Seismic wave

“ So, too, earth-waves striking the sea-border may thrust the waters off shore by their sudden impact, and the reaction may develop a wave which overwhelms the coast. Such waves may doubtless arise from a sudden stroke of seismic vibrations on the sea-bottom. The great gaping fissures that sometimes open during earthquakes occur oftenest where the surface on one side is less well supported than on the other, as on a slope, or near a bluff-face or a river-channel. ”

“ In the process of breaking, vibrations called “seismic waves” are generated. These waves travel outward from the source of the earthquake along the surface and through the Earth at varying speeds depending on the material through which they move. Some of the vibrations are of high enough frequency to be audible, while others are of very low frequency. These vibrations cause the entire planet to quiver or ring like a bell or a tuning fork.
A fault is a fracture in the Earth’s crust along which two blocks of the crust have slipped with respect to each other. ”

“ Whatever the nature of the impulse which originates the earthquake, it gives rise to a series of waves which are propagated through the earth’s substance and also superficially. In one kind, known as normal or condensational waves, Earthquake waves. or waves of elastic compression, the particles vibrate to and from the centre of disturbance, moving in the direction in which the wave travels, and therefore in a way analogous to the movement of air in a sound-wave. ”

“ Thus, for example, if the impulse (whatever may be its cause) be delivered somewhere beneath the bed of the sea, all four classes of earthquake waves may reach an observer on shore in succession. The elastic wave of shock passing through the earth generally reaches him first: its velocity of propagation depending upon the specific elasticity and the degree of continuity of the rocky or the incoherent formations or materials through which it passes. ”

“ The period of a wave is the time required for the vibrating particle to complete an oscillation. As the rocks of the earth’s crust are very heterogeneous, the earthquake-waves suffer refraction and reflection as they pass from one rock to another differing in density and elasticity. In this way the waves break up and become much modified in course of transmission, thus introducing great complexity into the phenomena. It is known that the normal waves travel more rapidly than the transverse. ”

“ 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. ”

“ It was felt from Boston to Cuba, and from eastern Iowa to the Bermudas, over a circular area whose diameter was a thousand miles.
An earthquake is transmitted from the focus [234] through the elastic rocks of the crust, as a wave, or series of waves, of compression and rarefaction, much as a sound wave is transmitted through the elastic medium of the air. Each earth particle vibrates with exceeding swiftness, but over a very short path. ”

“ These are based on the assumptions that the velocity increases with the depth below the surface, and that it is always the same at the same depth. From the focus of the earthquake, wave-paths diverge in all directions. Those which start horizontally curve upwards, and intersect the surface of the earth in a circle dividing the whole surface into two areas of very unequal size. Within the small area, the surface-velocity is infinite at the epicentre, and decreases outwards until it is least on the boundary-circle. ”

“ It is impossible to contemplate the effects which followed the great earthquake—the passage of a sea-wave of enormous volume over fully one-third of the earth’s surface, and the force with which, at the farthermost limits of its range, the wave rolled in upon shores more than 10,000 miles from its starting-place—without feeling that those geologists are right who deny that the subterranean forces of the earth are diminishing in intensity. ”

“ An earthquake is a vertical vibration, having an undulatory progression. An example of the simple bounding movement occurred in 1797, when the city of Riobamba, in the Quito Valley, was buried under part of a mountain shaken down by the violent concussion, and men were tossed several hundred feet. We saw one massive structure which had nearly turned a somersault. The ordinary vibrations seldom exceed two feet in height. The wave-movement has a rate of from twenty to thirty miles a minute, depending on the elasticity of the rock and the elevations on the surface. ”

“ By the initial impulse of explosion or lifting of the ground, a ‘great sea wave’ is generated, which travels 175shorewards with a velocity dependent upon its size and the depth of the ocean. At the same instant, a ‘sound wave’ may be produced in the air, which travels at a quicker rate than the ‘great sea wave.’ A third wave which is produced, is an ‘earth wave,’ which will reach the shore with a velocity dependent on the intensity of the impulse and the elasticity of the rocks through which it is propagated. ”

“ If the wave emerge with a steep angle to the horizon, the distortion is that of compression in the diagonal of the wall's plane, nearest parallel to the line of wave transit; and the fissures, if they occur, are also diagonal to the horizon, and approximate to directions perpendicular to the lines of pressure, i.e., to the line of wave transit. If the velocity of the wave be sufficient, in relation to the density and cohesion of the wall, a triangular mass may be projected from the end at which the wave passes out from it. ”

“ 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. ”

“ Were we at liberty to consider the vibration of the wave with vertical emergence performed in like manner, the movement of any point of matter upon the surface due to it would be limited to two motions—one directly up and down, due to the amplitude, and the other forward and backward horizontally, due to its altitude and to the senses. ”

“ 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. ”

“ Where the phenomena are clear, we may, on the contrary, always conclude that it lies along the wave-path, towards the end that presents the widest fissures. Very few large and massive cardinal buildings will be found that will not give, as respects a normal or slightly subnormal wave, a decisive response, from some or other of its parts, by this means. ”

“ The rate which waves are estimated to travel varies with the wind-propelling force. The average hurricane wave travels at about forty-five miles an hour. But earthquake waves—those set in motion by subaqueous disturbance—have been known to travel at the rate of six hundred feet in a second for thousands of miles across the ocean. ”

“ From the point where the earthquake originates, two sets of waves proceed in the solid crust of the earth, viz., the wave of elastic compression, propagated in every direction with a velocity proportioned to the elasticity and density of the parts of the earth-crust in its path. In different sorts of rock the velocity will not be the same: it will be greatest in the solid, and least in the loose aggregations of matter. ”

“ We never feel and observe the earthquake shocks themselves, never directly in their simplicity or multiplicity, but only the wave movements that are sent out from them in the elastic crust of the earth. These, however multifold their origin, proceed in an immense spherical wave which moves in more or less numerous repetitions through the earth's interior. It is this shaking of the earth by the spherical waves that our instruments represent as earthquakes. ”

“ Another stream of lava has appeared.... The noise of the explosion and of the rocks striking together is deafening. The ground is shaken by strong and continuous seismic movements, and the seismic instruments [instruments employed to record the time, direction, and intensity of earthquake movements] threaten to break. It will probably be necessary to abandon the observatory, which is very much exposed to the shocks. ”

“ It will be seen at once that within this region, or rather along that portion of the sea-coast which falls within the central region of disturbance, the true character of the sea-wave generated by the earthquake could not be recognized. If a rock fall from a lofty cliff into a comparatively shallow sea, the water around the place where the rock has fallen is disturbed in an irregular manner. The sea seems at one place to leap up and down; elsewhere one wave seems to beat against another, and the sharpest eye can detect no law in the motion of the seething waters. ”

“ The transmission of an earthquake shock through the earth takes place with wonderful rapidity. The elastic wave varies in velocity from 800 to 1,000 feet per second in sand or clay to three miles per second in solid granite. ”

“ Now, we occasionally find walls that are extremely massive in proportion to their altitude, and built of small stones or brick laid in bad masonry, and with almost bondless mortar; such walls have little or no resilience, and, when thrown more or less out of plumb, or fissured, with suitable conditions, afford a rude approximate measure of the horizontal amplitude of the earth wave, by the range of movement impressed at the level of the centre of oscillation. ”

“ If the earthquake wave ran through a uniform and highly elastic substance, such as glass, it would move everywhere with equal speed, and, in the case of the greater disturbances, the motion might be felt over the whole surface of the earth. But as the motion takes place through rocks of varying elasticity, the rate at which it journeys is very irregular. Moving through materials of one density, and with a rate of vibration determined by those conditions, the impulse is with difficulty communicated to strata which naturally vibrate at another speed. ”

“ There may be no displacement whatever of loose objects, nor any dislocation of large masses, such as churches, &c., though exposed to violent shock, if its emergence be quite or very nearly vertical, and that the maximum velocity of the wave does not exceed—
being equal , the amplitude of the wave, the masses being in such case rapidly lifted up and let fall again without the withdrawal of the support of the base. ”

“ 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. ”

“ From the data expressed in seismograms, scientists can determine the time, the epicenter, the focal depth, and the type of faulting of an earthquake and can estimate how much energy was released.
The two general types of vibrations produced by earthquakes are surface waves, which travel along the Earth’s surface, and body waves, which travel through the Earth. Surface waves usually have the strongest vibrations and probably cause most of the damage done by earthquakes. ”

“ And in descriptive Seismology the present distribution of the earthquake bands or regions of greatest seismic prevalence and activity are tolerably ascertained, and their connection with volcanic lines and those of elevation rendered more evident. Viewed alone, nothing can yet be said to be absolutely ascertained as to the immediately antecedent cause or causes of the impulse. The function of Earthquake, as part of the cosmical machine, has become more clear, as the distinctive boundaries between Earthquake and permanent elevation of the earth have been made evident ”

“ The balls were thrown off in the second semiphase of the wave; therefore, and as the time of oscillation of the tower (from its altitude) was large, and greater than that of the whole phase of the wave they were projected with a velocity less than that due to the shock; the top of the tower moving, by its elasticity and inertia, towards the east, at the same time that the forward movement of the whole mass towards the west, by the wave of shock, projected the balls with a velocity equal the difference. ”

“ One of the most practical problems which can be suggested to the seismologist is the determination of the district or districts in any given country from which earthquake disturbances originate. With a map of a country before us, shaded with tints of different intensity to indicate the relative frequency of seismic disturbance in various districts, we at once see the localities where we might dwell with the least disturbance, and those we should seek if we wish to make observational seismology a study. ”