Shock wave

“ In addition to seismic sea waves, earthquakes below the surface of the sea may produce a longitudinal pressure wave that travels upward at the speed of sound. When a ship encounters such a wave, it is felt as a sudden shock which may be so severe that the crew thinks the vessel has struck bottom. ”

“ In no instance has it ever before been known that a well-marked wave of enormous proportions should have been propagated over the largest ocean-tract on our globe, by an earth-shock whose direct action was limited to a relatively small region, and that region not situated in the centre, but on one side of the wide area traversed by the wave. ”

“ A breaking wave concentrates its transmitted force on a portion of the water forming the undulation, which, consequently, strikes a more powerful blow over a limited area against any structure than the more distributed shock of a simple undulation beating against a vertical wall. Moreover, the recoil of broken waves down a sloping beach or rubble mound produces a greater scour than the simple reflection of an undulation from a vertical wall, especially where the depth is sufficient to provide a cushion of water below the undulation, protecting the toe of the wall from the wash of recoil. ”

“ It is rarely, however, that a shock of an emergent wave, sufficient to throw back one end, and forward the other, occurs without the side walls being also thrown, in or out, or both, either by transversal wave motion or by secondary actions of the falling end walls upon them. ”

“ The shock was like no earthquake I've ever known. And this tidal wave—" He was reaching for a small switch. He turned a dial to the words: "News Service—General," and the instrument broke into hurried speech.
It told of earth shocks in many places—the whole world had felt it—some tremendous readjustment among the inner stresses of the earth—most serious on the Atlantic seaboard—the great Harkness Terminals destroyed—some older buildings in the business district shaken down—loss of life not yet computed.... ”

“ When our earth is shaken by one of those occasional adjustments of the crust which I have described, the wave that spreads like a pulsation from the centre of agitation extends all over our globe and, indeed I may say, is transmitted right through it. At the surface lying immediately over the centre of disturbance there will be a violent shock. In the surrounding country, and often over great distances, the earthquake may also be powerful enough to produce destructive effects. ”

“ Often have sailors been mystified, on receiving such shocks and hastily heaving the lead, to find the ocean unfathomable. Again, shocks which are most violent on land are not perceptible at sea, unless a great sea wave be produced; but such a wave in the open sea, as often experienced, produces no shock but passes under a vessel like a heavy swell. And a shock at sea is sometimes severe enough to snap a spar, or wrench loose bolts like the blow of a reef, yet no trace{488} is perceptible on shore. Lastly, earthquakes often happen in inland regions, and affect but a small area. ”

“ Although to the inhabitants of a shaken district the undulatory wave or vibration of an earthquake appears to radiate horizontally outwards from the spot on the surface where it is first felt, the force does not really operate in a horizontal direction, like a wave caused by a pebble on the 105surface of a pond; for at every point, except that immediately above the focus of the shock, it comes up obliquely from below, causing the ground to move forwards and then backwards in a, more or less horizontal direction. ”

“ If the liquid space is free, the vibrations are gradually extinguished; if they meet an obstacle, there is a shock. If the obstacle is a shore, they form a tidal-wave and raise large billows, while a few miles away from the shore the sea is quiet. When the obstacle is a shoal or a contrary vibration, heavy waves are raised on the surface of the sea; they seem to start from the bottom, and put ships in great danger. The waves produced in both cases are waves of transmission, as also are those which beat on reefs in pleasant weather, and those which prolong the swell after storms. ”

“ If the particular discharge of lightning with which we are concerned in this investigation were of a nature to produce a strong pressure wave, the pilots might have suffered from acoustical shock or concussion of a severity depending upon the proximity of the lightning to the cockpit and whether or not the cockpit windows were open. Experience has revealed that such pressure waves have produced severe concussion upon human beings which might result in unconsciousness. ”

“ And it appears not to have been recognized that there exists a special class of shock pulses for which the problem of decay admits of an explicit solution. The case in question arises when shock waves of moderate intensities (i.e., shocks with Mach numbers less than 1.5) are considered. For, under these circumstances (when the ratio of pressures on either side of the shock front is less than 2.5) the increase in entropy of an element of gas as it crosses the shock front can be ignored. ”

“ British Association,' 1858) , to be established prior to the shock; and the only known method of determination is based upon observation of the time of arrival of the wave, at each of three or more distant stations, within the shaken area.
In the facts which we can usually collect in the field, after the shock, we are limited to the casual observation by the ordinary time measurers (clocks and watches) of the moment of observed shock at several places. ”

“ In the same way as the discharge from an excited tube occasions a commotion in the human body, so the shock produced by the discharge between the cloud and many miles in compass of solid earth, must be an earthquake, and the snap from the contact the noise attending it. ”

“ Lastly, the vibrations which affect an observer at any moment are of various strength and period. One may hear all perhaps, while a second may be able to hear some and not others. Thus, to one observer, the sound may be like a rising wind, to another like a heavy traction-engine passing; one may hear the crashes which accompanied the strongest part of the shock, while a second may be deaf to the same vibrations; to one the sound may become continually louder and cease abruptly, to another it may increase to a maximum and then die away. ”

“ If we have a long line of billiard-balls arranged in contact one with another, and give a quick blow to the first one, the last one will roll away. The shock is transmitted from the first ball to the last one, without the intermediate ones suffering any appreciable motion. Marine disturbances caused by direct waves, tides, earthquakes, etc., may in the same way be transmitted through molecules of still water without agitating them. ”

“ I saw the worst assault at night on a seaside parade where the sea smote on the doors of England with the hammers of earthquake, and a white smoke went up into the black heavens. There one could thoroughly realise what an awful thing a wave really is. I talk like other people about the rushing swiftness of a wave. But the horrible thing about a wave is its hideous slowness. ”

“ Suppose a barrel of powder exploded in a cavity situated a hundred metres underground. At the surface we would hear a rumbling explosion, and feel a vertical shock within a limited space, and an undulatory thrill over a wider circle. The phenomenon will be much like that of an earthquake, except that the essential element of repetition will be wanting; for all will be over with the first shock. But, in the majority of earthquakes, the shocks come in succession, as if the cause of them were a regenerating one. ”

“ If we take 4 inches for the amplitude of the largest variation, and 12 feet per second for the maximum velocity, and assume the motion to have been of a simple harmonic character, the period of a complete vibration would be less than one-fifth of a second. [17] Now, we know from seismographic records that this is roughly the period of the small tremors that form the commencement of an earthquake-shock, while the period of the largest vibrations may amount to as much as one or two seconds. ”

“ The sounds, too, which so frequently accompany earthquakes are likewise simply results of this division of the waves and their escape into the air, for we perceive wave motions in the air as sound. The admirable delicacy of our sense of hearing is here manifested, for seismic movements are not rarely perceptible, or heard, as air waves, which we can not perceive as movements of the ground. Earthquake thunder is caused, like storm thunder, by shocks to the air, of which we hear the nearest and latest first, and the farthest and earliest last. ”

“ The object of the instrument is twofold: first, to measure the direction and intensity of a shock; and, second, to write down a history of the earthquake. The shock may be either vertical or horizontal, or partly vertical and partly horizontal. For the vertical shocks a fine metallic point is suspended by a coil of wire over a cup of mercury. The coil of wire acts as a spring, and the slightest upward motion of the earth is sufficient to cause the point to dip into the cup of mercury. ”

“ But, however overwhelming and disastrous in reference to our persons and buildings earthquakes may be, it must be borne in mind that the amplitude of their movements is wholly insignificant in proportion to the dimensions of the globe whose epidermis they shake. The phenomenon is rarely limited to a single shock. Generally, several shocks follow one upon another at short intervals. In many cases, the movements are repeated for months and years, with pauses of a variable duration, so as to form as a whole, till they are totally extinguished, what might be called a seismic period. ”

“ On account of this great multiplication of the intensity of the phenomenon, merely a small rearrangement of the rocks in the crust of the earth, in pursuance of the necessary work of accommodating its volume to the perpetual shrinkage, might produce an excessively violent shock, extending far and wide. The effect of such a shock would be propagated in the form of waves through the globe, just as a violent blow given at one end of a bar of iron by a hammer is propagated through the bar in the form of waves. ”

“ An internal disturbance should, if the earth can be regarded as solid, give rise to three wave-motions: two propagated through the interior of the earth with different velocities, and a third propagated over the surface. The results of seismographic observations have independently led to the recognition of three phases of the recorded vibrations: a set of “preliminary tremors” which are received at different stations at such times as to show that they are transmitted directly through the interior of the earth with a velocity of about 10 km. ”

“ But, in contending with subterranean forces, man is hitherto balked. Numbers of instruments exist for measuring the force and direction of earthquake shocks, but these can be made of little practical use; for we have seen that the vibrations travel from forty to one hundred and fifty miles a minute, according to the nature of the soil. Hence, could we know a certain shock would travel around the world, it would not be possible, after it was first felt, to send warning ahead in time to be of any especial value. ”

“ To most observers, the sound seems to increase and decrease in intensity with the shock, and so gradually and smoothly does this change take place that the sound is frequently mistaken for that of an underground train approaching the observer's house, passing beneath it, and receding in the opposite direction. Some persons, especially if situated within the meizoseismal area, hear also loud crashes in the midst of the rumbling sound and simultaneously with the strongest vibrations. At a moderate distance, say from 30 to 40 miles, the sound becomes more harsh and grating while the shock is felt ”

“ He imagines that the rent would start at or near the central point of the focus and then extend rapidly outwards in all directions. In the initial stage, vibrations of very small amplitude would alone be transmitted, and these would give rise to the early sounds and tremors. As the rending proceeded, the vibrations would increase in strength up to a certain point when they produced the shock itself. After this, they would decrease; and, in the final stage, would give place to the small vibrations corresponding to the sounds and tremors that marked the close of the earthquake. ”

“ Scientists all along the Pacific Coast of the United States reported that in the night their seismographs had recorded a series of light earthquakes; and it was noted that these tremors had occurred precisely eleven minutes and six seconds apart—simultaneously with the sounding of the mysterious wireless calls!
After that the aerial signals did not stop during any part of the twenty-four hours. And the earth shocks continued, gradually increasing in severity. They kept perfect time with the signals through the ether—a shock for every whisper, a rest for every pause. ”

“ Each drop as it strikes the water becomes a centre of disturbance, from which a series of ring-ripples expand outwards. Gravity and inertia are the agents by which this wave-motion is produced, and a rough experiment will suffice to show that the rate of propagation does not amount to a foot a second. A series of slight mechanical shocks is experienced by a body plunged in the water, as the wavelets reach it in succession. But a finer motion is at the same time set up and propagated. ”