Earthquake, a shaking of the solid ground by more or less violent movements, produced by natural forces. Such shakings, to which every part of the world is liable, are often imperceptible except to very sensitive instruments, but on the other hand are frequently productive of fearful destruction of life and property. These phenomena claim our special attention, since we must hope through them to make our next steps toward a full knowledge of the condition of the interior of our globe. After briefly considering the history of our knowledge of this subject as shown in its bibliography, we shall recount successively a few of the remarkable earthquakes, and shall then proceed to a short summary of our present knowledge of the subject, giving first some general views on the distribution of earthquakes, and then reviewing some of the many theories that have been broached in connection with this subject; in conclusion we shall give with some minuteness the remarkable results attained by Mallet and others who have lately established the science of seismology (Gr. earthquake) upon a firm basis.
Among the classical writers, Aristotle, Strabo, Seneca, Pliny, Josephus, etc, give numerous facts mixed with fanciful theories on the subject of earthquakes. In the earlier works of the modern revival of knowledge, Beccaria, Bylandt, Flamsteed, Percival, Priestley, Stuke-ly, and others, have sought with but little success by observations and theories to advance our knowledge of the origin of earthquakes; and with these we may class the memoir by Michell published in 1760. Of the works belonging to the present age of philosophical inquiry may be mentioned, first, those that treat specially of the phenomena of individual earthquakes, such as Kant, Roche, and Pereira, on the Lisbon earthquake of 1755; the report to the academy of Naples on the great earthquake of 1783 in Calabria; that of Pal-mieri and Scacchi on the Melfi earthquake of 1851; that of Volger on the earthquake of Viege, 1855 ; the report by Gilliss on his astronomical expedition to Chili, 1849-'52 ; and that of Mr. Mallet to the royal society at London on the Neapolitan earthquake of 1857, which was published in 1862 under the title of "First Principles of Observational Seismology." Numerous special or general catalogues of earthquakes have been published, of which we note those of Von Hoff, Cotte, Hoffmann, Merian, and especially the great catalogue of Mallet, "The Earthquake Catalogue of the British Association," which very complete record embraces between 6,000 and 7,000 earthquakes recorded between the years 1606 13. 0. and A. D. 1842. For the interval 1842 to 1872 the catalogues compiled by Perrey and published from time to time in the transactions of the Belgian royal academy form an almost exhaustive record.
The special catalogues of Per-rey for southern Europe, of W. T. Brigham for the United States, etc, may also be consulted. The catalogue by Scrope in his treatise on volcanoes (2d ed., revised and enlarged, 1872) is quite trustworthy for the most recent dates. Of theoretical works on the nature and origin of earthquakes there is scarcely any end, but among the few who have really advanced this branch of knowledge we may mention Dr. Thomas Young and Gay-Lussac, who seem to have remarked the similarity between earthquake shocks and the vibrations of sounding bodies, and Mr. Mallet, who, viewing all the phenomena with the eye of an expert mechanical engineer and by the application of rigidly exact measurements and inductive reasoning, has undoubtedly succeeded to a remarkable degree in perfecting our views of the forces acting during an earthquake, and advanced our knowledge of the ultimate origin of the initial disturbance. The intimate connection between the phenomena of geology on the one hand and of volcanoes and earthquakes on the other has been treated of with more or less fulness by Babbage, Elie de Beaumont, Leopold von Buch, Dana, Darwin, Daubeny, Forbes, Herschel, Hopkins, Hottingen, Humboldt, Hunt, Lyell, Murchison, Phillips, Ritter, Rogers, and Scrope; and the established laws of dynamics, of heat and of strength of materials, have been applied with much success to the question of the origin of these disturbances by Haughton, Hopkins, Oldham, Pr6vost, Thomson, and especially by Mallet (see his preface to Palmieri's "Vesuvius," London, 1873). Of general treatises, that by Prof. J. L). Whitney, " Earthquakes, Volcanoes, and Mountain Building" (New York, 1871), and that by Prof. Boccardo, Seismopirologia (Genoa, 1869), are among the most recent.
The essay by Mallet lately presented to the royal society promises when printed in full to mark an epoch in the science of seismology. Interesting matter will also be found presented in a popular style in Reclus on "The Earth." - Memorable Earthquakes. In southern Italy and Sicily no century has elapsed since the earliest periods of history that has not been distinguished by severe if not frequent earthquakes. From 1773 to the end of 1776 this region was almost constantly disturbed; no fewer than 947 shocks were experienced in the first of these years, of which 501 were of the first degree of force. Lyell observes that great importance attaches to these from the minuteness of the observations of men competent to collect and describe with accuracy the physical facts that throw light on geological questions. The great earthquake of 1783 in Calabria probably caused the death of 100,000 persons, and was felt in a great part of Europe; it was fully described by the commission of the Neapolitan academy.
The origin of the shock was at a spot under the centre of Calabria; the disturbance passed under the sea without producing any great sea wave, but on reaching the opposite coast of Sicily destroyed the city of Messina. The destructive sea wave that entered the harbor of the latter city was not propagated across the sea, but was probably caused by the dislocation of large masses of rock that fell into the waters near Messina. In central Italy, among the earliest of the recorded earthquakes is that of the year A. D. 63, which resulted in the partial destruction of Herculaneum and Pompeii, 16 years previous to the time when those cities were buried under the lava and ashes accompanying an eruption of Vesuvius. The earthquake of 1857 in the kingdom of Naples is the most noteworthy that has occurred there during the present century, not only because of its extent and fatal violence, but even more so because of the masterly investigation to which its phenomena were submitted by Mallet on behalf of the British association for the advancement of science. This earthquake was felt throughout the kingdom of Naples; in the city itself comparatively slight damage was done, the chief scene of the destruction being in the provinces.
The velocity with which the wave of shock spread in all directions from its origin near the town of Potenza was about 775 ft. per second. Of the many suggestive results of Mallet's investigation we shall have occasion to speak further on. - The earthquake of 1855, known as the earthquake of Viege, has been carefully studied by Volger. It was felt slight-ly in Paris, Mentz, and Geneva; the region of greatest violence was between Bern, Lugano, and Chamouni; the velocity of translation of' the wave of shock was northward 2,861 ft. per second, but southward only 1,391 ft. On Nov. 14, 1861, another great earthquake occurred in Switzerland, simultaneously with which it was noticed that in the artesian wells at Passy, Paris, the sediment suddenly increased from 956 to 2,268 grains per cubic metre, after which it began to decrease. In general it has been noted that the springs of this place are similarly affected by almost every earthquake of western Europe. - In Portugal, the city of Lisbon was visited on the morning of Nov. 1, 1755, by one of the most memorable earthquakes recorded in history. The rumbling sound that precedes most earthquakes was immediately followed by the great shock which threw down the principal portion of the city.
The sea retired, leaving the bar dry, and returned in a minute as a great wave or breaker 50 ft. or more in height. It is believed that 60,000 persons perished in the space of six minutes. The part of the city that was permanently engulfed beneath the waters of the bay was covered to the depth of 600 ft. The portion of the earth that was shaken by this earthquake was estimated by Humboldt as equal to four times the extent of Europe; but many of the reports and estimates in reference to it have been greatly exaggerated, and from more exact considerations Reclus states that probably an area equal to six times that of France was sensibly disturbed. The shock was felt in the Alps and on the coast of Sweden; in Bohemia the warm springs of Teplitz disappeared for a time, and again burst forth, deluging the region with ochre-stained waters. Many towns were destroyed in northern Africa. The waves of shock reached Scotland, probably passing under it, causing temporary changes in the waters of Loch Lomond, which suddenly rose more than two feet and then fell to below their usual level.
Almost if not quite simultaneously there began the great eruption of the volcano of Kotluggia in Iceland. The velocity of propagation of the wave of shock was about 2,000 ft. per second, and its origin was probably under the ocean some distance west of Portugal. The great sea wave that swept over the coast of Portugal was nearly 60 ft. high at Cadiz, and extended to Madeira, and possibly to the West Indies. The numerous other earthquakes and earthquake waves experienced during the same month throughout Europe and America may very probably have had some connection with that at Lisbon, but are not to be confounded with the shock of Nov. 1. - In Syria, the Biblical records preserve notices of earthquakes in the reign of Ahab, about 900 B. C, and again in the reign of Uzziah, about 800. The earthquake that devastated Judea at the time of the battle of Actium, 31 B. C, was such as according to Josephus had never happened before, and caused the death of 10,000 persons. That which occurred at the crucifixion was accompanied by a darkness very similar to that recorded Jan. 22, 1835, in Central America, on the occasion of the eruption of the volcano Coseguina and the attendant earthquake. The ancient city of Anti-och has been peculiarly visited from time immemorial.
Besides many other instances, the following may be especially noted: The city was almost destroyed A. D. 115, at the time of the visit of the emperor Trajan, who was himself hurt. In 458 it was again visited, and in 526 occurred the most disastrous one of which any record has been preserved; Gibbon states that 250,000 persons are said to have perished at this time. Sixty years later, in 587, an earthquake destroyed 30,000 persons. The last occurred there in April, 1872. - In India, by the earthquake of 1819, a tract of 2,000 sq. m. of country near the mouth of the Indus was submerged, and a neighboring region elevated into a mound. The great earthquake of Jan. 10,1869, has been studied by Oldham with results of great value. The origin of the shock was a point under Asaloo, somewhat deeper down than in the Neapolitan earthquake as investigated by Mallet. Eruptions of sand and hot water took place through the fissures, forming cones. The entire lower valley of the Ganges was embraced in the field of disturbance, and the shock was felt quite decidedly at Calcutta. - In mid ocean there are certain regions frequently visited by earthquakes, if we may rely on the testimony of those navigating them.
Among these may be specially designated the portion of the Atlantic ocean near the equator and about half way between Guinea and Brazil. - In the United States, in 1811, occurred the famous earthquake of New Madrid in Missouri, in the valley of the Mississippi river. Humboldt remarks that it presents one of the few examples of the incessant quaking of the ground for several successive months far from any volcano. Over an extent of country stretching for 300 miles southward from the mouth of the Ohio river the ground rose and sank in great undulations, and lakes were formed and were again drained. The surface burst open in fissures that generally trended N. E. and S. W., and were sometimes more than half a mile long; from these fissures mud and water were often thrown as high as the tops of the trees. During the continuance of these convulsions the inhabitants distinguished two classes of movements, the vertical and the horizontal; the latter were regarded as far more desolating than the former.
The disturbances continued over what has since been called " the sunk country" until March 26, 1812, when they ceased coincidently with the great earthquake of Caracas. The most severe earthquake that has been recorded in the middle and eastern states was that of Nov. 18, 1755. The shock then felt in New England was undoubtedly a wave promulgated from either the same centre whence emanated the tremendous disturbance that had destroyed Lisbon on the first day of the month, or from a centre whose activity had been stimulated by the continual quakings that then prevailed from Iceland to the Mediterranean. This earthquake of the 18th began in Massachusetts with a roaring noise like that of thunder; after a minute's continuance of this there came the first severe shock with a swell like that of a long rolling sea - a swell so great that men in the open fields ran to seize something by which to hold on lest they should be thrown down. After two or three lesser shocks there came the most violent of all, producing a quick horizontal tremor with sudden jerks and wrenches; this continued two minutes, and after a slight revival died away. Numerous other shocks followed in the course of a month.
In Boston the main shock threw down or dislocated and twisted many chimneys, wind vanes, brick buildings, etc, and throughout the country it threw down the rude stone walls bounding the farms; new springs of water were opened; the vessels in the harbor felt the shock; large numbers offish were killed and floated on the surface of the water. Nine hours afterward, at 2 o'clock P. M., a sea wave 20 ft. high, that had undoubtedly originated at the source of this earthquake, arrived at the harbor of St. Martin's in the West Indies. On Oct. 19, 1870, occurred the most considerable shock that has been observed in the middle and eastern states during the present century. The source of this disturbance has been traced with some probability to the volcanic region 50 to 100 m. N. E. of Quebec; from this region the shock spread to St. Johns, New Brunswick, and thence was felt westward to Chicago and southward to New York. The velocity of the wave of shock was about 14,000 ft. per second. The occurrence of the shock felt at Quebec was telegraphed to Montreal by the operators of the Montreal telegraph company in time to call the attention of those at the latter city to the phenomena about 30 seconds before the shock reached them.
In California, the earthquake of 1852 destroyed one of the southern missions. That of March 26, 1872, was the most severe that has occurred there during many years; special damage was done in San Francisco by the cracking of the walls of fine public buildings. In Nevada, the mining regions suffered in 1871 by the destruction of Lone Pine and other settlements. In Mexico a disastrous earthquake occurred June 19, 1858. It extended throughout the valley of Mexico, demolishing the aqueduct that supplies the city with water; it was felt in most of the surrounding provinces, where it also did great damage, and was in some places recorded as being the most severe ever experienced there. - In the "West Indies, subject as they are to very frequent shocks, we need only note that of June 7,1692, by which Port Royal, the capital of Jamaica, was in less than three minutes sunk beneath the sea. The fissures produced in the earth opened and closed so rapidly that in some cases, it is said, the lower parts of the bodies of persons were buried while the upper portions remained above ground. - In San Salvador, near the capital, is a volcano that was thrown up in 1770, and which is very similar to Stromboli in the regularity of its intermittent eruptions; it has remained in a state of constant activity, and continues to increase in size by means of the accumulation of ejected lava, ashes, etc.
Its origin was preceded by earthquakes for several months, and on Feb. 23, 1770, by the opening of a crevice whence issued lava, ashes, smoke, and flame. On March 19, 1873, San Salvador was utterly destroyed. Three successive severe shocks were experienced; but the inhabitants were by the previous noises so well warned that a comparatively small loss of life (less than 500) took place. The frequency of earthquakes in this region is expressively told by the name given to it by the aboriginal Indians, Cuscatlan, "the land that swings like a hammock." - In South America, the city of Caracas, Venezuela, was entirely destroyed by three shocks within 50 seconds on March 26, 1812. In Ecuador, the city of Quito was almost destroyed on March 22, 1859. On Aug. 16, 1868, a terrible earthquake devastated a large part of Ecuador; this was one feature in the series of severe shocks that between the 13th and 16th were experienced over nearly the whole of the southern half of the eastern coast of the Pacific ocean. In Peru, Callao was destroyed in 1586; the accompanying sea wave must have been about 90 ft. high, and resulted from the joint action of two shocks.
It was again destroyed in 1746. On Aug. 13 and 14, 1868, the seaport of Arica suffered severely; and during several days there occurred numerous shocks throughout the coast region from lat. 10° to 25° S. The earthquake sea wave was specially destructive (see Arica), and it sped thence across the Pacific ocean, reaching the Hawaiian islands on the 14th and Yokohama on the 15th; it was also felt on the coasts of Australia and of Alaska. Among the numerous earthquakes that have visited Chili we will mention three very instructive ones. That of 1822 is interesting historically, because the accounts given of certain twisting effects led Mallet to enter upon those dynamical studies that have done so much to elucidate the obscure points in seismology. The earthquake was itself a very severe one, and specially interesting, since in connection with it there occurred a permanent elevation to the extent of from two to seven feet of fully 100,000 square miles of land lying between the Andes and the coast; lines of sea beaches at higher levels and further inland indicate the previous lifting up of the same region at different times along the same lines.
About midnight of Feb. 20, 1835, the city of Concepcion was for the fourth time destroyed; there were felt over 300 successive shocks within two weeks; the accompanying sea wave was 30 ft. high, and probably originated near the island of Juan Fernandez, where there also simultaneously broke forth a submarine volcano, which sent up a column of fiery ejecta through a depth of 400 ft. of water; about the city of Concepcion the sea water was black and of an offensive smell, killing many fish. Among the numerous earthquakes observed by Gilliss during his astronomical expedition to Santiago was that of April 2, 1851, the most severe since 1822. The heaviest portion continued a minute and a half, within which interval were experienced continuous rumbling and innumerable severe shocks or oscillations." The movements of pendulums and other free bodies were nearly in the meridian. The warning rumbling noises preceded the shock by about 15 seconds. The earthquake was severely felt over a region extending 200 m. N, and S., and at least 100 m. E. and W.; the point of greatest severity was apparently 30 m. S. of Santiago, though at all places visited by it it seemed to come from some point further south.
This earthquake was one of the cases, extremely rare in Chili, in which the origin of the disturbance was under the mainland, and where therefore no great sea wave was produced. In the Argentine Republic, Mendoza was overturned in March, 1861; the shock was very severe, and continued from the 20th to the 23d over Buenos Ayres and the entire confederacy, and 12,000 persons are estimated to have perished. - In the Hawaiian islands, the most severe earthquake ever known occurred on Feb. 19, 1871. At Honolulu it lasted 50 or 60 seconds, the motion being chiefly vertical with a N. E. and S. W. rocking movement; a roaring sound that was also heard far out at sea preceded the shock, and it was noticed that vessels that lay furthest from the wharves were first struck, and afterward those close to the shore. At Lahaina the direction of the vibration was S. and N. On the W. coast of Hawaii the shock was as severe as on the S. coast of Oahu; it seemed to come from the west. The various observed phenomena point to a centre of disturbance near the centre of the circle in the circumference of which is the group of islands. - In the East Indies is a seat of almost perpetual volcanic and earthquake activity.
In 1772, during an eruption of Papandayang, one of the loftiest volcanoes of Java, an area of 100 sq. m. was overwhelmed with ashes (not submerged under the sea), destroying about 40 villages and 3,000 inhabitants. Simultaneously eruptions took place from two volcanoes respectively distant 184 and 352 m. from Papandayang, although the many neighboring volcanic vents were quiet. On July 3, 1863, the city of Manila was much damaged by a violent earthquake; two systems of shocks were experienced, trending respectively in a N. to S. and E. to W. direction. The islands of Japan are subject to numerous violent disturbances. The earthquake of Dec. 23, 1854, which destroyed the ports of Simoda and Osaka and injured the city of Yedo, was accompanied by a sea wave 30 ft. high, which made the destruction more complete, and then swept eastward over the Pacific ocean in a series of five or seven waves that were felt a few hours later at the Bonin islands and on the California coast, affording Bache an opportunity to determine approximately the average depth of the water. - Registration of Earthquakes. The observations of the nature of earthquake shocks have heretofore been generally indefinite and unsatisfactory.
Every disturbed stone or other object forms of itself a permanent record of the force and direction of the shock, but the difficulty of properly interpreting the often complex results has led to the suggestion of a system of automatic registration by means of permanent simple instruments called seismographs, of which the following are some of the principal: 1. Horizontal plates of glass, wood, or stone, strewn with sand or loose blocks of various sizes, densities, and proportions, show by the changes impressed on the positions of the movable materials the direction and force of the horizontal components of a shock. 2. Tide gauges record not only the great sea wave, but also the smaller forced waves that precede this, and therefore give the means of determining the time of the occurrence as well as other data. 3. Similarly, the self-recording mercurial barometer, if sufficiently sensitive, records the time and also the force of the vertical component of the shock. 4. The delicate levels of the accurate astronomical instruments give information of the slightest change of level, and have in the hands of Wagner recorded vibrations that would otherwise have wholly escaped the senses. 5. An accidental observation of Gilliss, who happened to be viewing the moon during a slight earthquake, shows that by means of a telescope sighted on celestial or distant terrestrial objects we may become aware of the slightest imaginable vibrations, which may be recorded photographically. 6. The telegraph as used by Gilliss at Santiago in 1852, and by the operators in Montreal in 1871, may be used especially with a chronograph to record the times of transit of a shock as it passes several stations successively. 7. The self-recording magnetometers afford a very delicate record of the changes affecting the suspended needle, and also of the effect of the shock on the magnetic condition of the globe. 8. Pendulous bodies (neither barometers nor magnetometers, but simple pendulums) have long offered a favorite mode of observing the direction of the shock, which is the same as the azimuth of the axis of the ellipse described by the pendulum in consequence of the disturbance; the force of the shock is recorded by the extent of the arc of vibration. 9. Vertical spiral springs, from which weights are suspended or upon which they press, afford very delicate means of observing and recording the vertical component of the force. 10. The Cacciatore seismometer consists simply of a basin filled to the brim with mercury; at the eight points of the compass are small holes through which the liquid may be projected by the force of the earthquake shock. 11. Lamont's apparatus consists of finely balanced needles, to which may be attached small mirrors throwing a reflected beam of light into a telescope, so that the slightest movement may become visible. 12. The most complete instruments for self-registration are those employed by Palmieri, Kreil, and Mallet, but the last is far more perfect than any other.
Palmieri's apparatus consists of a fillet of paper drawn along uniformly, on which are made marks corresponding to the times and extent of the disturbances communicated by the vertical component of the shock to certain weights hung on spiral springs; for the horizontal component Palmieri employs four open U-shaped tubes partly filled with mercury; they are so set up that the planes of the tubes are directed toward different points of the compass; when the earthquake shock disturbs the quiescent mercury, a galvanic current is made or broken, by which the time record is made on a moving fillet of paper; light floats are also disturbed, and show by the extent of their movements the force of the shock. 13. The preceding instrument has some points of resemblance with that first proposed in 1846 by Mallet; the latter is however calculated to resolve with greater ease and accuracy all the problems connected with terrestrial waves of shock. Mallet employs one vertical and four horizontal L-shaped glass tubes, each separated from the other and filled for a short distance with mercury; the mercury being originally in a quiescent state, the linear motion that is communicated to it relative to the sides of the tubes by the wave of shock breaks or makes a galvanic circuit by which the time is recorded on a revolving cylinder; the extent of the vibrations of the mercurial column is also recorded by appropriate dots or lines, and the apparatus is so placed that we are able to deduce the movements of the earth in three directions, vertically, meridionally, and at right angles thereto, whence the angle of emergence and all the other local phenomena maybe calculated. 14. Kreil of Vienna proposed in 1855 to employ a pendulum suspended by two independent springs, of which the lower for instance allowed of motion in the meridian, but the upper in a plane at right angles thereto; the movement of the pendulum is recorded by its lower end marking on a revolving disk of paper or metal. - Connection between Earthquakes and Volcanoes. The intimate relation of these phenomena is apparent at once to even the most careless observer, but correct and exact ideas on the subject have been attained only within the present age.
On the one hand, earthquakes are most numerous in the volcanic regions of the globe; on the other, they are not at all confined to these centres; they are sometimes attended by the growth or formation of volcanoes, and in general no volcanic activity is manifested without being attended by more or less notable earthquakes, the quakings being however to a great degree confined to the volcano in eruption; the only exception to the latter statement can indeed be shown to be explicable in such a way as to give further confirmation of the general statement that the strains, pressures, and explosions occurring within the depths of the solid earth give rise at the surface of the globe, if feeble, to slight earthquakes, and if stronger, to more severe shocks accompanied by upheavals and depressions; if yet stronger, though not paroxysmal, to the formation of fissures, volcanic vents, and eruptions of steam, gases, hot water, mud, scoriae, ashes, lava, and flying stones. A slight earthquake central in a non-volcanic region is "an uncompleted effort to establish a volcano." The presence of a volcano is demonstrative of a previous earthquake, and the volcanic vent, offering as it does a way of easy escape for gases, etc, is, unless it becomes clogged up, to some extent an assurance that subsequent earthquakes in its immediate region will be of less severity.
The area thus protected may extend to a distance of 100 miles from the volcano, but is usually much less, showing that within the earth's surface the centres of the origin of earthquake disturbances are often quite insulated. The exceedingly great variety in the strata of the earth and the nature of the volcanic eruptions necessitates a special study of the individual cases, in order to perceive how intimately and yet how diversely these phenomena are associated with every variety of earthquake. - Atmospheric Relations. The connection between the atmosphere and the earthquake, though probably incidental, is an important one. It is found that there is a decided increase in the frequency and severity of shocks during the rainy season, at least in certain localities, and especially, as Mallet says ("First Principles of Seismology," 1862), in very dry countries such as Asia Minor and Syria, and in the volcanic regions where eruptions of steam prevail, and where the melted snows and heavy rains are rapidly drained off into deep fissures.
There seems to be no reasonable doubt that in these cases the pressure of the drainage water directly affects the liquid in the interior of the earth, and increases the supply of both steam and lava for the volcanoes, as well as the liability to such internal explosions of steam as may originate the earthquake shocks. That the diminution of atmospheric pressure such as prevails over the central regions of some severe storms, or during certain seasons, cannot directly be of importance in comparison with the above mentioned more powerful hydrostatic pressure, seems quite evident; yet there are volcanoes, such as that of Stromboli, where the conditions of equilibrium are so delicately adjusted that, according to the testimony and traditions of those living in their neighborhood, their activity increases perceptibly on the approach of every storm. - Geographical Distribution of Earthquakes. Besides the preceding general relations between earthquakes and volcanoes, it is important to consider the distribution of the former, or rather their segregation in certain well marked regions.
It is not necessary in this place to enumerate the regions of volcanic activity, although these are peculiarly also subject to earthquakes. (See Volcano.) The regions that are far removed from active volcanoes, yet specially subject to earthquake shocks, are, in the order of the severity of the phenomena, the Himalaya mountains and India, Syria, Algeria, the W. coast of North America, the Mississippi valley, Scotland, New England, and the Saint Lawrence valley. The regions that are at present comparatively free from sensible earthquakes are Egypt and the eastern and southern portions of Africa, northern Europe and Asia, Australia, many portions of North America, the eastern portion of South America, and Greenland. - Chronological Distribution of Earthquakes. The contortions of geological strata, their faults and fissures, and still more the presence of old trap dikes, etc, that break through every known geological formation, sufficiently show that in every part of the world earthquake phenomena have been common at some time throughout the entire history of the globe since the formation of its solid crust; indeed, we have in these phenomena the record of the condition of the interior of the earth at successive stages separated by long intervals of time.
The periodicity of earthquakes has been studied by many writers, of whom among the earliest was Merian, whose conclusion, confirmed by Mallet, Perrey, Vol-ger, Kluge, Bridgman, etc, was that these occur more frequently in the winter than in the summer months, as for instance in New England, Bridgman finds that between 1638 and 1869, of 227 earthquakes, 148 occurred during the winter half of the year, and 74 during the summer half. If this were an annual period independent of the wet and dry climatological seasons before alluded to, it would be of deep import; but on the contrary it is less marked in some countries than in others, its laws being apparently peculiar to the various centres of shock; it has therefore been treated in the preceding section as one depending on climatological considerations. Mallet has indeed shown that, considering the whole world in one view, there is a preponderance of shocks recorded in certain months; but it is possible, as he states, that this preponderance results from the greater fulness of the records relating to them for the northern hemisphere. A daily period has also been recognized, such that more shocks are recorded during the day than during the night.
This periodicity, which has been demonstrated for Switzerland, if it (as thus enunciated) be a law of nature, will be so neutralized by the corresponding daily period of the antipodal regions, that in the total sum of the earthquakes over the whole earth no such period will be noticed. The gradual diminution and ultimate cessation of earthquakes, as maintained in 1771 by Castilhon, and the periods depending on hurricanes and on electric phenomena, are all too poorly established to be worthy of more than a passing mention. Thus also must we dismiss the attempts of Falb and others to predict earthquakes for any considerable period in advance. The other periods of really cosmical importance are as follows: 1. That depending on the variations in the solar spots. 2. That depending on or connected with terrestrial magnetism. These two classes of phenomena, however, have as yet not been so well established as is necessary to their full acceptance. Lamont (1840), Varley (1870), and Sumichrast (1871) trace a connection between terrestrial magnetism and earthquakes, though they differ as to whether the former is to be considered as the cause or the consequence of the latter; most likely is it that both are consequences of a deeper common cause; in either case, however, there results a periodicity of earthquakes allied to the known periods in terrestrial magnetism. 3. The periods demonstrated by Perrey to be connected with the moon's phases. 4. That discovered by Palmi-eri for Mount Vesuvius, the violence of whose eruptions perceptibly increases twice in the solar day in a manner similar to the ebb and flow of the tide.
Both these latter may possibly be actual laws, though the modes of their connection are quite unknown and are likely to prove very complicated. 5. The quasi-periodical alternation of force announced by Mallet in his fourth report, such that a season of many severe shocks is followed by one of very few and sometimes slight shocks. 6. Mallet also detects indications of an increase in the frequency of earthquakes about the middle and end of each century. 7. The periodicity suggested by the facts first published by Humboldt, who remarked that in Mexico, South America, and the United States, the centres of earthquake influence seem to change their positions. Perrey and Mallet, agreeing with Miraldi and with the ancient traditions of Italy, announce the same fact for that peninsula; in it the numerous earthquake centres follow the general line of the ridges of the Apennines, as indeed Mallet has shown to be the rule over the whole world; but further it is shown that in Italy each of these centres has apparently a slow secular movement along the general line, so that its greatest energy is not recurrent at the very same spot, or not until after a long period. - Physical Changes produced by Earthquakes. Permanent upheavals and depressions affecting the physical geography of large regions of the earth are considered as being on an extended scale manifestations of the same force that suddenly elevates and depresses smaller districts at the moment of an earthquake.
Dana, Lyell, and others, presume that extended changes take place with extreme slowness and comparative quiet; and Prevost, Dana, Mallet, and others, attribute these to the influence of tangential pressures and to the slow cooling and contracting of the earth. Beaumont and others ascribe the elevation of the great mountain chains to single upheaving shocks, with of course accompanying earthquakes of the greatest severity. The former views have now far more adherents, they being more in accordance with the customary modes of operation in nature; it is however not to be denied that both methods of operation are admissible. Of sudden changes may be noticed the elevations during the earthquake at Concepcion in 1835, and the depressions attending the shock at Lisbon in 1755. Of slow movements we find examples in the upheaval of the Scandinavian peninsula, which is now proceeding at the rate on its eastern coasts of live feet per century. About one half of the islands of the Pacific ocean are believed to be rising, as also the island of Spitzbergen, the West Indies, and the whole of the W. coast of South America. Among the most remarkable slow depressions are the case of the southern portion of Greenland, the numerous coral-reefed islands and atolls of the Pacific, and the countries of Holland, Belgium, Denmark, and the S. shore of the Baltic sea.
In North America the most notable well established case of depression is that now in progress along the Atlantic coast from Cape Cod to Cape Hatteras, which region is calculated to be sinking at the rate of about two feet per century. - Earthquake Waves. This term is commonly applied to the great oceanic waves that attend such earthquakes as originate under the ocean. These waves while in deep water are merely very long but decided swells; in traversing shoal water they become more marked, and on nearing the shores often break most disastrously upon them, being in this case preceded by a rapid fall of the water, as is the case in ordinary breakers. The numerous cases that have been cited in a former section of this article suffice to show how prominent a feature of a disastrous earthquake are these sea waves. On account of their sudden definite origin and their vast size, these waves have on several occasions afforded a reasonable basis, in connection with the received principles of wave motion, for an approximate computation of the average depth of the sea over which they may have passed.
A fine example of such a computation was given by Prof. Bache in the case of the waves that rolled into the harbors of Simoda, Osaka, Peel's island, San Diego, and San Francisco, in December, 1854. These waves apparently started from a point near Simoda, and on reaching the American coast were recorded on the self-registering tide gauges of the coast survey. The crest of the highest wave experienced at Simoda was 30 ft. above the average sea level at 9h. 30m. A. M., local time; the highest at San Francisco was 0.45 of a foot above the normal height of the water, and occurred about nine hours after the wave at Simoda. From these and other data Prof. Bache concluded the mean depth of the intermediate Pacific ocean to be between 2,100 and 2,500 fathoms. The wave attending the Arica earthquake of Aug. 13, 1808, also extended over the entire Pacific ocean; from the records at four tidal stations of the United States coast survey, and the observations made at eight other points, including Australia, Hilgard (1872) concluded that the eastern equatorial portions of the Pacific are the deepest, while the depth of the northern portions seems quite small. - Nature of the Earthquake Shock. The discovery of the true nature of the earthquake, to say nothing of its ultimate origin or cause, for a long time baffled the labors of mankind; the application of a purely inductive train of reasoning has always been difficult, because of our own inability to experiment on the earth on so large a scale as was necessary, and because of the absence of exact observations and records.
The principal results arrived at previous to or independently of Mallet's dynamical studies have been already given in the generalizations under the head of geographical distribution, etc. The history of the numerous hypotheses and crude theories that have been advanced from time to time in reference to the nature of the earthquake shock may be thus briefly summarized. Up to the beginning of the 17th century no views other than the most indefinite and superstitious notions seem to have been propounded and accepted. In 1679 Travagini suggested that the successive shocks are pulses of force. Flamsteed in 1693 considered that vibrating air caused the earth to tremble, as in ordinary thunder. During the 18th century Buffon and others used the term concussion in speaking of the movement of the earth; and an anonymous French author in 1756 maintained that chains of mountains are long levers, so that a slight movement at one end will be felt as a blow or earthquake shock at the other. Since this period all students have recognized the phenomena as those of a concussion, except perhaps Michell, who in 1760 announced his theory that the surface of the earth heaved like the swell of the ocean, or even like a wave; a view adopted in some respects in 1843 by H. D. and W. B. Rogers to explain the plication of the strata of the Alleghanies. In 1807 Dr. Thomas Young suggested the probability that the motion of the earth at any point is a vibratory one, and that it is propagated through the earth in a manner analogous to that of waves of sound; but he appears to have made no clear distinction between the subterraneous waves of shock and sound.
Similar views were adopted in 1823 by Gay-Lussac. In 1846 Mallet published his work on "The Dynamics of Earthquakes," in which, without specially indicating the ultimate origin of the earthquake, he very clearly presents views that are now. widely accepted as to the true nature of the phenomena observed at the surface of the earth. These views are substantially as follows : An earthquake is the passage past the observer of a wave of elastic compression, in any direction from vertically upward to horizontally through the crust and along the surface of the earth, from any centre of impulse or from more than one, and which may be attended with sound and tidal waves depending upon the circumstances of the original impulse. When the wave of compression is passing through a solid stratum, each particle of the earth performs a vibratory movement similar to that made on the passage of a • wave of sound, moving forward and returning in an elliptical or a more complicated curve. When the shock reaches the earth's surface the vibration of the particles at any point may be somewhat to the right or left of the line of propagation from the origin of the impulse, owing to unequal strains or other disturbing causes; but in the main it is simply forward during the first half of the orbit (the first semiphase), and backward during the latter half, and therefore nearly in a straight line.
The forward and the return movements when at their maximum velocities are generally almost precisely in a line with the direction of the propagation, and it is at these moments of maximum velocity that the destruction of walls, etc, by the earthquake is accomplished. During the forward movement occurs also the greatest velocity in the upward direction. The downward movement of the earth accompanies the backward or the latter half of the pulse; but the backward movements are generally slower than the forward, and therefore have a less destructive effect. The twisting of isolated blocks, which had by many been ascribed to a circular or vorticose movement of the earth, is in reality, as Mallet shows, the effect of a simple direct stroke acting in connection with the inertia of the body and the friction at its base as a mechanical couple. When the origin of the impulse is vertically beneath the observer, the movement of each particle of the earth about him will be mainly in the vertical direction; when the origin is at a distance, the movement will be more or less inclined to the horizon; and if by a proper seismometer we determine for several places the azimuthal direction and the angle of emergence of the pulse, it becomes possible to fix exactly the position of the origin.
The destruction of walls, moving of solid bodies, etc, are therefore due not to the velocity of transit or that with which the shock passes along over the earth's surface, but to the velocity of the movement of each particle of the surface of the earth in its small elliptical orbit. The velocity of translation depends on the elasticity of the rock or earth, but rapidly diminishes in proportion as the rock is discontinuous or non-homogeneous; it is therefore generally different in each direction. The wave of shock or the pulse of compression is reflected and refracted on encountering a new stratum of different elasticity, precisely as in the case of sound. So also phenomena of interference can arise, and it is in such cases that the pulses cease to be rectilinear or elliptical, and become more complicated curves. "When the earthquake originates under the bed of the sea, the vertical shock communicated to the waters causes a wave or a swell, often of large extent, that spreads outward in all directions with a velocity varying with the depth of the ocean at the point over which it may be passing, and which on reaching shallow water rushes along as a breaker preceded by a fall or a recession of the water.
In earthquakes of this kind, therefore, there may proceed from a single origin (the seismic focus) within the earth a great wave of compression (the wave of shock) and, accompanying it, a wave of sound; both waves will before they reach the observer be transformed by the influence of discontinuity and non-homogeneity into a complicated mixture of strong and weak shocks and loud and feeble noises. The sea water first affected by the shock will receive and propagate waves of sound, small forced waves of water, and the large swell which ultimately becomes the great earthquake wave, all of which will reach the observer on the land at different moments, depending on their respective velocities. The disturbance having passed through the ocean upward to the atmosphere must communicate to it the three sorts of waves already existing in the water, but of these the sound wave is the only one that is perceived. This completes the entire phenomena of an earthquake so far as relates to the shocks and sounds.
When the impulse originates under the land, there is wanting from the preceding enumeration of six classes of waves or pulses only the great sea wave; all the others may be present, but if the shock extends with severity to the shore of a sea, the small forced waves that will arise may be of considerable magnitude, while the vertical stroke that was in the former case expended in producing the great sea wave now expends itself on terrestrial objects. The preceding views, as elaborated by Mallet, and in part adopted by Hopkins in his report of 1847 to the British association, were very generally accepted, but partook somewhat of a hypothetical nature, because of the scarcity of accurate observations of actual earthquakes, till 1862, when Mallet published the details of the methods and results of his investigation of the Neapolitan earthquake of 1857, to the study of which he had devoted much time at the expense of the royal society. In this investigation, the great importance of which as well as its complete success justifies our detailed attention, Mallet has established the science of seismology upon an immutable basis of accurate observation.
By using the fissures in buildings, the disturbances of heavy objects, etc, as natural seismometers, and by proper selections of representative cases, he was able, with the mathematical assistance of the Rev. Samuel Haughton, to fix from 177 determinations the position of the seismic focus or origin of the disturbance as being a cavity beneath the village of Caggiora near Potenza. From 26 determinations of the angle of emergence he found the mean depth of the cavity to be 5| m. beneath the sea. Lines of equal seismic disturbance being drawn, he showed the earthquake to have desolated a region larger than any of which we have exact knowledge. The refractions and reflections of the wave of shock in encountering various strata were clearly showm. From the number and directions of the sounds and shocks he deduced the general form and position of the focal cavity, and showed that it must have been of the nature of a curved fissure whose height was 3 m., length 9 m., and thickness very small. The amplitude of the vibrations of the particles at the surface of the earth near the seismic vertical was deduced as 2.5 in.; at a greater distance it was found to he somewhat greater, and Mallet remarks that it thus becomes evident that the earthquake is itself not an agent capable of directly producing any considerable permanent elevation of the land, but that it occurs along with and as part of a train of circumstances that do occasionally produce such elevations.
The velocity with which the shock is propagated along the surface, or the velocity of transit, was found from seven determinations to vary between 989 and G58 ft. per second, depending on the nature of the strata concerned. • This agrees well with other determinations made by Mallet in England and Schweitz in Hungary; but the velocities are 300 ft less than those found by Noggerath in southern Germany, where however different strata come into consideration, and Mallet has shown that owing to discontinuity and other causes the actual velocity of translation may be so low as one tenth of that which would take place were the rock continuous. These velocities refer to the surface of the ground, and require a slight increase in order to obtain the true velocity normal to the front of the wave. The maximum velocity of the wave particle in its orbit, or the velocity of the shock itself, was determined in 13 cases; the results vary from 9.8 to 21.2 ft. per second, their mean being 12.4 ft., and only three of the determinations materially differ from this. There were strong indications of a diminution of wave velocity with increase of distance from the focus.
It is therefore evident that the velocity of the shock that causes the destruction during an earthquake is far less than the rapidity with which it is propagated. It is not necessary to presume that a similar velocity prevails in all other earthquakes; indeed, in the case of the tremendous shock at Riobamba in 1797, Mallet shows that the velocity of the shock must have been about 80 ft. per second, and this is very probably one of the most violent on record. Owing to the imperfect elasticity of the earth, the return movement in the orbit of each particle is not so great or rapid as the forward movement; in two cases Mallet was able to determine the existence of a slight difference of this kind. A definite idea of the severity of the maximum shock experienced during this earthquake may be formed by considering that it was such as would have been given to an obstacle run into by a locomotive moving at the rate of 10 m. per hour. The nature of the fleeting earthquake forces that instantaneously desolate the earth having been thus most successfully investigated by Mallet, the same path has been followed by Oldham in an equally successful and instructive manner in reference to the earthquake of Cachar, India, on Jan. 10,1869; but the details of this work have as yet been only partially published. - The Origin of Earthquakes. If our ideas as to the true nature of earthquakes have but recently come into some degree of accordance with the truth, still more is this the case with our views as to their ultimate cause and origin.
On this difficult subject it might still be hazardous to offer any conjectures, were it not that the many unsuccessful attempts of the past have resulted to a certain degree in limiting our views to one or two causes that are not improbably in continual and effective operation. The theories relative to the origin of earthquakes may be divided into two classes: first, those that relate to the immediate active cause of all classes of earthquake motions; second, those that relate to the origin of the internal heat of the earth. The views of the first class have already been incidentally partially indicated in the preceding section. Considering them chronologically, we come first to the ancient indefinite notion of a blow or concussion somewhere within the earth, which was more definitely explained by Fromondi (1527) and by Travagini (1679). In 1693 Flamsteed suggested that explosions of gas communicated tremblings to the air and thus to the earth; in which view he was followed by Amontons (1703), who invoked heated air as the exploding gas. Lister regarded lightning and earthquakes as the results of the firing of "the inflammable breath of pyrites," i. e., sulphur; and Lemery (1700) considered earthquakes to be due to the explosions of any chemical mixtures containing sulphur and iron.
Bouguer also invokes the inflammation and explosion of gases. Stukely (1750) finds their origin in electrical discharges within the earth, in which he is followed by Beccaria, Donati, Percival, and Priestley; and even Humboldt and Poey do not deny the possibility of some connection with electricity. Buffon held that caverns exist in the interior of the earth, and that the falling of their ceilings down upon their floors produced the initial concussions and noises; Humboldt, Darwin, and others, have shown that such caverns may have some connection with earthquakes, though not necessarily in the manner indicated by Buffon. The theory of the Rev. J. Michell (1760) demanded the sudden formation or condensation of aqueous vapor between the crust and the molten interior of the earth, and the passage of waves of this vapor in between the sedimentary strata of the crust, lifting the upper strata in waves like those of a carpet when it is gently shaken on the floor. In 1785 Dolo-mieu explained how water penetrating into the interior of AEtna might flash into steam, producing eruptions and earthquakes; this idea, extended by Bakewell (1813), has since become a favorite with many, and in some shape or other probably has been the prevailing one of the past 30 years.
To meet certain very serious objections to Dolomieu's theory, Scrope (in 1825 and 1872) considers that the water is already in place, being confined in cavities within the earth, long before a given earthquake, but that it is exploded into steam by the heat conducted from some source perhaps at a great horizontal distance; he thinks that no concussion in the proper sense of the word takes place, but that when the expansive power of the water and vapor overcomes the adhesion of the rock and splits it, then the sudden tearing is accompanied by a reaction, giving rise to waves of shock, sound, etc. The idea of the formation of fissures as the result of protracted increasing tension seems also to have been adopted by Hopkins, Darwin, and others. In 1834 Keferstein, and in 1836 Sir John Herschel, independently of each other, advocated certain views that have been more elaborately treated by many geologists, such as Hunt (1858-'72), Sorby (1863), Deville, Nau-mann, Forbes, and Daubree. Starting with a suggestion of Scrope and Babbage that the heat slowly conducted from the solid interior of the earth would be concentrated near the surface under the increasing accumulation of sedimentary deposits that are poor conductors of heat, these chemical geologists show that such heat would probably slowly bring about chemical changes, metamorphism, evolution of gases, the transformation of the strata into a pasty mass of lava, and the explosion of the confined water, if such existed or were formed by the chemical changes.
In 1854 Zantedeschi maintained that the fluid nucleus of the earth and the exterior thin crust are subject to solar and lunar tides, the crust being flexible like a thin sheet of iron; and that the upward pressure against the crust of a high tide in the interior ocean of lava elevates and cracks the solid strata, causing earthquakes and volcanoes. These hypothetical tides have been rendered quite improbable by the researches of Hopkins and Thomson. Lamont (1862), Var-ley (1870), and Sumichrast (1871) have connected together the phenomena of terrestrial magnetism and earthquakes, and have expressed the belief that the latter are due to magnetic action. The earlier papers of Mallet (1846 and 1862) made highly probable the general truth of the theory that the sudden expansion of steam under high tension is the immediate cause of the earthquake; he showed that the phenomena of the earthquake of 1857 may be thus explained in a very reasonable manner; but in 1873 he concludes that more causes than one may and probably do produce these concussions, as will be seen in our next section. - The second class of theories of the origin of earthquakes includes those that seek for the source of the internal heat of the globe.
Passing by the ancient idea of internal fire, we come to Lemery (1700), Lister, Boyer, Breis-lak, Sir "William Hamilton, Sir Humphry Davy, Daubeny, and many others, who attributed the heat to some form or other of internal chemical changes. Stukely (1750) and his followers sought its origin in electric currents. The nebular hypothesis of Laplace (1780) afforded Poisson, Scrope, Hopkins, Lyell, Phillips, and others, a base for the idea, now widely accepted, that the present internal heat is but the remnant of that which prevailed when the earth was in its original condition of entire fluidity, and which it has slowly lost by radiation. Finally, when the mathematical investigations of Sir William Thomson had confirmed the idea of Hopkins that the earth must be in general solid, perhaps even to the centre, and when the ideas of permanent internal lakes of lava and of areas of special heat were beginning to be looked upon as too purely hypothetical, Mallet (1873) has with great success undertaken to solve the problem, and his views, covering as they do the whole field of vulcan-icity, seem to meet with universal approbation.
He shows that the secular cooling of the earth, supposing it to have been once a hot fluid globe, gave rise at first to a thin crust, whose contraction while thin was more rapid than that of the interior, thus causing it to be torn into segments by forces of tension; but that as the crust thickened it became subject to strains of the nature of tangential pressures as soon as the interior began to contract more rapidly than the exterior (the existence of such pressures was first pointed out by Prevost in 1835); the bending and crushing consequent upon these crushing pressures formed the chief mountain chains and ocean beds, as urged by Prevost, Dana, and others; the nucleus shrinking more than the shell, and the latter not being able to resist the strain upon it, it must crush and sink down upon the shrinking nucleus, the crushing being more or less paroxysmal; this crushing is accompanied by an immense evolution of heat, the quantity of which may be calculated by means of the theorems of the mechanical theory of heat, in connection with experiments instituted by Mallet (the heat resulting from crushing is a reality; that which Vose in 1866 suggested as resulting from pressure only cannot exist unless the pressure produce crushing or sudden compression); this heat, acting on the already heated interior, abundantly suffices to melt the adjacent portions there of, producing lava, or to convert water into the steam that produces both earthquakes and volcanic eruptions.
Mallet has made such experiments on the crushing of rocks and such calculations for all the volcanoes of the earth as prove that of the telluric heat known by the observations of the temperature of the earth to be annually dissipated into space, one fourth part is sufficient to account for the annual volcanic energy. He discerns therefore in the secular cooling of the earth and crushing of its shell a true physical cause for the earthquake; the crushing goes on per saltum, and is attended with either partial permanent elevations or with heat sufficient to melt rock and produce volcanoes and earthquakes. The function of the earthquake and volcano is thus seen to be preservative and not destructive; these are the instruments by means of which the solid crust of the globe is kept in a state to follow down after the shrinking nucleus; little by little they accomplish in each locality a work that would otherwise be accomplished by sudden and terrible paroxysms overspreading the whole earth. - Earthquake-proof Buildings. The views of Mallet as to the nature of the movement of the surface of the earth during an earthquake shock have enabled him to define certain principles according to which buildings may be constructed so that they shall be able to resist any shock that is likely to visit them.
These principles have been already applied by Mallet and Stevenson in the construction of lighthouses for the Japanese government, and by Mr. W. Lloyd in the construction of the new custom house at Valparaiso.