separate earthquakes, and not the lighter
shocks which follow all grander disturbances, would occasion him
concern.
[Illustration: FIG. 49.—View of a portion of the ruins of Messina
after the earthquake of December 28, 1908.]
Earthquakes are the sensible manifestations of changes in level or of
lateral adjustments of portions of the continents, and the seismic
disturbances upon the sea—seaquakes and seismic sea waves—relate to
similar changes upon the floor of the ocean.
During the grander or catastrophic earthquakes, the changes are
indeed terrifying, and have usually been accompanied by losses to
life and property, which are only to be compared with those of great
conflagrations or of inundations on thickly populated plains. The
conflagration has all too frequently been an aftermath of the great
historic earthquakes. The earthquake of December 28, 1908, in southern
Italy, destroyed almost the entire population of a great city, and left
of its massive buildings only a confused heap of rubble (Fig. 49). Two
years later a heavy earthquake resulted in great damage to cities in
Costa Rica (Fig. 50), while two years earlier our own country was first
really awakened to the danger in which it stands from these convulsive
earth throes; though, as we shall see, these dangers can be largely met
through proper methods of construction.
[Illustration:
FIG. 50.—Ruins of the Carnegie Palace of Peace at Cartago, Costa Rica,
destroyed when almost completed by the great earthquake of May 4, 1910
(after a photograph by Rear-Admiral Singer, U.S.N.).]
Earthquakes are usually preceded for a brief instant by subterranean
rumblings whose intensity appears to bear no relation to the shocks
which follow. The ground then rocks in wavelike motions, which, if of
large amplitude, may induce nausea, prevent animals from keeping upon
their feet, and wreck all structures not specially adapted to withstand
them. Heavy bodies are sometimes thrown up from the ground (Fig. 51),
and at other times similar heavy masses are, apparently because of
their inertia, more deeply imbedded in the earth. Thus gravestones and
heavy stone posts are often sunk more deeply in the ground and are
surrounded by a hollow and perhaps by small open cracks in the surface
(Fig. 52). When bodies are thrown upward, it would imply that a quick
upward movement of the ground had been suddenly arrested, while the
burial of heavy bodies in the earth is probably due to a movement which
begins suddenly and is less abruptly terminated.
[Illustration: FIG. 51.—Bowlders thrown into the air and overturned
during the Assam earthquake of 1897 (after R. D. Oldham).]
[Illustration:
FIG. 52.—Heavy post sunk deeper into the ground during the Charleston
earthquake of August 31, 1886 (after Dutton).]
=Seaquakes and seismic sea waves.=—Upon the ocean the quakes which
emanate from the sea floor are felt on shipboard as sudden joltings
which produce the impression that the ship has struck upon a shoal,
though in most instances there is no visible commotion in the
water. The distribution of these shocks, as indicated either by the
experiences of neighboring ships at the time of a particular shock, or
by the records of vessels which at different times have sailed over an
area of frequent seismic disturbance, appears to be limited to narrow
zones or lines (Fig. 53). The same tendency of under-sea disturbances
to be localized upon definite straight lines has been often illustrated
by the behavior of deep-sea cables which are laid in proximity to one
another and which have been known to part simultaneously at points
ranged upon a straight line.
[Illustration:
FIG. 53.—Map showing the localities at which shocks have been reported
at sea off Cape Mendocino, California.]
Far grander disturbances upon the floor of the ocean have been revealed
by the great sea waves—the so-called “tidal waves”, properly referred
to as _tsunamis_—which recur in those sea districts which adjoin the
special earthquake zones upon the continents (p. 86). The forerunner of
such a sea wave approaching the shore is usually a sudden withdrawal
of the water so as to lay bare a portion of the bottom, but this is
well-recognized to be the premonition of a gigantic oncoming wave
which sweeps all before it and is only halted when it has rolled over
all the low-lying country and encountered a mountain wall. Such
seismic waves have been especially common upon the Pacific shore of
South America and upon the Japanese littoral (Fig. 54). These waves
proceed from above the great deeps upon the ocean bottom, and clearly
result from the grander earth movements to which these depressions owe
their exceptional depth. The withdrawal of the water from neighboring
shores may be presumed to be connected with a descent of the floor
of the depression and the consequent drawing-in of the ocean surface
above. The later high wave would thus represent the dispersion of the
mountain of water which is raised by the meeting of the waters from the
different sides of the depression.
[Illustration: FIG. 54.—Effect of a seismic water wave at Kamaishi,
Japan, in 1896 (after E. R. Scidmore).]
[Illustration: FIG. 55.—A fault of vertical displacement.]
=The grander and the lesser earth movements.=—Upon the land the
grander and so-called catastrophic earthquakes are usually the
accompaniment of important changes in the surface of the ground that
will be discussed in later sections. Those shocks which do little
damage to structures produce no visible changes in the earth’s surface,
except, it may be, to shake down some water-soaked masses of earth upon
the steeper slopes. Still other movements, and these too slight to be
felt even in the night when the animal world is at rest, may yet be
distinguished by their sounds, the unmistakable rumblings which are
characteristic alike of the heaviest and the lightest of earthquake
shocks.
[Illustration:
FIG. 56.—Escarpment produced by an earthquake fault of vertical
displacement which cut across the Chedrang River and thus produced a
waterfall, Assam earthquake of 1897 (after R. D. Oldham).]
=Changes in the earth’s surface during earthquakes—faults and
fissures.=—Each of the grander among historic earthquakes has been
accompanied by noteworthy changes in the configuration of the earth’s
surface within the district where the shocks were most intense. A
section of the ground is usually found to have moved with reference
to another upon the other side of a vertical plane which is usually
to be seen; we have here to do with the actual making of a fault
or displacement such as we find the fossil examples of within the
rocks. The displacement, or throw, upon the fault plane may be either
upward or downward or laterally in one direction or the other, or
these movements may be combined. A movement of adjacent sections of
the ground upward or downward with reference to each other (Fig. 55)
has been often observed, notably at Midori after the great Japanese
earthquake of 1891, and in the Chedrang valley of Assam after the
earthquake of 1897 (Fig. 56).
[Illustration: FIG. 57.—A fault of lateral displacement.]
[Illustration:
FIG. 58.—Fence parted and displaced fifteen feet by a transverse fault
formed during the California earthquake of 1906 (after W. B. Scott).]
[Illustration:
FIG. 59.—Fault with vertical and lateral displacements combined.]
A lateral throw, unaccompanied by appreciable vertical displacement
(Fig. 57), is especially well illustrated by the fault in California
which was formed during the earthquake of 1906 (Fig. 58). A combination
of the two types of displacement in one (Fig. 59) is exemplified by the
Baishiko fault of Formosa at the place shown in plate 3 A.
=The measure of displacement.=—To afford some measure of the
displacements which have been observed upon earthquake faults, it may
be stated that the maximum vertical throw measured upon the fault in
the Neo valley of Japan (1891) was 18 feet, in the Chedrang valley of
Assam (1897) 35 feet, and of the Alaskan coast (1899) 47 feet. Large
sections of land were bodily uplifted in these cases within the space
of a few seconds, or at most a few minutes, by the amounts given. The
largest recorded lateral displacement measured upon an earthquake fault
is about 21 feet upon the California rift after the earthquake of 1906;
though an amount only slightly less than this is indicated in the
shifting of roads and arroyas dating from the earthquake of 1872 in the
Owens valley, California. Fault lines once established are planes of
special weakness and become later the seat of repeated movements of the
same kind.
┌──────────────────────────────────────────────────────────────────────┐
│ PLATE 3. │
│ │
│ [Illustration: _A._ An earthquake fault opened in Formosa in 1906, │
│ with vertical and lateral displacements combined (after Omori).] │
│ │
│ [Illustration: _B._ Earthquake faults opened in Alaska in 1889, on │
│ which vertical slices of the earth’s shell have undergone individual │
│ adjustments (after Tarr and Martin).] │
└──────────────────────────────────────────────────────────────────────┘
[Illustration:
FIG. 60.—Diagram to show how small faults in the rock basement may be
masked at the surface through adjustments within the loose rock mantle.]
The greater number of earthquake faults are found in the loose rock
cover which so generally mantles the firmer rock basement, and it is
almost certain that the throws within the solid rock are considerably
larger than those which are here measured at the surface, owing to
the adjustments which so readily take place in the looser materials.
Those lighter shocks of earthquake which are accompanied by no visible
displacements at the surface do, however, in some instances affect
in a measure the flow of water upon the surface, and thus indicate
that small changes of surface level have occurred without breaks
sufficiently sharp to be perceived (Fig. 60). Intermediate between the
steep escarpment and the masked displacement just described is the
so-called “mole-hill” effect,—a rounded and variously cracked slope or
ridge above the position of a buried fault (Fig. 61).
[Illustration:
FIG. 61.—Diagram to show the appearance of a “mole hill” above a
buried earthquake fault (after Kotô).]
The escarpments due to earthquake faults in loose materials at the
earth’s surface can obviously retain their steepness for a few years
or decades at the most; for because of their verticality they must
gradually disappear in rounded slopes under the action of the elements.
Smaller displacements within a rock which rapidly disintegrates under
the action of frost and sun will likewise before long be effaced. In
those exceptional instances where a resistant rock type has had all
altered upper layers planed away until a fresh and hard surface is
exposed, and has further been protected from the frost and sun beneath
a thin layer of soil, its original surface may be retained unaltered
for many centuries. Upon such a surface the lightest of sensible
shocks, or even the smaller earth movements which are not perceived
at the time, may leave an almost indelible record. Such records
particularly show that the movements which they register occur upon the
planes of jointing within the rock, and that these ready formed cracks
have probably been the seats of repeated and cumulative adjustments
(Fig. 62).
[Illustration:
FIG. 62.—Post-glacial earthquake faults of small but cumulative
displacement, eastern New York (after Woodworth).]
[Illustration: FIG. 63.—Earthquake cracks in Colorado desert (after a
photograph by Sauerven).]
=Contraction of the earth’s surface during earthquakes.=—The wide
variations in the amount of the lateral displacement upon earthquake
faults, like those opened in California in 1906, show that at the
time of a heavy earthquake there must be large local changes in the
density of the surface materials. Literally, thousands of fissures may
appear in the lowlands, many of them no doubt a secondary effect of
the shaking, but others, like the _quebradas_ of the southern Andes or
the “earthquake cracks” in the Colorado desert (Fig. 63), may have a
deeper-seated origin. Many facts go to show, however, that though local
expansion does occur in some localities, a surface contraction is a
far more general consequence of earth movement. In civilized countries
of high industrial development, where lines of metal of one kind or
another run for long distances beneath or upon the surface of the
ground, such general contraction of the surface may be easily proven.
Comparatively seldom are lines of metal pulled apart in such a way as
to show an expansion of the surface; whereas bucklings and kinkings of
the lines appear in many places to prove that the area within which
they are found has, as a whole, been reduced.
[Illustration: FIG. 64.—Diagrams to show how railway tracks are either
broken or buckled locally within the district visited by an earthquake.]
[Illustration: FIG. 65.—The Biwajima railroad bridge in Japan after
the earthquake of 1891 (after Milne and Burton).]
[Illustration:
FIG. 66.—Diagrams to show how the compression of a district and its
consequent contraction during an earthquake may close up the joint
spaces within the rock basement and concentrate the contraction of the
overlying mantle where this is partially cut through and so weakened in
the valley sections.]
Water pipes laid in the ground at a depth of some feet may be bowed up
into an arch which appears above the surface; lines of curbing are
raised into broken arches, and the tracks of railways are thrown into
local loops and kinks which imply a very considerable local contraction
of the surface (Fig. 64). With unvarying regularity railway or other
bridges which cross rivers or ravines, if the structures are seriously
damaged, indicate that the river banks have drawn nearer together at
the time of the disturbance. In such cases, whenever the bridge girder
has remained in place upon its abutments, these have either been broken
or back-tilted as a whole in such a manner as to indicate an approach
of the foundations which was prevented at the top by the stiffness of
the girder (Fig. 65).
[Illustration:
FIG. 67.—Map of the Chedrang fault which made its appearance during
the Assam earthquake of 1897. The figures give the amounts of the local
vertical displacement measured in feet (after R. D. Oldham).]
The simplest explanation of such an approach of the banks at the
sides of the valleys cut in loose surface material is to be found in
a general closing up of the joint spaces within the underlying rock,
and an adjustment of the mantle upon the floor mainly in the valley
sections (Fig. 66).
[Illustration:
FIG. 68.—Map giving the displacements in feet measured along an
earthquake fault formed in Alaska in 1899 (after Tarr and Martin).]
=The plan of an earthquake fault.=—In our consideration of earthquake
faults we have thus far given our attention to the displacement as
viewed at a single locality only. Such displacements are, however,
continued for many miles, and sometimes for hundreds of miles; and when
now we examine a map or plan of such a line of faulting, new facts of
large significance make their appearance. This may be well illustrated
by a study of the plan of the Chedrang fault which appeared at the
time of the Assam earthquake of 1897 (Fig. 67). From this map it
will be noticed that the upward or downward displacement upon the
perpendicular plane of the fault is not uniform, but is subject to
large and _sudden_ changes. Thus in order the measurements in feet are
32, 0, 18, 35, 0, 8, 25, 12, 8, 2, 0. The fault formed in 1899 upon
the shores of Russell Fjord in Alaska (Fig. 68) reveals similar sudden
changes of throw, only that here the direction of the movement is often
reversed; or, otherwise expressed, the upthrow is suddenly transferred
from one side of the fault to the other. Such abrupt changes in the
direction of the displacement have been observed upon many earthquake
faults, and a particularly striking one is represented in Fig. 69.
[Illustration:
FIG. 69.—Abrupt change in the direction of throw upon an earthquake
fault which was formed in the Owens valley, California, in 1872. The
observer looks directly along the course of the fault from the left
foreground to the cliff beyond and to the left of the impounded water
(after a photograph by W. D. Johnson).]
=The block movements of the disturbed district.=—The displacements
upon earthquake faults are thus seen to be subdivided into sections,
each of which differs from its neighbors upon either side and is
sharply separated from them, at least in many instances. These points
of abrupt change of displacement are, in many cases at least, the
intersection points with transverse faults (Fig. 69). Such points of
abrupt change in the degree or in the direction of the displacement may
be, when looked at from above, abrupt turning points in the direction
of extension of the fault, whose course upon the map appears as a
zigzag line made up of straight sections connected by sharp elbows
(Fig. 70).
[Illustration:
FIG. 70.—Map of the faults within an area of the Owens valley,
California, formed in part during the earthquake of 1872, and in part
due to early disturbances, In the western portions the displacements
cut across firm rock and alluvial deposits alike without deviation of
direction (after a map by W. D. Johnson).]
Such a grouping of surface faults as are represented upon the map is
evidence that the area of the earth’s shell, which is included, has at
the time of the earthquake been subject to adjustments as a series of
separate units or blocks, certain of the boundaries of which are the
fault lines represented. The changes in displacement measured upon the
larger faults make it clear that the observed faults can represent but
a fraction of the total number of lines of displacement, the others
being masked by variations in the compactness of the loose mantling
deposits. Could we but have this mantle removed, we should doubtless
find a rock floor separated into parts like an ancient Pompeiian
pavement, the individual blocks in which have been thrown, some upward
and some downward, by varying amounts. Less than a hundred miles away
to the eastward from the Owens Valley, a portion of this pavement has
been uncovered in the extensive operations of the Tonapah Mining
District, so that there we may study in all its detail the elaborate
pattern of earth marquetry (Fig. 71) which for the floor of the Owens
valley is as yet denied us.
[Illustration:
FIG. 71.—Marquetry of the rock floor of the Tonapah Mining District,
Nevada (after Spurr).]
[Illustration:
FIG. 72.—Map of a portion of the Alaskan coast to show the adjustments
in level during the earthquake of 1899 (after Tarr and Martin).]
=The earth blocks adjusted during the Alaskan earthquake of 1899.=—For
a study of the adjustments which take place between neighboring earth
blocks during a great earthquake, the recent Alaskan disturbance
has offered the advantage that the most affected district was upon
the seacoast, where changes of level could be referred to the datum
of the sea’s surface. Here a great island and large sections of the
neighboring shore underwent movements both as a whole in large blocks
and in adjustments of their subordinate parts among themselves (Fig.
72). Some sections of the coast were here elevated by as much as 47
feet, while neighboring sections were uplifted by smaller amounts (Fig.
73), and certain smaller sections were even dropped below the level
of the sea.
[Illustration:
FIG. 73.—View on Haencke Island, Disenchantment Bay, Alaska, revealing
the shore that rose seventeen feet above the sea during the earthquake
of 1899, and was found with barnacles still clinging to the rock (after
Tarr and Martin).]
The amount of such subsidence is, however, difficult
to ascertain, for the reason that the former shore features are now
covered with water and thus removed from observation. In favorable
localities the minimum amount of submergence may sometimes be measured
upon forest trees which are now flooded with sea water. In Fig. 74
a portion of the coast is represented where the beach sand is now
extended back into the spruce forest, a distance of a hundred feet or
more, and where sedgy beach grass is growing among trees whose roots
are now laved in salt water. At the front of this forest the great
storm waves overturn the trees and pile the wreckage in front of those
that still remain standing.
[Illustration:
FIG. 74.—Partially submerged forest upon the shore of Knight Island,
Alaska, due to the sinking of a section of the coast during the
earthquake of 1899 (after Tarr and Martin).]
[Illustration:
FIG. 75.—Settlement of a section of the shore at Port Royal, Jamaica,
during the earthquake of January 14, 1907, adjacent to a similar but
larger settlement of the near shore during the earthquake of 1692
(after a photograph by Brown).]
Upon the glaciated rock surfaces of the Alaskan coast, exceptionally
favorable opportunities are found for study of the intricate pattern
of the earth mosaic which is under adjustment at the time of an
earthquake. Upon Gannett Nunatak the surface was found divided by
parallel faults into distinct slices which individually underwent small
changes of level (plate 3 B).