Popular Science Monthly/Volume 29/July 1886/Earthquakes and Other Seismic Movements
|EARTHQUAKES AND OTHER SEISMIC MOVEMENTS|
WE are accustomed to think of the land of the earth as something solid and fixed; and, as a testimonial of this impression, the Latin phrase terra firma, firm land or solid ground, has been naturalized in the languages of nearly all civilized peoples. On the other hand, we speak of water as unstable. But the geological history of the earth and the more careful observations of modern times have taught us that these ideas do not correctly represent the qualities of the land-masses and water-masses of the globe as compared with one another. The ancient shore-marks on the continents and the phenomena of elevation and subsidence that have been observed in historic times, confirming their evidence, show that the land and the ocean are continually changing their level as to one another; and it has further been made evident, by experiment, as well as by a priori reasoning, that it is not the ocean that changes, but the land which undergoes alternate movements of elevation and depression. An earthquake-shock is a phenomenon well adapted to destroy the faith of any person who feels one in the fixedness of the earth; and such, by the evidence, is the effect for the time on all who experience these shocks. Even the light pulsations which sometimes pass over parts of the United States occasion panic and excite a momentary impression that everything is falling over or sinking away; while the more violent shocks that are felt in earthquake-infested countries produce indescribable terror; and such catastrophes as those historical earthquakes of Lisbon and Caracas, and the more recent ones of Ischia and the Strait of Sunda amount to a demonstration that the reason for such terrors is real, and that the continents also can not escape the general law of change and perishability.
Earth-movements—the name by which these phenomena may be most conveniently described are various, and comprise, so far as they are now considered, earthquakes, or sudden violent movements of the ground; earth-tremors, or minute movements which usually escape attention by the smallness of their amplitude; earth pulsations, or movements which are overlooked on account of the length of their period; and earth oscillations or movements of long period and large amplitude—like the shifting of levels of land-masses—which attract attention from their geological importance. Some of these movements have only recently begun to attract attention. They are all intimately associated in their occurrence and their origin.
The study of earthquakes is of interest to the geologist in many ways. As they seem to be connected with volcanic action, the study of them may help to throw light on that, and vice versa. As an earthquake-wave travels along from strata to strata, the study of its reflections and changes in transit may lead to the discovery of peculiarities in rocky structure, of which we should otherwise have no accurate knowledge. It may teach us something about the transmission of disturbances in elastic media, about the earth's magnetism, the electric currents of the earth, and other kindred problems. It is of interest to the meteorologist to know the connections which probably exist between earthquakes and the fluctuations of the barometer, the changes of the thermometer, and the quantity of rainfall. In a practical point, we may ask ourselves what are the effects of earthquakes upon buildings, and how, in earthquake-shaken countries, the buildings are to be made to withstand them.
A typical earthquake consists of a series of small tremors succeeded by a shock, or of a series of shocks separated by more or less irregular—both in period and in amplitude—vibrations of the ground. Man can take but little account of these movements, for they come upon him by surprise, and, by the time he is ready to begin to observe, they are over. Hence we must have recourse to instruments. It is easy enough to construct an instrument that shall move at the time of an Fig. 1. earthquake, and leave a record of its motion a—seismoscope; but an instrument that shall record the period, extent, and direction of each of the vibrations constituting the earthquake a seismometer or seismograph is a more complicated affair.
The earliest seismoscope of which we find any historical record is that of the Chinese Chôko, which was invented in a. d. 136. According to the historical account given of it, it consisted of a spherically formed copper vessel (Fig. 1), eight feet in diameter. "Its outer part," the account says, "is ornamented by the figures of different kinds of birds and animals, and old, peculiar-looking letters. In the inner part of this instrument is a column so suspended that it can move in eight directions. Also, in the inside of the bottle, there is an arrangement by which some record of an earthquake is made according to the movement of the pillar. On the outside of the bottle there are eight dragon-heads, each of which holds a ball in its mouth. Underneath these heads there are eight frogs so placed that they appear to watch the dragon's face, so that they are ready to receive the ball if it should be dropped. All the arrangements which cause the pillar to knock the ball out of the dragon's mouth are well hidden in the bottle. When an earthquake occurs, and the bottle is shaken, the dragon instantly drops the ball and the frog which receives it vibrates vigorously. Any one watching this instrument can easily observe earthquakes. With this arrangement, although one dragon may drop a ball, it is not necessary for the other seven dragons to drop their balls unless the movement has been in all directions: thus we can easily tell the direction of an earthquake. Once upon a time a dragon dropped its ball without any earthquake being observed, and the people therefore thought the instrument of no use, but after two or three days a notice came, saying that an earthquake' had taken place at Ròsei. Hearing of this, those who doubted the use of this instrument began to believe in it again. After this ingenious instrument had been invented by Chôko, the Chinese Government wisely appointed a secretary to make observations on earthquakes." This, the most ancient of the whole class, is closely resembled by some of the instruments of modern times.
The Japanese have an instrument consisting of a magnet holding up a nail, which, when shaken off, starts the train of an alarum, but this does not seem to have ever acted with success. Other seismoscopes depend upon the overthrow of a round column of wood or metal, the projection of balls which are connected with electric circuits, or the disturbance of liquids. Some seismographs depend upon the motions of a pendulum, which may be made to show whether the direction of the shock has been constant or variable, and the maximum extent of its motion in various directions. Other instruments are formed by various adjustments of movable bodies, or with springs and adaptations of clock-work. For a complete seismograph we require three distinct sets of apparatus—an apparatus to record horizontal motion, one to record vertical motion, and one to record time. These principles are all embodied in the Gray and Milne seismograph, which is now in use in Japan. In this apparatus (Fig. 2) two mutually rectangular components of the horizontal motion of the earth are recorded on a sheet of smoked paper wound round a drum, D, kept continuously in. motion by clock-work, W, by means of two conical pendulum seismographs, C. The vertical motion is recorded on the same sheet of paper by means of a compensated-spring seismograph, S. L. M. B. The time of occurrence of an earthquake is determined by causing the circuit of two electro-magnets to be closed by the shaking. One of these magnets relieves a mechanism, forming part of a time-keeper, which causes the dial of the time-piece to come suddenly forward on the hands and then move back to its original position. The hands are. provided with ink-pads, which mark their positions on the dial, thus indicating the hour, minute, and second when the circuit was closed. The second electro-magnet causes a pointer to make a mark on the paper receiving the record of the motion. This mark indicates the part of the earthquake at which the circuit was closed. The duration
of the earthquake is estimated from the length of the record on the smoked paper and the rate of motion of the drum. The nature and period of the different movements are obtained from the curves drawn on the paper.
It may be said, as the result of experiences and observations, that an ordinary earthquake consists of a number of backward-and-forward motions of the ground following each other in quick succession. Sometimes these commence and die out so gradually that those who have endeavored to time the duration of an earthquake have found it difficult to say when the shock began and when it ended. Sometimes the motions gradually increase to a maximum and then die out as gradually; sometimes the maximum comes suddenly; and at other times during an earthquake the observer's feelings distinctly tell him that there are several maxima. The chief results which investigators have aimed at have been the measurement of the amplitude, period, direction, and duration of the motions; and attention has been given to the velocity with which the disturbance is propagated.
If we were to ask the inhabitants of a town which had been shaken by an earthquake the direction of the motion they had experienced, it is not unlikely that their replies would include all the points of the compass. Many, in consequence of their alarm, have not been able to make accurate observations. Others have been deceived by the motion of the building in which they were situated. Some tell us that the motion was north and south, while others say that it was east and west. A certain number have recognized several motions, and among the rest there will be a few who have felt a wriggling or twisting. Leaving out exceptional cases, the general result obtained from personal observation as to the direction of an earthquake of moderate intensity is extremely indefinite, and the only satisfactory information to be got is that derived from instruments or from the effects of the earthquake as exhibited in shattered buildings and bodies which had been overturned or projected. By the use of seismographs it has been shown that during an earthquake the ground may move in one, two, or several directions, and it is only when a decided shock is experienced that we can determine with any confidence the direction in which the motion has been propagated. The apparently twisting or wriggling motions are supposed to be the result of combinations of linear movements in different directions. It is often difficult, when reading accounts of earthquakes, to determine the length of time a shaking was continuous. Disturbances which succeed one another with sufficient rapidity to cause an almost continual trembling of the ground may be regarded as collectively forming one great seismic effort, which may last a minute, an hour, a day, a week, or even several years. Strictly speaking, they are a series of separate earthquakes, the vibrations of which more or less overlap. Whenever a large earthquake occurs, it is generally succeeded by a considerable number of smaller shocks. Disturbances of this character are compared by Mallet to "an occasional cannonade during a continuous but irregular rattle of musketry." Continuous motions perceptible to our senses without the aid of instruments usually last from thirty seconds to about two or three minutes. The principal vibrations or shocks of the disturbance occur at unequal intervals; and in the periods of vibration there are irregularities in any given earthquake, and different earthquakes differ from one another. The extent of the movement is much less than the feelings of one experiencing a shock would lead him to estimate it. It is usually within the fraction of an inch in either direction. According to Dr. Wagener, the earth's horizontal motion at the time of a small earthquake is usually only the fraction of a millimetre, and it seldom exceeds three or four millimetres. Mallet believes that the displacement may in some instances be equal to a foot; and M. Abella records a rough observation, in the Philippine Islands, of a motion of the earth to a distance of two metres, when fissures were formed, and seen to open and shut. The velocity of propagation of the wave may vary, even in the same country, between several hundreds and several thousands of feet per second. The same earthquake travels faster across districts near to its origin than it does across districts which are far removed; and, the greater the intensity of the shock, the greater is the velocity.
If we were suddenly placed among the ruins of a large city which had been shattered by an earthquake, it is doubtful whether we should at once recognize any law as to the relative position of the masses of rubbish and the general destruction around. The results of observation have, however, shown that, among the apparently chaotic ruin produced by earthquakes there runs more or less of law governing the position of bodies which have fallen, the direction and position of cracks in walls, and the other effects. Usually, walls of buildings at right angles to the shock will be more likely to be overthrown than those which are parallel to it. It is said that in Carácas every house has its lado securo, or safe side, where the inhabitants place their fragile property. It is the north side, and has been chosen because about two out of three destructive shocks traverse the city from west to east, so that the walls in those sides of the building take them broadside on. This appears to be the rule in destructive earthquakes. But, when a building is subjected to a slight movement, it is assumed that the walls at right angles to the direction of the shock move backward and forward as a whole, and there is little or no tendency for them to be fractured at their weaker parts. The walls, however, which are parallel to the direction of the movement are extended and contracted along their length, and may consequently be expected to give way over the door-and window-openings. The results of the examination of more than three hundred foreign-built brick houses in Tokio, Japan, all similar in their construction, are typically illustrated in Fig. 3. They show that in the upper windows nearly all the cracks ran from the springing of the arches, which formed an angle with the abutment. In the lower arches, which curved into the abutments, not a single crack was observed at the spring-way. The cracks in those arches were near the crown, where beams projected to carry the balcony; and in many instances they proceeded from such beams, even if there were no arches beneath. The houses which were most cracked were in the streets running parallel to the direction in which the greater number and most powerful set of shocks cross the city. From the fact that cracks once made in a building did not appear to extend under the repetition of shocks similar to the one that produced them, it has been inferred that buildings thus cracked acquire a degree of flexibility, and that, by providing cracks or joints between the parts of buildings which have different periods of vibration, some of the strain might be taken off from them, and they might be made more stable. In stone-work, the cracks have been observed generally to run through the mortar-joints; in brick-work, through either bricks or mortar, often preferring the bricks.
As fractures in walls seem most likely to take place above openings like doors or windows, it follows that where architecture demands that openings should be placed one above another in heavy walls, there will be lines of weakness running through the openings. As arches are only intended to resist vertical thrusts, special construction must be adopted to make them strong enough to resist horizontal pulls. This might be given by inserting iron girders or wooden lintels in the arches. Mr. Perry, of Tokio, has suggested a plan of building so that the openings of each tier would occupy alternate positions. Such a line is shown in Fig. 4, where the dotted lines run through openings
|Fig. 3.—Brick Buildings in Tokio, showing Fractures.||Fig. 4.||Fig. 5.|
representing the direction of the lines of weakness. If we compare this with Fig. 5, we shall see that in the case of a horizontal movement, a b, or a vertical movement, c d, fractures would more probably occur in a house built like Fig. 5 than in one built like Fig. 4. If, how-ever, these two buildings were shaken by a shock which had an angle of emergence of about 45°, in the direction of e f, the effects might be reversed. Fractures following a vertical line of weakness are shown in the accompanying drawing (Fig. 6) of the church of St. Augustin, at Manila, shattered by the earthquake of 1880.
When an earthquake shock enters and proceeds along a line of buildings, the last building in the row will, of course, suffer the most, and will exhibit the greatest tendency to fly away from its neighbors. If the house stands on the edge of a canal, or cliff, this tendency is increased by the similar motion of the escarpment. The fate of an end-building thus stricken is shown in Fig. 7, which is taken from the photograph of a house that was shattered in 1868 at San Francisco. Houses may also be rocked on their foundations, or even quite overturned, as appears to have happened to the stud-mill at Hayward, California (Fig. 8).
In any building which may be affected by an earthquake, we have to consider the vibration of a number of parts, the periods of which, if they were independent of each other, would be different. On account of this difference in period, while one portion of a building is
endeavoring to move toward the right, another is pulling toward the left, and either the bonds which join them or the parts themselves will be strained or broken. This was illustrated by many of the chimneys in the houses at Yokohama, which, in the earthquake of February 20, 1880, were shorn off just above the roof. Since then, builders have learned to let chimneys pass freely through the roof without coming in contact with any of the main timbers.
In trying to make structures earthquake-proof, we may build our house weak and flexible, so that the shock shall pass over it as the wind over a reed, or we may attempt to make it stronger than the shock. The native Japanese houses, with their flexible framing, are built on the former plan; some of the European houses essay the latter. In Italy the houses are left to take their chances. In South America, where much exposed to earthquakes, they are built of only one story, or of bamboo and ropes, similarly to the Japanese plan. One of the safest houses for an earthquake country would probably be a one-storied, strongly framed timber house, with a light, flattish roof,
made of shingles or sheet-iron, the whole resting on a quantity of small cast-iron balls carried on flat plates bedded in the foundations. The chimneys might be made of sheet-iron, carried through holes free of the roof. The ornamentation ought to be of light materials. The nature of the ground on which the house is built does not always
appear to be in itself a matter of prime moment. Its relations with other foundations are more important. In some places solid strata, in others soft strata, appear to afford the more favorable situations; and the superiority of either probably depends on a variety of local circumstances. Places near the junction of the two kinds of formations are the worst. The progress of the wave may be interrupted by the interposition of a mountain-range or a hill, in which case we have behind the barrier the phenomenon called an earthquake-shadow; it may be cut off by a deep ditch, as a canal; and in certain parts of South America there appear to exist tracts of ground which are practically exempt from the shocks, while the whole country around is violently shaken. It would seem as if the shock passed beneath such a district as water passes beneath a bridge; and for this reason such tracts have been christened "bridges." In the Syrian earthquake of 1837, neighboring villages, and even neighboring houses, suffered differently. In one case a house was entirely destroyed, while in the next house nothing was felt. In Japan, at a place called Choshi, about fifty-five miles east of the capital, earthquakes are seldom felt, although the surrounding districts may be severely shaken. At this place a large basaltic boss rises in the midst of alluvial strata. The immunity of the district from earthquakes has probably given rise to the myth of the Kanam rock, which is a stone supposed to rest upon the head of a monstrous cat-fish, whose writhings cause the shakings so often felt.
Possibly something may be done in arranging the surroundings of buildings to ward off the destructive effects of earthquakes. The Temple of Diana, at Ephesus, was built on the edge of a marsh for this object. Pliny says that the Capitol of Rome was saved by the Catacombs. Elisée Reclus says that the Romans and Hellenes found out that caverns, wells, and quarries retarded the disturbance of the earth, and protected edifices in their neighborhood. The Tower of Capua was saved by its numerous wells. Vivenzis asserts that in building the Capitol the Romans sank wells to weaken the effects of terrestrial oscillations; and Humboldt relates the same of the inhabitants of San Domingo. Quito is said to receive protection from the numerous canons in the neighborhood, while Lactacunga, fifteen miles distant, has often been destroyed. Similarly, it is extremely probable that many portions of Tokio have from time to time been protected more or less from the severe shocks of earthquakes by the numerous moats and deep canals which intersect the city.
Various causes have been assigned for the production of earthquakes, and, although they may all singly or in combination contribute to the effect, we must conclude, after considering the whole subject, that the primary cause is endogenous to our earth, and that exogenous causes, like the attraction of the sun and moon, and barometric fluctuations, play but a small part in the actual production of the phenomena, their greatest effect being to cause a slight preponderance in the number of earthquakes at particular seasons. The majority of earthquakes are due to explosive efforts at volcanic foci. The greater number of these explosions take place beneath the sea, and are probably due to the admission of water through fissures to the heated rocks beneath. A smaller number of earthquakes originate at actual volcanoes. Some earthquakes are produced by the sudden fracture of rocky-strata or the production of faults. This may be attributable to stresses brought about by elevatory pressure. Lastly, we have earthquakes due to the collapse of underground excavations; and these may have been produced by evisceration caused by volcanic eruptions, by the washing away or solution of the earth by chemically charged waters or hot springs, or by other causes.
Considerable attention has been drawn lately toward the study of small vibratory motions of the ground which, to the unaided senses, are usually passed by without recognition. They are called earth-tremors, and were only discovered when difficulties caused by them were encountered in the adjustment of extremely delicate astronomical and other instruments. These movements have been most carefully Fig. 9.—Normal, Tromometer. B. bob of pendulum; P, prism; M, microscope; S, rim of scale. studied in Italy by Father Bertelli, of Florence; le Conte Malvasia, at Bologna; M. di Rossi, at Rome; and le Baron Puet, at Nice. Delicate instruments have been devised for detecting and recording them, the most important of which is the normal tromometer of Bertelli and Rossi. It consists of a pendulum (Fig. 9) one and a half metre long, carrying, by means of a very fine wire, a weight of one hundred grammes. To the base of the bob a vertical stile is attached, and the whole is inclosed in a tube, terminated at its base by a glass prism of such a form that, when looked through horizontally, the motion of the stile can be seen in all azimuths. In front of this prism a microscope is placed. Inside the microscope is a micromatic scale, so arranged that it can be turned to coincide with the apparent direction of oscillation of the point of the stile. In this way not only can the amplitude of the motion of the stile be measured, but also its azimuth. The extent of vertical motion is measured by the up-and-down motion of the stile due to the elasticity of the supporting wire.
Another instrument, the microseismograph of Professor Rossi, gives automatic records of slight motions. It consists of four pendulums, each about three feet long, suspended so that they form the corners of a square platform. In the center of this platform a fifth but rather longer pendulum is suspended. The four pendulums are each connected just above their bobs to the central pendulum with loose silk threads. Fixed to the center of each of these threads, and held vertically by a light spring, is a needle, so adjusted that each thread is depressed to form an obtuse angle of about 155°. These needles form the terminals of an electric circuit, the other termination of which is a small cup of mercury placed just below the lower end of the needle. By a horizontal swing of one of the pendulums this arrangement causes the needle to move vertically, but with a slightly multiplied amplitude. By this motion the needle comes in contact with the mercury, and an electro-magnet with a lever and pencil is caused to make a mark on a band moved by clock-work. The five pendulums being of different lengths, the apparatus is adapted to respond to seismic waves of different velocities.
Professor Rossi's microphone consists of a metallic swing arranged like the beam of a balance. By means of a movable weight at one end of the beam, this is so adjusted that it falls down until it comes in contact with a metallic stop. The beam and the stop form two poles of an electric circuit, in which is a telephone. The slightest motion in a vertical direction causes a fluctuation in the current passing between the stop and the beam, and announces itself in the telephone.
By observations made with instruments like these, it has been shown that the soil of Italy is in incessant movement, with periods of excessive activity, called seismic storms, that usually last about ten days. The storms are separated by periods of relative calm. They are more regular in winter, and exhibit sharp maximums in spring and autumn. In the midst of such a period or at its end there is usually an earthquake. They have been observed to be generally related to barometric depressions.
Earth-pulsations are slow but large undulations that appear to travel over or disturb the surface of the globe. They are made manifest through variations in the movement of pendulums, changes in the position of the bubbles of levels, eccentricities in the behavior of clocks, the swinging of chandeliers in churches, unusual disturbances in bodies of water, and even of water in tubs, irregularities in the flow of springs, and other phenomena, the occurrence of which, or the peculiar manner of it, while it is consistent with the hypothesis of such movement, can not be accounted for on any other probable supposition.
- Earthquakes and other Earth Movements. By John Milne, Professor of Mining and Geology in the Imperial College of Tokio, Japan. International Scientific Series. No. LV. New York: D. Appleton & Co., pp. 348.