Fig. 38.—Diagram to show the different combinations of the series composing two double sets of master joints, and in a, a, a additional disorderly fractures.
The system of the fractures.—In referring to experiments made upon the fracture of solid blocks under compression (p. 41), it was shown that two series of parallel fractures develop perpendicular to each free surface of the block, and that these series are each of them inclined by half of a right angle to the direction of compression, and thus perpendicular to each other. The fragments into which a block with one free surface would thus tend to be divided should be square prisms perpendicular to the free surface. It would be interesting, if it were practicable, to learn from experiment how these prisms would be further fractured by a continuation of the compression. From mechanical considerations involving the resolution of forces with reference to the ready-formed fractures, it seems probable that the next series of fractures to form would bisect the angles of the first double series or set. Wherever rocks are found exposed in their original attitudes, they are, in fact, seen to be intersected by two parallel series of fractures which are perpendicular to the earth’s surface and to each other and are described as joints. In many cases more than two series of such fractures are found, yet even in these cases two more perfectly developed series are prominent and almost exactly perpendicular to each other as well as to the earth’s surface. This omnipresent double series or set of joints is the well-known set of master joints, and very often it is found developed practically alone (Fig. 36). Over large areas, the direction of the set of master joints may remain practically constant, or this set may quite suddenly give place to a similar set which is, however, turned through half a right angle from the first (Fig. 37). Not infrequently two such sets of master joints are found together bisecting each other’s angles within the same rocks, and to them are sometimes added additional though less perfect series of joint planes.
Studied throughout a considerable district, the various series which make up these two sets of master joints may be seen locally developed in different combinations as well as in association with additional fissure planes which are not easily reduced to any simple law of arrangement (Fig. 38 a, a, a). Only rarely are regular joint series observed which do not stand perpendicular to the original attitude of the rock beds. In a few localities, however, rectangular joint sets have been discovered which divide the rock into prisms parallel to the earth’s surface and with the joint series inclined to it each by half a right angle. Where the rock beds have been much disturbed, the complex of joints may be such as to defy all attempts at orderly arrangement.
The space intervals of joints.—The same kind of subequal spacing which characterizes the fractures near the surface of the block in Daubrée’s experiment (Fig. 19, p. 41) is found simulated by the rock joints (Fig. 39). Such unit intervals between fractures may be grouped together into larger units which are separated by fractures of unusual perfection. We may think of such larger space units as having the smaller ones superimposed upon them (Fig. 40).
The displacements upon joints—faults.—In the vast majority of cases, the joint fractures when carefully examined betray no evidence of any appreciable movement of the two walls upon each other. Generally the rock layers are seen to cross the joints without apparent displacement. Joints are therefore planes of disjunction only, and not planes of displacement.
Fig. 41.—Faulted blocks of basalt divided by joints near Woodbury, Connecticut. To show the structure of the rock, some of the foliage has been removed in preparing the sketch from a photograph.
Within many districts, however, a displacement may be seen to have occurred upon certain of the joint planes, and these are then described as faults. Such displacements of necessity imply a differential movement of sections or blocks of the earth’s crust, the so-called orographic blocks, which are bounded by the joint planes and play individual rôles in the movement. A simple case of such displacements in rocks intersected by a single set of master joints is represented in the model of plate 4 C. The most prominent fault represented by this model runs lengthwise through the middle, and the displacement which is measured upon it not only varies between wide limits, but is marked by abrupt changes at the margins of the larger blocks. This vertical displacement upon the fault is called its throw. Though not illustrated by the model, horizontal displacements may likewise occur, and these will be more fully discussed when the subject of earthquakes is considered in the following chapter. An actual example of blocks displaced by vertical adjustment is represented in Fig. 41, a simple type of faulting which has taken place in rocks but slightly disturbed from their original attitude, but intersected by a relatively simple system of master joints. In those regions where the beds have been folded and perhaps overthrust before their elevation into the zone of fracture, and which are further intersected by disorderly fissure planes, the results are far more complex. In such cases the planes of individual displacement may not be vertical, though they are generally steeper than 45°. For their description it is necessary to make use of additional technical terms (Fig. 42). The inclination of a sloping fault plane measured against the vertical is called the hade of the fault. The total displacement is measured along the plane of the fault from a point upon one limb to the point from which it was separated in the other. The additional terms are made sufficiently clear by the diagram.
Fig. 42.—A fault in previously disturbed strata. AB, displacement; AC, throw; BD, stratigraphic throw; BC, heave; angle CAB, hade.
Methods of detecting faults.—The first effect of a fault is usually to produce a crack at the surface of the earth; and, provided there is a vertical displacement or throw, an escarpment which rises upon the upthrown side of the fault. In general it may be said that escarpments which appear at the earth’s surface as plane surfaces probably represent planes of fracture, though not necessarily planes of faulting. In many cases the actual displacements lie buried under loose rock débris near to and paralleling the escarpment, and in some cases as a result of the erosional processes working upon alternately hard and soft layers of rock, the escarpment may later appear upon the downthrown side or limb of the fault (Fig. 43). As an illustration of a fault escarpment, the façade of El Capitan and many other rock faces of the Yosemite valley may be instanced.
The most satisfactory evidence of a fault is the discovery of a rock bed which may be easily identified, and which is actually seen displaced on a plane of fracture which intersects it (Fig. 42, p. 59). When such an easily recognizable layer is not to be found, the plane of displacement may perhaps be discovered as a narrow zone composed of angular fragments of the rock cemented together by minerals which form out of solution in water. Such a fractured rock zone which follows a plane of faulting is a fault breccia. If the fault breccia, or vein rock, is much stronger than the rock on either side, it may eventually stand in relief at the surface like a dike or wall. At other times the displacement produces little fracture of the walls, but they slide over each other in such a manner as to yield either a smoothly corrugated or an evenly polished surface which is described as “slickensides.” It may be, however, that during the movement either one or both of the walls have “dragged”, and so are curled back in the immediate neighborhood of the fault plane (Fig. 44).
When, as is quite generally the case, the actual plane of displacement of a fault is not open to inspection, the movement may be proven by the observation of abrupt, as contrasted with gradual, changes in the strikes and dips of neighboring exposures (Fig. 45); or by noting that some easily recognized formation has been sharply offset in its outcrops (Fig. 46).
Fig. 45.—Map to show how a fault may be indicated in abrupt changes of the strike and dip of neighboring exposures.
Fig. 46.—A series of parallel faults indicated by successive offsets in the course of an easily recognizable rock formation.
There are in addition many indications rather than proofs of the presence of faults, which must be taken account of in every general study of the geology of a district. Thus the outcrops of all neighboring formations may terminate abruptly upon a straight line which intersects all alike. Deep-seated fissure springs may be aligned in a striking manner, and so indicate the course of a prominent fracture, though not necessarily of a fault. Much the same may be said of the dikes of cooled magma which have been injected along preëxisting fractures.
The base of the geological map.—Modern topographic maps form an important part of the library of the serious student of physiography; they are the gazetteer of this branch of science. Every civilized nation has to-day either completed a topographic atlas of its territory, or it is vigorously prosecuting a survey to furnish maps which represent the relief with some detail, and publishing the results in the form of an atlas of quadrangles. Thus a relief map will erelong be obtainable of any part of the civilized world, and may be purchased in separate sections. Nowhere is this work being taken up with greater vigor than in the United States, where a vast domain representing every type of topographic peculiarity is being attacked from many centers. Here and elsewhere the relief of the land is being expressed by so-called contours or lines of equal altitude upon the earth’s surface. It is as though a series of horizontal planes, separated by uniform intervals of 20 or 40 or 100 feet, had been made to intersect the surface, and the intersection curves, after consecutive numeration, had been dropped into a single plane for printing.
The colored areas upon the geological map are each supposed to indicate where a certain rock type or formation lies immediately below the surface, and this distribution represents the best judgment of the geologist who, after a study of the district, has prepared the map. Unfortunately the conventions in use are such that his observation and his theory have been hopelessly intermingled in the finished product. Armed with the geological map, the student who visits the district finds spread out before him, it may be, a landscape of hill and valley, of green forest and brown farming land, which is as different as may be from the colored puzzle which he holds in his hand. Hidden under the farm vegetation or masked by the woods are scattered outcroppings of rock which have been the basis of the geologist’s judgment in preparing the map. Experience shows that in order to bridge the wide gap between the geology in the landscape and the patches of color upon the map something more than mere examination of the colored sheet is necessary. We shall therefore describe, with the aid of laboratory models, the various stages necessary to the preparation of a geological map, and every student should be advised to follow this by practical study of some small area where rocks are found in outcrop.
Laboratory models for the study of geological maps.—In order to represent in the laboratory the disposition of rock outcrops in the field, special laboratory tables are prepared with removable covers and with fixed tops, which are divided into squares numbered like the township sections of the national domain (Fig. 47). To represent the rock outcrops, blocks are prepared which may be fixed in any desired position by fitting a pin into a small augur hole bored through the table. The outcrop blocks for the sedimentary rock types are so constructed as to show the strike and dip of the beds. (See Appendix D.)
Fig. 47.—Field map prepared from a laboratory table.
The method of preparing the map.—To prepare the map, use is made of a geological compass with clinometer attachment, a protractor, and a map base divided into sections like the top of the table, and on the scale of one inch to the foot. Each exposure represented upon the table is “visited” and then located upon the base map in its proper position and attitude. The result is the field map (Fig. 47), which thus represents the facts only, unless there have been uncertainties in the correlation of exposures or in determining the position of the bedding plane.
Fig. 48.—Areal geological map constructed from the field map of Fig. 47, with two selected geological sections.
To prepare the areal geological map from the field map, it is first necessary to fix the boundaries which separate formations at the surface; and now perhaps for the first time it is realized how large an element of uncertainty may enter if the exposures were widely separated. It is clear that no two persons will draw these lines in the same positions throughout, though certain portions of them—where the facts are more nearly adequate—may correspond. In Fig. 48 is represented the areal geological map constructed from the field map, with the doubtful area at one side left blank.
Some conclusions from this map may now be profitably considered. The complexly folded sandstone formation at the left of the map appears as the oldest member represented, since its area has been cut through by the intrusive granite which does not intrude other formations, and is unconformably overlaid by the limestone and its basal layer of conglomerate. The limestone in turn is unconformably overlaid by the merely tilted sandstone beds at the right of the map. These three sedimentary formations clearly represent decreasing amounts of close folding, from which it is clear that each earlier formation has passed through an episode not shared by that of next younger age. Of the other intrusive rocks, the dike of porphyry is younger than all the other formations, with the possible exception of the upper sandstone. Offsetting of the formations has disclosed the course of a fault, and from its relations to the dikes we may learn that of these the porphyry is younger and the basalt older than the date of the faulting.
The dashed lines upon the map (AB and CD) have been selected as appropriate lines along which to construct geological sections (Fig. 48, below map), and from these sections the exposed thicknesses of the different formations may be calculated. In one instance only, that of the conglomerate, can we be sure that this exposed thickness measures the entire formation.
Fold versus fault topography.—The more resistant or “stronger” rock beds, as regards attacks of the atmosphere, in the course of time come to stand in relief, separated by depressions which overlie the “weaker” formations. Simple open folds which are not plunging exercise an influence upon topography by producing generally long and straight ridges. More complex flexures, since they generally plunge, make themselves apparent by features which in the map are represented by curves. Fracture structures, and especially block displacements, are differentiated from these curving features by the dominance of straight or nearly rectilinear lines upon the map. The effect of erosion is to reduce the asperity of features and to mold them with flowing curves. The fracture structures are for this reason much more likely to be overlooked, and if they are not to elude the observer, they must be sought out with care. Fold and fracture structures may both be revealed upon the same map.
Reading References to Chapter VI
CHAPTER VII
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.
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.
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.
Fig. 51.—Bowlders thrown into the air and overturned during the Assam earthquake of 1897 (after R. D. Oldham).
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.
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.
Fig. 54.—Effect of a seismic water wave at Kamaishi, Japan, in 1896 (after E. R. Scidmore).
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.
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).
Fig. 57.—A fault of lateral displacement.
Fig. 58.—Fence parted and displaced fifteen feet by a transverse fault formed during the California earthquake of 1906 (after W. B. Scott).
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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.
A. An earthquake fault opened in Formosa in 1906, with vertical and lateral displacements combined (after Omori).
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).
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).
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).
Fig. 62.—Post-glacial earthquake faults of small but cumulative displacement, eastern New York (after Woodworth).
Fig. 63.—Earthquake cracks in Colorado desert (after a photograph by Sauerven).
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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.
Fig. 64.—Diagrams to show how railway tracks are either broken or buckled locally within the district visited by an earthquake.
Fig. 65.—The Biwajima railroad bridge in Japan after the earthquake of 1891 (after Milne and Burton).
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).
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).
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.
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).
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.
Fig. 71.—Marquetry of the rock floor of the Tonapah Mining District, Nevada (after Spurr).
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.
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.
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).
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).
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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).