Popular Science Monthly/Volume 2/March 1873/Earthquake-Phenomena
|←Notes||Popular Science Monthly Volume 2 March 1873 (1873)
By Elias Lewis Jr.
|Electricity and Life→|
IN the afternoon of the 1st day of June, 1638, 18 years after the landing of the pilgrims, there occurred the first earthquake in New England, of which we have an authentic record.
It is 234 years since that event, and, according to a catalogue prepared by W. T. Brigham, published in the Memoirs of the Boston Society of Natural History, it appears that, down to October 20, 1870, 231 earthquakes are recorded as having taken place in New England. From this able paper we learn that, in 1663, portions of Canada, New England, and New York, were convulsed by earthquake-shocks.
In 1727, at Newbury, and near the mouth of the Merrimac River, an earthquake took place during the evening when the atmosphere was perfectly calm and clear. The sound preceded the shock. The earth opened, and a sulphurous blast threw up mounds of calcined dust. Several days previous to the earthquake, water in the wells became fetid, and of a pale brimstone color. In 1755, on the 18th day of November, a hollow, roaring noise was heard in various parts of New England. In a minute the earth seemed to undulate as if a wave were passing. This was followed by a vibratory and jerking motion, familiar in earthquake countries. The first shock of this earthquake occurred 17 days after the terrible one at Lisbon, the vibrations of which had not yet ceased.
The great earthquake at New Madrid, in Missouri, took place in 1811-'12. The shocks here were vertical, proving, as we shall see hereafter, that the centre of energy was directly underneath. At other times, the shocks, which continued many months, were undulatory. The ground rose in huge waves, which burst, and volumes of water, sand, and pit-coal, were thrown high as the tops of the trees. The forests waved like standing corn in a gale of wind, and an area 70 miles long by 30 miles wide was submerged, and became a swampy lake.
On the 13th of August, 1868, a fearful earthquake took place in Peru, which laid waste much of the country lying between the Andes and the Pacific. The shocks were felt through a distance of 1,400 miles north and south, and three important cities were destroyed. At Arequipa, in Peru, 40 miles from the sea, a slight undulatory shock was felt, followed by others so violent that in five minutes not a house was standing in that city of 44,000 inhabitants. A subterranean rumbling, like the rush of an avalanche, was heard above the crash, and a cloud of dust rose in the still air over the city. On the sea-coast were situated Iquique and Arica—both were destroyed by the shocks, and overwhelmed by a tremendous wave. The ocean thus took up the vibrations of the land, and waves of tremendous volume were put in motion, which rolled, not only upon the coast, but away from it with a velocity in the deep ocean of not less than 400 miles an hour. The great wave—for one was of much greater volume than the others—has been estimated at upward of 200 miles breadth, with a length along its curved crest of 8,000 miles. This rolled into the harbor of Yokohama, in Japan, 10,500 miles distant, and was felt at Port Fairy, in South Victoria, distant nearly one-half of the earth's circumference.
In 1797, a province of Ecuador, about 100 miles south of Quito, was visited by what is described by Humboldt as "one of the most fearful phenomena recorded in the physical history of our planet." The shocks were vertical, and occurred as "mine-like explosions." The town of Riobamba was over the central area, and many of its inhabitants were thrown 100 feet into the air.
The shocks, in this instance, were not announced by any subterranean thunder, but, just 18 minutes after, a terrific roar was heard beneath Quito. It thus appears that shocks are not always preceded by sounds, nor do the sounds increase with the violence of the shock.
Sometimes, says Humboldt, there is a "ringing noise, as if vitrified masses were struck; again, a continuous rumbling and hollow roar; at others, a rattling and clanking as of chains or near thunder." With the lightning's flash we know that the danger is over, and await the coming thunder without alarm; but thunder, rolling deep in the earth, announces possible if not certain calamity.
Throughout the region of the Andes a connection between volcanic and earthquake action has been recognized by the people. It was supposed by Strabo that volcanoes are safety-valves, and scientific observation suggests that they may relieve the pressure and tension which would otherwise lay the earth in ruins.
For two years previous to 1538 earthquakes had been violent and frequent at Pozzuoli on the Bay of Baise, and elsewhere in the vicinity of Naples. On the 27th and 28th of September they did not cease day or night. On the night of the 29th, flames issued from the ground near the baths of Nero, the earth rose and burst at the top with tremendous roar, and discharged steam, gas, pumice, mud, and ashes. A mountain 1,000 feet high was formed, known as Monte Nuovo, which, at the present time, is 8,000 feet in circumference, and 440 feet above
the bay. The present aspect of Monte Nuovo is represented in Fig. 1, and the region around is shown in the frontispiece, from the last edition of Lyell's "Principles of Geology."
The Phlegræan Fields, of which Monte Nuovo now forms a part, have, in the opinion of Sir Charles Lyell, a "mutual relation with Vesuvius—a violent disturbance in one district serving as a safety-valve to the other—both never being in full activity at once."
In the Sandwich Islands, in 1868, Mauna Loa and the craters of Kilhauea on its flank were in active eruption. The valleys of the mountains were filled with rivers of fire, and a cloud of smoke and vapor arose, it is said, over the mountain, to a height of eight miles. During these fearful phenomena, which continued more than a month, 1,500 earthquake-shocks occurred, 300 of which were counted in five days. But whether shocks occur in the immediate vicinity of volcanoes during eruptions, or whether activity of the one diminishes the violence of the other, it is certain that they have a mutual relation, and probably a common origin.
The opening and closing of fissures and chasms in the ground during earthquakes is a common phenomenon. Men, animals, and dwellings, are sometimes swallowed in them, and forever disappear. In 1848 an earthquake shook a large portion of New Zealand, and a fissure of great depth opened along a chain of mountains from 1,000 to 4,000 feet high, extending 60 miles, but of not more than 18 inches in average width.
During the Calabrian earthquake of 1783 the surface of the ground opened and closed in immense fissures, by means of which new lakes were formed and others drained or were dried up.
At Jerocarne the earth is described, by Sir Charles Lyell, as lacerated in an extraordinary manner. "Fissures ran in every direction, like cracks in a broken pane of glass."
In another instance, several dwellings were engulfed in a fissure, and were found to be jammed with their contents into a compact mass. Chasms of immense length and depth were formed. Some were crescent-shaped, and a mile in length.
The plains of Calabria were covered in many places with circular hollows from one foot to three or four feet in diameter. Some of these were filled with water, others with dry sand.
Fig. 4 is a section of one of these circular holes, which appears to be funnel-shaped.
But changes in the earth's crust occur during earthquakes, on a still grander scale. Evidences of local disturbance, however disastrous it may have been, are often effaced if not forgotten in a few centuries frequently in a few years. But the slow upheaval of mountain-chains and the dislocation of strata through profound depths are results which alter at last the physical aspect and contour of the surface of the globe. It would not be proper, however, to say that these changes are caused by earthquakes, but rather that the earthquake vibration is a concomitant of the displacement by which they are produced.
Humboldt, Lyell, Dana, and other authorities, consider earth quakes to be the dynamic result of action of the earth's heated interior upon its cooled exterior. Whether the central portions of the earth be fluid or not, it is quite certain that heat increases as we descend; and it is estimated by Sir Charles Lyell that the heat at a depth of 25 miles would be sufficient to melt granite, and at 34 miles to render fluid or incandescent every known substance. We have no means of knowing the condition of matter under the enormous pressure which prevails at a depth of 34 miles, and are most concerned with the fact that the heat of fusion exists at no very great depth beneath the surface. The earth's crust is, therefore, its cooled exterior.
It is found that nearly all rocks contract by cooling and expand by heat. Lyell estimates that sandstone a mile in thickness, and heated to 200° Fahr., would expand so as to lift a mass of rock upon it 10 feet above its former level; and if a mass of the earth's crust equally expansible, 50 miles in thickness, be heated to 800°, it would rise 1,500 feet. From cooling we have the reverse effect—shrinkage, contraction, lateral pressure, and ultimately bending of the strata.
The strain thus produced will at last cause fracture, and the vibration that results is an earthquake. In Fig. 5 we have an illustration of fracture and displacement.
This form of tension is being continually and everywhere produced in the earth's crust, and there is probably no instant of time when that crust is entirely free from vibrations.
"There is nothing," observes Darwin, "not even the wind that blows, so unstable as the level of the crust of the globe."
Prof. Tyndall observes that, "where the acting force is small and the time great, the result is a slow and almost inappreciable change." Thus, great areas of land may be elevated or depressed. "But where the intensity is great and the time small, sudden convulsion must ensue." Thus, in an instant, mountains may undergo a change of elevation, or be shaken to fragments, or tracks of land sunken and over-flowed. In the delta of the Indus are extensive areas of level ground, over which native villages were scattered, with fortifications and other defences. Of these, the fort at Sindree is shown in Fig. 7 as it was
before the disastrous earthquake of 1819. 2,000 square miles of the delta sank from six to 12 feet, and was thus overflowed by the sea. The village of Sindree and its fortifications were upon the sunken area.
Northward, about 5½ miles from Sindree, a range of very low hills was elevated during the earthquake. It was seen over the expanse of waters, and extended about 50 miles, with a breadth in places of 16 miles, and was called by the natives, "Ulla Bund, or the Mound of God."
In 1822, just half a century ago, an earthquake occurred in Chili, of terrific violence, even for that region of convulsions. It was estimated that 100,000 square miles of land were elevated from two to seven feet, the rise being greatest inland, and probably included a portion of the Chilian Andes. The location of the force must have been at great depth, perhaps not less than 20 miles below the base of the Andes; and it is probable that the entire superincumbent mass underwent a change of level of from two to seven feet of perpendicular elevation.
The earthquake at Lisbon, in 1755, has impressed the public mind more than any other in modern times. The shocks, one of which exceeded all the others in violence, continued six minutes. The mountains near were shaken to their foundations, and everywhere split and rent. No part of the city was seriously injured which was built on the limestone or basaltic formations; but the shocks were most violent and disastrous in the tertiary and blue clay on which the ruined portion of the city stood.
The sea-wave put in motion by this earthquake exceeded in volume all others of which we have a record, except the one already noticed, which traversed the Pacific Ocean in 1868. It was observed, during this convulsion, that the sea retired from the shore before the great wave rolled in.
It was Darwin who first suggested that waves first draw the waters from the shore on which they are advancing to break. He calls attention to the familiar fact that waves thrown up by the paddles of a steamer, as they approach the shore, are alway preceded by a receding of the water. An under-draught seems to first suck the water back, and such actually is the fact. Now, in the sea-wave raised by the earthquake, what takes place? We have remarked that an
earthquake is a vibration of the earth's elastic crust, and moves with tremendous velocity. When it occurs beneath the sea, or when the undulations reach the surface beneath the sea, the motion is communicated to the water, which it elevates in a wave. Simultaneously with this lifting of the water, an under-draught toward that point takes place. Were it not so, the elevation of the wave could not be sustained. Directly the great wave moves from the area of disturbance at the rate before stated, of 400 miles an hour, or about 6½ miles in a minute, in the deep ocean. It is described by Mallet as "a low, broad swell of the sea. It might pass beneath the vessel unobserved." Approaching the shore, the front becomes elevated. The under-draught has continually preceded it, and has withdrawn the water from the shore, so that vessels at anchor are frequently grounded, and the wave seems to stand upon the bottom like a gigantic wall. At Arica it was unbroken by a ripple, and "shone in the sun like burnished silver."
A notion prevails that earthquakes are always preceded by unusual conditions of the atmosphere, but careful observations have shown that they occur during all kinds of weather. The Lisbon earthquake,
which took place in the morning of the 1st of November, was preceded by a "period of clear autumnal weather," but the morning was calm, foggy, and warm. At Arica, as we have learned, the sky was serene and the atmosphere tranquil. Some of the greatest convulsions have been preceded by a close, hazy sky. Sir Charles Lyell observes that "irregularities in the seasons frequently precede and follow shocks. Sudden gusts of wind interrupted by dead calms, violent rains at unusual seasons, or in countries where they seldom occur, are phenomena often attending earthquakes."
The number of important earthquakes up to the year 1881, of which we have a reliable account, is, according to Prof. Ansted, 7,000. So meagre are early records that only 787 of these are spoken of previous to the year 1500. There is a catalogue of 3,340 which occurred from 1800 to 1850, or one in about five days. The means of detecting and recording shocks are now so perfected, that, when applied in all parts of the globe, they will, doubtless, fully justify our statement that in no instant of time is the earth's crust free from vibrations. The seismograph is an instrument for the "automatic registration of earthquake-shocks."
An interesting account of this instrument, by George Forbes, was published in the September number of The Popular Science Monthly.
Earthquakes have been defined to be a "travelling zone of vibration." The movement is in every direction from the area of disturbance, and the velocity depends on the substance and structure of the material through which it is transmitted. In New Zealand, in 1848, people on the shore witnessed the disastrous progress of the earthquake along the mountains before they felt the shocks. At Messina, during the Calabrian earthquake, the terrified inhabitants saw villas overthrown upon the coast by shocks which they had not felt, but which in a moment laid in ruins a portion of their own city. The velocity with
which the vibrations travel has been a subject of careful investigation. The Lisbon earthquake moved about 20 miles in a minute; that which occurred in 1819, in the delta of the Indus, appears to have moved at the rate of 53 miles in a minute, or nearly 5,000 feet in a second. Other observations show that the movement may be from 1,000 to 5,000 feet per second. It has been ascertained that in blasting rocks the vibrations move in a second from 1,000 to 1,700 feet. The sound-waves move more rapidly, and, for this reason, shocks are usually preceded by subterranean rumbling. The velocity of sound through uniform strata is ascertained to be from 8,000 to 10,000 feet in a second. Tyndall found that sound-waves moved through burnt clay nearly ten times more rapidly than through air at a temperature of 32° Fahr. From this the phenomena of earthquake movement might occur in the following order: Supposing the centre of the disturbance to be beneath the ocean, as at Lisbon, an observer on the shore might expect to experience—
1. The underground rumble, moving at the rate of 8,000 to 10,000 feet per second.
2. The shock, moving from 1,000 to 5,000 feet per second.
3. The sea-wave, moving about 528 feet per second.
4. Sound, through the air moving at the rate of 1,090 feet per second. It should be noted, however, that the velocity of the sea-wave depends on depth of water.
The vibrations of an earthquake, it is evident, differ in no respect from those produced by other causes, excepting in intensity. The jar arising from a discharge of artillery, by a carriage rolling over pavements, or slamming of heavy doors, puts in motion a series of moving waves just as truly as does the rending of rocks, or an explosion of steam or gas in a fracture thus produced. But, a question arises, What moves when the earthquake is progressing. The phenomena maybe explained thus: Around the source of disturbance the rock is pressed outward in every direction as air is pressed outward around a vibrating bell, forming what is called a zone or shell of compressed rock. The extent of this compression is the width of the earthquake-wave, and depends on the force exerted and the elasticity of the rock. In each zone or shell there is always a point of maximum density—and that is where the energy of compression and the rock's elastic force are equal.
As the wave passes, another zone is formed, and the particles behind return by their elasticity to their former position. From this it is obvious that, as the wave is passing, the individual particles of the rock have first a forward and then a backward motion—a swing or excursion to and fro. The extent of this motion is the amplitude of vibration, and may be very small compared with the breadth of the wave.
Mallet found by computation that, given a certain depth of fissure, and a certain heat of steam, the expansive force would produce a wave of nine inches amplitude at the surface. His observations of the Neapolitan earthquake of 1857 show that the maximum amplitude at the surface was only 2.5 inches. In his elaborate and beautiful volume on the eruption of Vesuvius, in 1872, just published, Mr. Mallet reaffirms a statement previously made by him, that "it is the vibration of the wave itself, i. e., the motion of the wave-particles, that does the mischief, not the transit of the wave from place to place on the surface."
We understand, then, that there is motion of particles as well as a transit-wave; that the "travelling zone or shell of vibration" is a zone or shell of "elastic compression."
Fig. 9 illustrates the "shells" as they move away from B, the focus of disturbance. The transit-wave, with its interstitial vibrations, reaches the surface in the manner shown in the diagram Fig. 10.
The diagram shows the waves radiating from the earthquake focus A to c d e f and g successively, and reaching the surface at B, where the shocks would be vertical. At 1 2 3 they become more and more oblique, and at greater distances appear almost horizontal.
Now, while the movement of the transit-wave may be very rapid, that of the particles of matter is surprisingly small. At Lisbon the velocity of the wave was 20 miles a minute, or 1,200 miles an hour. According to Mallet, where the velocity of the transit wave was 1,000
feet per second, the movement of the particles was only 12 feet per second, or eight miles an hour, and he states that three columns of the Temple of Serapis, on the shore of the Bay of Baiæ (see frontispiece), a region subject to earthquake-shocks, would be overthrown by a shock "whose wave-particles had an horizontal velocity of 3½ feet per second." The shock which threw human beings 100 feet in the air, at Riobamba, must have had a velocity of 80 feet per second. The theory of Mr. Hopkins, published in 1847, was that the disastrous results of earthquakes were caused by the velocity of the wave of translation, and that theory is probably accepted by many who will distrust the conclusions of Mr. Mallet. But it is obvious to every observer that the enormous velocity of 1,200 miles an hour is not communicated to objects on the surface as the wave passes. They are rarely thrown to any considerable distance. Buildings are overthrown, but they fall where they stood.
We have already remarked that objects standing directly on the uniform strata are seldom injured by earthquake-shocks. Such was the case, as we have seen, with that portion of Lisbon which was built on the limestone and basalt. But where the surface, perhaps hundreds of feet deep, is of loose unelastic material, the transit-wave, with its vibrations, in passing through it, becomes broken into oscillations, its force is dissipated and motion reduced, but the vibratory swing which it communicates is sufficient to fissure the earth's surface and strew it with ruins.
On the coast of Dublin Bay, Mallet exploded gunpowder buried several feet beneath the surface, in the sand, and ascertained the intensity and velocity of the shock by a delicateThe filling of the fissure with water, and its conversion into superheated steam, may have produced the subsequent shocks. By calculation, the same author shows the enormous pressure and rending power of steam if admitted without limit into such a fissure. "If the temperature increase 1° Fahr. for 60 feet depth, then, at the focal centre of the fissure, the temperature would be 883.4° Fahr., and the pressure on the walls of the cavity not less than 640,528,000,000 tons. But the pressure would be vastly increased if the temperature be near that of melted rock." That this may be the case is rendered probable from recent investigations of Mallet, by which he shows that the heat . Other experiments gave the rate and intensity of movement in more compact formations with the following results: In sand, 824.9 feet in a second; in divided granite, 1,306.4 feet; in compact granite, 1,664.6 feet. It is found by observation that objects, as walls and chimneys, fall backward or forward, but generally in a line with the direction in which the wave travels, while fractures of walls occur in a line transverse to the direction of the wave. And by diagram, Fig. 10, it will be obvious that, given the position of the ruins over an extended area, not only the centre but depth of disturbance may be ascertained. If the focus be C, as shown in the diagram, the wave would reach the surface at an angle other than if the focus be at A, and the result would appear in the manner of displacement on the surface. By the principle here indicated, Mallet was able to locate the central area and depth of the Neapolitan earthquake of 1857, and states that the great fissure, the forming of which caused the first shock, was 7½ miles in length, and 5¾ miles in average depth.
which melts the great lava-beds, and fills cavities in the earth's crust with steam and gases, may not arise directly from the earth's central heat, but from the crushing of strata as it contracts and settles upon the cooling interior.
By a series of experiments and observations made by Mr. Mallet, it is shown that the "annual loss of heat into space of our globe at present is equal to that which would liquefy, at 32° Fahr., about 777 cubic miles of ice; and this is the measuring unit for the amount of contraction of our globe now going on."
The amount of shrinking depends, therefore, on the amount of heat lost—a view long since insisted on by Prof. Dana; and this, according to Mallet, is sufficient to account for all the phenomena. To this cause, then, we refer the never-ending oscillations of the earth's cooled exterior, and the enormous lateral strain by which it is bent and fractured, and its broken ridges made to grind and crush with terrific vibrations.
In many areas the earthquake energies of former times have been long at rest, but, according to Sir Charles Lyell, the total energy may not have diminished.
He finds evidence of convulsions as great and obvious in recent as in earlier time. Mallet, however, remarks that "seismic energy may be considered as possibly constant during historic time, but is probably a decaying energy viewed in reference to much longer periods."
Everywhere we see, in exposed portions, crevices open or filled—ejections of trap and basalt; and wall-like dikes stand out upon the slopes of mountains. These are legible and significant chapters in the earth's dynamic history.
Do earthquakes occur with any order or system, so that their coming may be foretold?
Prof. Palmieri, in his observatory on Mount Vesuvius, is able, says George Forbes, "to predict eruptions." "This is a small eruption," remarked the professor, "but there is going to be a greater one; it may be a year hence, but it will come." "In almost exactly a year," continues Mr. Forbes, "the great eruption did come."
From Mallet's catalogue of European earthquakes it appears that, during 15½ centuries, 1,157 took place during the winter, against 875 in the summer months.
Although science has cleared up some of the mystery which hung over earthquakes in less enlightened times, it has not divested them of their sublimity and terrible reality.
Their work of destruction is done in a moment. The great battles of the world have scarcely been so destructive of human life.
We read that 250,000 persons perished during the earthquake at Antioch in 526. At Lisbon 60,000 people were destroyed. During one of the Calabrian earthquakes 35,000; and during the one at Arequipa in 1868. 40,000 persons perished. Pestilence, famine, and fire, add to the fatality. Visitations so severe and disastrous permanently affect the inhabitants of earthquake regions. Their minds lose their calm equipoise — they become nervous, and the first considerable shock sends them to the street or cathedral for safety.
Humboldt remarks that, when "we feel the ground move beneath us, our deceptive faith in the repose of Nature vanishes, and we feel ourselves transferred into a realm of unknown and destructive forces. Every sound, the faintest motion of the air, arrests attention. To man, the earthquake conveys the idea of unlimited danger." And Von Tschudi adds his testimony, that "no familiarity with earthquakes can blunt this feeling of insecurity. The traveller from the north of Europe waits with impatience to feel the movement of the earth, and with his own ear to listen to the subterranean sounds, but, soon as his wish is gratified, he is terror-stricken, and is prompted to seek safety in flight." Thus it is that physical phenomena aid in moulding the mental and moral character of a people. The earthquake records itself, not only on the inorganic world, but in man's spiritual nature.