Fault line

“ Oftentimes a great line of fault has been repeated, or the movements have been continued through successive periods, so that, newer deposits having covered the old line of displacement, the strata both newer and older have given way along the old line of fracture. ”

“ It is true that now and again streams are found flowing in the direction of a fault, but that is simply because the dislocation is a line of weakness, along which it is easier for the denuding forces to act. ”

“ 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. ”

“ 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. ”

“ As we might infer from what we know as to their causes, earthquakes are most frequent in regions now undergoing deformation. Such are young rising mountain ranges, fault lines where readjustments recur from time to time, and the slopes of suboceanic depressions whose steepness suggests that subsidence may there be in progress. ”

“ Faults afford a valuable indication of the conditions of stress to which a region has been subjected, but some caution must be exercised in their interpretation. Normal faults usually indicate an extension of the surface sufficient to permit the fault-blocks to settle down unequally. Reversed faults usually signify a compression of the surface which requires the blocks to overlap one another more than they did before the faulting. In other words, normal faulting usually implies tensional stress, and reversed faulting compressional stress. ”

“ When the principal movement along a highly inclined fault-plane has been approximately horizontal, the fault has been variously styled a lateral-shift, transcurrent fault, transverse thrust or a heave fault. The horizontal component in faulting movements is more common than is often supposed. ”

“ Faults are the most common of all the forms of disturbed stratification: but, except in particular cases, they are the least influential on the physical configuration of the country. All the rocks which are disturbed by any fault have experienced on one side the same movement, and to the same extent, excepting only those portions which have been subjected to violent pressure; and the bottom of the faults has never been reached, except when they terminate in another dislocation.
Relation of Faults, Mineral Veins, Dykes, &c., to the great Lines of disturbed Rocks. ”

“ That is what we often see when examining the geological earth strata. One stratum, instead of lying evenly with another where a crack has occurred, rests a little above it or below it. This inequality or unevenness geologists call a fault. Now we can easily see that whenever, and by whatever causes, a fault is produced, there is probably at the same time an earthquake. The fault cannot be produced without a great and shaking disturbance. ”

“ In regions where movements along great faults have long since ceased, the original steep slopes may be completely obliterated by erosion.
Fig. 10.—Vertical sections through strata illustrating common kinds of faults: a, “normal faults” where one mass simply sinks below another; b, a “thrust fault” where one mass is shoved over another. (After U. S. Geological Survey.)
How does the geologist determine the actual amount of displacement, especially in the case of a large fault in stratified rocks? ”

“ A third fault, the effect of which is not so evident at first sight, throws the strata down roughly 50 ft. to the south. These are only small faults, but sometimes faults occur, in which the strata have been moved on opposite sides of the fault thousands of feet away from one another. We might think we should see a wall of rock rising up on the surface of the ground where a fault occurs; but the faults have mostly taken place ages ago; and, when they do happen, the rocks are generally moved only a little way at a time. ”

“ Lines of faults extend sometimes for hundreds of miles. The great Pennine Fault of England is 130 miles long, and others extend for much greater distances. The surfaces on both sides of a fault are often smooth and highly polished by the movement which has taken place in the strata. ”

“ Imagine forces of some kind, acting laterally, to compress those layers. There will be corrugations, sometimes actual fractures, the two sides of which, sliding one against the other, will be either raised or depressed. Faults is the name which we give to the fractures that penetrate the terrestrial shell and separate two masses of rock, one of which slides over the plane of fracture. The fault, therefore, reveals an edge, a lower lip produced by the subsidence of the soil, and an upper lip produced by an elevation. ”

“ An original horizontal rock layer may be upfolded into anticlines, downfolded into synclines, and broken by either reverse or normal faults. A fault is a fracture or a zone of fractures within the earth’s crust along which movement has taken place. ”

“ Faults range in length up to hundreds of miles, those from one to twenty miles in length being very common. Where an earth block has been displaced thousands of feet along a fault surface, it is not to be understood that the whole displacement resulted from a single movement, but rather from a series of sudden movements separated by greater or less intervals of time. Each sudden movement along a fault surface produces a vibration of the earth near by. ”

“ One of the puzzles of Grand Canyon, Bradshaw Mountains, and Searchlight—if not also of New England, the Black Hills, and the Yellowstone—is faulting. A fault is what a geologist means by a crack down deep where the country rock has dropped down on one side so as to make a discordance across country. Earthquake faults make a visible bank or step or sidewise slip, changing the surface after an earthquake.
The northwestern states are partly mapped as fault block mountains. The island of Hawaii has a series of fault step blocks southeast, slipping toward the ocean. ”

“ Thus, a miner working along the coal-seam S, from a to b, would describe the fault, f, as an upcast, since he would have to mine to a higher level to catch his coal again. But, had he approached the fault from c to d, he would then have termed it a downthrow, because he would see from the hade of the fault that his coal-seam must be sought for at a lower level. Faults are of all sizes, from a foot or two up to vertical displacements of thousands of feet. Powerful dislocations can often be followed for many miles across a country, running in a more or less linear direction. ”

“ It needs no argument to show that we have here the top of a fault such as produced the shock, or it may be one of a secondary nature, such as any earthquake is likely to bring about in the strata which it traverses. In certain cases two faults conjoin their action, so that a portion of the surface disappears beneath the earth, entombing whatever may have stood on the vanished site. ”

“ Strike-slip (lateral) faults occur in response to either type of stress; the blocks move horizontally past one another. Most faulting along spreading zones is normal, along subduction zones is thrust, and along transform faults is strike-slip.
Normal Fault. Blocks are pulled apart
Thrust Fault. Blocks are pushed together
Strike-Slip Fault. Blocks slide past each other
Geologists have found that earthquakes tend to reoccur along faults, which reflect zones of weakness in the Earth’s crust. ”

“ Where strata and normal fault surfaces lie at various angles, and also in thrust faults, those angles must be determined in addition to the data above named. In many mining regions, where valuable deposits are affected by faulting, accurate knowledge of the direction and amount of displacements of faults is of great economic importance. ”

“ In all thrust faults one mass has been pushed over another, so as to bring the underlying and older strata upon younger beds; and when the fault planes are nearly horizontal, and especially when the rocks have been broken into many slices which have slidden far one upon another, the true succession of strata is extremely hard to decipher. ”

“ When a fault-slip occurs, the displacement is [335] obviously greatest in the central region, and dies out gradually towards the margins of the focus. The phenomena described above show that the evanescent displacement within these margins generate sound-vibrations only; and that the greater slip within the central region produces also the more important vibrations that compose the shock. ”

“ Fig. 47.—The Fault-scarp at Midori. (Koto.) ToList
The general character of the fault-scarp changes with the surface features. On flat ground, where the throw is small, it cuts up the soft earth into enormous clods, or makes a rounded ridge from one to two feet high, so that it resembles, more than anything else, [193] the pathway of a gigantic mole (Fig. 46) . When the throw is considerable—and in one place it reaches from 18 to 20 feet—the fault-scarp forms a terrace, which from a distance has the appearance of a railway embankment (Fig. 47) . ”

“ In the present state of our ignorance of the causes of subsidence, an hypothesis which can explain the great amount of displacement in some faults, on sound mechanical principles, by a succession of movements, is far preferable to any theory which assumes each fault to have been accomplished by a single upcast or downthrow of several thousand feet. ”

“ At a very remote period of the past, the rocks comprising these various strata were subjected to some powerful natural force, which resulted in their fracture, dislocation, and the gradual upheaval of a large portion of them, producing at the point of disruption what is known to geologists as a fault. ”

“ On the other hand, a large area of rock is sometimes lifted up, or surrounded by a system of faults, which have let down the encircling ground; such a fault-block is known also as a horst; a considerable area of Greenland stands up in this manner.
Faults have often an important influence upon water-supply by bringing impervious beds up against pervious ones or vice versa, thus forming underground dams or reservoirs, or allowing water to flow away that would otherwise be conserved. Springs often rise along the outcrop of a fault. ”

“ Earthquakes are now looked upon generally as signs of abrupt yielding to intense pressure. Sometimes they indicate that a huge weight of material deep down has slid to a lower level, packing more closely or pushing aside other masses, and causing what miners call “a fault”—that is a break or dislocation in the regular lines of the stratified rocks. ”

“ The road, on both sides, was deep in mud and prolific of catastrophe; and even now, with conditions much better, there are numerous accidents. Cars all travel at frightful speed. There are no restrictions, and it is nothing to see machines upset and abandoned in the low-lying fields that border the road.
Conditions, however, are better than they were. Part of the conservation system has been the building of narrow ditches at right angles to the line of the road, to lead off the water. Every ten feet or so there is a gutter filled with fagots.
I had been in the general's car before. ”

“ The beds on each side of the central fault rise nearly S. at an angle of about 6°; but, in the immediate vicinity of the green-rock fault, the second coal and all the beds lying above it (as far as they have been explored) are thrown up at an angle so rapidly increasing as, in the space of about 100 yards, to amount to 25°. ”

“ Where the fault plane is not vertical the surfaces on the two sides may be distinguished as the HANGING WALL and the FOOT WALL. Faults differ in throw from a fraction of an inch to many thousands of feet. ”

“ When many faults occur, each producing only a moderate "throw," "shift," or displacement of the strata, their range is usually of only a few hundred yards, or a few miles, when they fall into and are stopped by some greater faults, or axes of movement. ”

“ Any one of our readers who has ever seen a train of luggage wagons being shunted is familiar with the way in which the shock of a sudden pull or push on the part of the engine travels all down the line of wagons, and we may think of the shock of an earthquake as travelling along and through the earth in the same way. Observation, however, shows that these waves are propagated farthest in one particular direction. ”

“ Or if we continue the crack to the edge of the plank, which we may look on as a great boundary fault, we may bend one side so as to make the dislocation gradually increase from its commencement up to its termination. Of the latter case we have examples in the Corbyn's Hall and Brockmoor faults, that gradually increase their "throw" as they approach the western boundary fault. ”