CHAPTER VII.ToC

While the unfelt earth-waves of the great earthquake were still wending their way over the zone that surrounds the disturbed area, the central regions were again shaken, at 6.29A.M., by a shock strong enough to produce fresh ruins in the stricken towns along the coast. Nearly two and a half hours of quiet followed, broken only by a few subterranean rumblings in the central part of the meizoseismal area. Then, at 8.51A.M., occurred another shock, short and sharp, and inferior in strength only to the principal earthquake. Both of these after-shocks were felt in Western Switzerland; indeed, they were perceptible nearly as far as the great shock; the second, however, a little farther than the first, for it alone was noticed at such places as Vicenza, Forlì, and Florence. The shock at 6.29 was usually described as long and its vibrations as undulatory only; that at 8.51 as rather subsultory than undulatory and of very brief duration. The latter, however, was followed after an interval of a few seconds by another shock so weak that it generally passed unobserved. Both shocks were preceded by a rumbling sound.

During the next two days, tremors and earth-sounds were frequent in the Riviera; once an hour, on an average, the greater part of the meizoseismal area was shaken by vibrations more or less slight. But, between one shock and another, at Diano Marina and Alassio, and even as far as Nice, it only required attention from a careful observer to perceive an almost continual throbbing of the ground.

Only one of these shocks, that of February 24th, at 2.10A.M., was strong enough to cause slight damage to buildings. It disturbed an area, not exceeded by any of the later shocks, the boundary of which, shown by the dotted line A in Fig. 33, extends to the north and east as far as Piacenza and Spezia, while to the west it includes Cannes. The centre of the curve so drawn lies on land, but, as the shock was not felt in Corsica, there is no evidence as to the southerly extension of the disturbed area; and it is probable, as Professor Mercalli suggests, that the shock originated in the eastern or Oneglia focus of the great earthquake.

After February 25th, slight shocks were felt during the next fortnight, at the rate of three or four a day, until March 11th, when the last after-shock resulting in slight damage occurred at about 3.12P.M.The boundary of its disturbed area, represented in Fig. 33 by the dotted line B, passes a little to the east of Savona, and then through Alessandria, Moncalieri, and Marseilles. The shock, however, was not observed in Corsica, so that the exact position of the epicentre is unknown; but Professor Mercalli believes it to coincide with the western or Nice epicentre of the principal earthquake. At themoment of the shock, the sea was observed from Alassio to curl and to rise slightly, while the tide-gauge at Nice, which had traced a continuous curve earlier in the day, showed a characteristic notch about 3.7P.M.

Of the remaining after-shocks, only two attained any notable degree of strength. One, on May 20th at about 8.15A.M., disturbed an area nearly concentric with that of the great earthquake, and with a boundary coinciding nearly with the isoseismal 2 in Fig. 33. Again, on July 17th at 11.30P.M., occurred a shock felt over an area nearly as large as that disturbed on February 24th at 2.10A.M., and situated in the same part of the country.

Altogether, during the year following the Riviera earthquake, Professor Mercalli records 190 after-shocks, most of them slight or only just felt. With the exception of the first two (on February 23rd), none was observed outside the isoseismal 4 of the principal earthquake (Fig. 33); and, of the rest, only the four whose dates are given above disturbed an area of more than one-eighth of that of the great shock. Some of them, like the shock of March 11th, were stronger in the western part of the meizoseismal area; but the majority affected most the eastern portion and seem to be closely associated with the Oneglia focus.

From February 26th to April 20th, Professor Rumi made observations on the after-shocks by means of the Foucault pendulum erected at Genoa for demonstrating the rotation of the earth. In nearly every case, the oscillations took place along a north-east and south-west line, or in the same direction as the first great shock—a resemblance which supportsthe inference that many of the after-shocks originated within the Oneglia focus.

Recent Movements in the Riviera.—The earliest movements that resulted in the great range of the Maritime Alps and the Ligurian Apennines date from pre-Carboniferous times, when the central crystalline massifs in part emerged. At the end of the Liassic epoch, the secondary formations of the district were uplifted, and it was at this time that the range assumed its characteristic curved form. Later still, at the close of the Eocene period, an elevation of more than 9000 feet took place, for upper Eocene beds are found at this height in the Maritime Alps.

Since that time, other important movements have occurred. Pliocene deposits have been found in the Riviera at an altitude of 1,800 feet. Recent soundings in the Gulf of Genoa have also shown that all the valleys of the Riviera between Nice and Genoa are continued far below the level of the sea to depths of not less than 3000 feet. Thus, at the end of the Pliocene or beginning of the Quaternary period, there was an elevation of nearly 5000 feet, accompanied or followed by the erosion of the valleys which, later on, during the Quaternary period, were submerged about 3000 feet. Even in still more recent times, probably in the Palæolithic age, minor movements continued. Traces of recent elevation, varying in amount from a few feet to sixty feet or more, occur at the Balzi Rossi in the Alpes Maritimes, near Bergeggi, and in Genoa;while evidences of submergence are to be found near Monaco, at Beaulieu and at Diano Marina. It is important to notice that the great movements dating from the end of the Eocene period are almost confined to the Maritime Alps and the western portion of the Riviera. In the parts of Piedmont lying to the north of Cuneo and in the eastern Riviera, they produced hardly any sensible effect.

Seismic History of the Riviera.—The movements just referred to are those which, in course of time, have become sensible to the eye. They represent the sum of a long-continued series of displacements that may once have been on a large scale, but are now comparatively small. The earthquakes that occur in the Riviera show, however, that the final stage has not yet been reached. Their epicentres indicate the regions in which slips are still taking place, and the magnitude of these slips is roughly measured by the intensity of the resulting shocks.

The map in Fig. 40 is one of a series drawn by Professor Mercalli to represent the distribution of seismic activity in Piedmont and the Riviera. It corresponds to the period from 1801 to 1895. The whole area is divided into a number of seismic districts, each of which is distinguished by a particular degree of activity. In estimating this quantity, Professor Mercalli takes intensity as well as frequency into account. Thus, the lowest degree, represented by the lightest tint of shading, corresponds to one or two strong earthquakes with a few moderate or slight shocks; the eighth and highest to four or five ruinous or disastrous earthquakes followed by trains of after-shocks. The map shows very clearly that, during the last century, theseismic activity was greatest in the Maritime Alps and the western Riviera—that is, in the very districts in which the recent mountain-making movements have been most conspicuous.[53]

Distribution of seismic activity in the Riviera.Fig.40.—Distribution of seismic activity in the Riviera. (Mercalli.)ToList

Fig.40.—Distribution of seismic activity in the Riviera. (Mercalli.)ToList

In all these districts, Professor Mercalli distinguishes several well-marked seismic centres, to each of which he traces the origin of two or more earthquakes. In the districts with which we are at present concerned, those of the Alpes Maritimes and the western Riviera, the most important centres aresituated near Oneglia (in the sea), near Taggia, in the valleys of the Vesubia and Tinea (near Nice), and in the sea to the south of Nice. To the first of these centres belongs the disastrous earthquake of February 23rd, 1887, as well as its after-shocks on February 24th, May 20th, July 17th, and September 30th of the same year, also the ruinous earthquakes of 1612 and 1854, and several others of a lesser degree of intensity. All of these were longitudinal earthquakes, the axes of their meizoseismal areas being parallel to the neighbouring mountain-ranges. A few miles to the west of Oneglia lies the Taggia centre, with which were connected the disastrous earthquake of 1831, the violent earthquake of 1874, and other strong or very strong shocks. These were for the most part transversal earthquakes, their axes being perpendicular to those of the Oneglia centre.

Some of the strongest earthquakes in this region originated in a centre lying to the north of Nice in the valleys of the Vesubia and Tinea. Among them may be mentioned the ruinous earthquakes of 1494, 1556, 1564, and 1644, and probably also the disastrous earthquake of 1227. A fourth centre, and one of considerable interest, is that which lies at sea, a short distance to the south of Nice, and nearly along the continuation of the valleys above-mentioned. This is the secondary centre of the earthquake of 1887, and probably also of that of December 29th, 1554. It is occasionally in action apart from the Oneglia centre, as on November 27th, 1771, June 19th, 1806, and December 21st, 1861; but such shocks, though rather strong, never reach a high degree of intensity.

Origin of the Earthquakes of 1887.—The most important feature in the principal earthquake of 1887 is its origination in two distinct foci, which are sometimes in action almost simultaneously, but more often separately. The earthquakes belonging to the two foci differ greatly in intensity and number, and the stronger part of the shock in 1887 originated in the focus associated with the more disastrous and more frequent earthquakes.

The existence of two foci would of course give rise to a meizoseismal area elongated in the direction of the line joining them. It is clear, however, that the Oneglia focus was also extended in the same direction; for, in the after-shock of February 24th, the isoseismals drawn by Professor Mercalli are parallel to this line; and this was also the case in the shock of March 11th. As both foci were under the sea, it is difficult to locate them with precision; but it seems very probable that they occupy portions of a submarine fault that runs parallel or nearly so to the Apennine axis between the meridians of Oneglia and Nice.

A brief period of preparation is a characteristic of the Riviera earthquakes. In 1887, two at least of the preliminary shocks on February 23rd (those of about 2 and 5A.M.) originated in the Oneglia focus. At 6.20A.M.the first and weaker movement took place in the western focus; and, a few seconds after the resulting vibrations reached the eastern focus, the second and greater slip took place there. The occurrence of seismic sea-waves is probably evidence of the formation of a small, though sensible, fault-scarp in the same region. To relieve the additional stresses thus brought into action along the fault-surface,numerous small slips took place in different parts, some as far to the west as the Nice focus, but the greater number probably within or close to the focus in the neighbourhood of Oneglia.

1.Bertelli, T.—"Osservazioni fatte in occasione di una escursione sulle Riviera Ligure di ponente dopo i terremoti ivi seguiti nell' anno 1887."Boll. Mens. dell' Oss. di Moncalieri, vol. viii., 1888, Nos. 6, 7, 8.2.Charlon, E.—"Note sur le tremblement de terre du 23 février 1887."Bull. del Vulc. Ital., anno xiv., 1887, pp. 18-23.3.Denza, F.—Alcune notizie sul terremoto del 23 febbraio 1887(Turin).4.Issel, A.—"Il terremoto del 1887 in Liguria."Boll. del R. Com. Geol. d'Italia, anno 1887, supplemento, pp. 1-207.5.Mercalli, G.—I terremoti della Liguria e del Piemonte. (Naples, 1897, 146 pp.)6.Oddone, E.—"I dati sismici della Liguria in rapporto alla frequenza ed alla periodicità."Boll. della Soc. Sismol. Ital., vol. ii., 1896, pp. 140-151.7.Offret, A.—"Sur le tremblement de terre du 23 février 1887. Discussion des heures observés dans la zone épicentrale." Paris,Acad. Sci., Compt. Rend., vol. civ., 1887, pp. 1150-1153.8. ——. "Tremblements de terre du 23 février 1887. Heures de l'arrivée des secousses en dehors de l'épicentre."Ibid., pp. 1238-1242.9.Rossi, M.S. de.—"Relazione sui terremoti del febbraio 1887."Bull. del Vulc. Ital., anno xiv., 1887, pp. 5-17.10. ——. "Bibliografia: Sul terremoto ligure del 23 febbraio 1887."Ibid., pp. 60-62, 107-112, 115-128.11.Taramelli, T., andG. Mercalli.—"Il terremoto ligure del 23 febbraio 1887."Annali dell' Uff. Centr. di Meteor. e di Geodin., vol. viii., parte iv., 1888. (Roma, 298 pp.)12.Uzielli, G.—Le commozioni telluriche e il terremoto del 23 febbraio 1887(Turin).13.Nature, vol. xxxv., 1887, pp. 438, 462, 534-535; vol. xxxvi., 1887, pp. 4, 151-152.14. Paris,Acad. Sci. Compt. Rend., vol. civ., 1887, pp. 556-557, 606-612, 634-635, 659-667, 744-745, 757-758, 759-760, 764-766, 822-823, 830-835, 884-890, 950-951, 1088-1089, 1243-1245, 1350-1352, 1416-1419; vol. cv., 1887, pp. 202-203; vol. cviii., 1889, p. 1189; vol. cix.; 1889, pp. 164-166, 272-274, 660.

2.Charlon, E.—"Note sur le tremblement de terre du 23 février 1887."Bull. del Vulc. Ital., anno xiv., 1887, pp. 18-23.

3.Denza, F.—Alcune notizie sul terremoto del 23 febbraio 1887(Turin).

4.Issel, A.—"Il terremoto del 1887 in Liguria."Boll. del R. Com. Geol. d'Italia, anno 1887, supplemento, pp. 1-207.

5.Mercalli, G.—I terremoti della Liguria e del Piemonte. (Naples, 1897, 146 pp.)

6.Oddone, E.—"I dati sismici della Liguria in rapporto alla frequenza ed alla periodicità."Boll. della Soc. Sismol. Ital., vol. ii., 1896, pp. 140-151.

7.Offret, A.—"Sur le tremblement de terre du 23 février 1887. Discussion des heures observés dans la zone épicentrale." Paris,Acad. Sci., Compt. Rend., vol. civ., 1887, pp. 1150-1153.

8. ——. "Tremblements de terre du 23 février 1887. Heures de l'arrivée des secousses en dehors de l'épicentre."Ibid., pp. 1238-1242.

9.Rossi, M.S. de.—"Relazione sui terremoti del febbraio 1887."Bull. del Vulc. Ital., anno xiv., 1887, pp. 5-17.

10. ——. "Bibliografia: Sul terremoto ligure del 23 febbraio 1887."Ibid., pp. 60-62, 107-112, 115-128.

11.Taramelli, T., andG. Mercalli.—"Il terremoto ligure del 23 febbraio 1887."Annali dell' Uff. Centr. di Meteor. e di Geodin., vol. viii., parte iv., 1888. (Roma, 298 pp.)

12.Uzielli, G.—Le commozioni telluriche e il terremoto del 23 febbraio 1887(Turin).

13.Nature, vol. xxxv., 1887, pp. 438, 462, 534-535; vol. xxxvi., 1887, pp. 4, 151-152.

14. Paris,Acad. Sci. Compt. Rend., vol. civ., 1887, pp. 556-557, 606-612, 634-635, 659-667, 744-745, 757-758, 759-760, 764-766, 822-823, 830-835, 884-890, 950-951, 1088-1089, 1243-1245, 1350-1352, 1416-1419; vol. cv., 1887, pp. 202-203; vol. cviii., 1889, p. 1189; vol. cix.; 1889, pp. 164-166, 272-274, 660.

[47]The above times and all others in this chapter are given in Rome mean time, which is 50m. earlier than Greenwich mean time.

[48]Professor Uzielli has also published a map of the isoseismal lines for the Italian part of the disturbed area.

[49]It seems doubtful whether this movement was connected with the earthquake. M. Offret does not include Nice in his list of observatories at which magnetographs were disturbed.

[50]This is the time given by M. Offret. According to M. Mascart, it should be 6h. 25m. 40s.

[51]In order to test the truth of this explanation, M. Moureaux suspended a bar of copper at the Parc Saint-Maur observatory by two threads in the same way as the horizontal force-magnet. The direction of this bar was also registered photographically, and it remained unmoved during the Verny earthquake of July 12th, 1889, and the Dardanelles earthquake of October 25th, 1889, while one or more of the magnets were disturbed. The experiment, however, was ineffective; for, in order that the magnet may rest in a horizontal position, its centre of gravity must be at unequal distances from the two points of support.

[52]The hour-marks in Fig. 38 refer to Paris mean time, and those in Fig. 39 to Genoa mean time.

[53]In the seventeenth century, the maximum seismic activity was manifested in the neighbourhood of Nice, and in the eighteenth century in Piedmont.

Although several years have elapsed since the occurrence of the greatest of Japanese earthquakes, the final report that will embody the labours of all its investigators is yet to be written. Several important contributions to it, however, have already been made. Professor Koto, in an admirable memoir, has traced the course of the great fault-scarp and discussed the origin of the earthquake; Professor Omori, with equal care and thoroughness, has investigated the unrivalled series of after-shocks; Mr. Conder studied the damaged buildings from an architect's point of view; Professor Tanakadate and Dr. Nagaoka devoted themselves to a re-determination of the magnetic elements of the central district,[54]while, by the compilation of his great catalogue of Japanese earthquakes during the years 1885-92, Professor Milne has provided the materials for a further analysis of the minor shocks that preceded and followed the principal earthquake.

The part of Japan over which the earthquake wassensibly felt is shown in Fig. 41. The small black area in the centre is that in which the shock was most severe and the principal damage to life and property occurred. The other bands, more or less darkly shaded according to the greater or less intensity of the shock, will be referred to afterwards. Fig. 45 represents the meizoseismal area on a larger scale; and, as the greater part of it lies within the two provinces of Mino and Owari, the earthquake is generally known among the Japanese themselves as the Mino-Owari earthquake of 1891.

Sketch-Map of Disturbed Area and Isoseismal Lines.Fig.41.—Sketch-Map of Disturbed Area and Isoseismal Lines. (Masato.)ToList

Fig.41.—Sketch-Map of Disturbed Area and Isoseismal Lines. (Masato.)ToList

More than half of the meizoseismal area occupies a low flat plain of not less than 400 square miles in extent. On all sides but the south, the plain, which is a continuation of the depression forming the Sea of Isé, is surrounded by mountain ranges, those to the west, north, and north-east being built up mainly of Palæozoic rocks, and those on the east side of granite. A network of rivers and canals converts what might otherwise have been unproductive ground into one of the most fertile districts in Japan. A great garden, as it has been aptly termed, the whole plain is covered with rice-fields, and supports a population of about 787 to the square mile—a density which is exceeded in only six counties of England. As a rule, the soil is a loose, incoherent, fine sand, with but little clayey matter; and it is, no doubt, to its sandy nature that the disastrous effects of the earthquake were largely due. In the northern half of the district, the meizoseismal area is much narrower, and here it crosses a great mountain-range running from south-west to north-east and separating the river-systems of the Japan sea from those of the Pacific. To the north, the meizoseismal area terminates in another plain, in the centre of which lies the city of Fukui, where the destructiveness of the earthquake was only inferior to that experienced in the provinces of Mino and Owari. There is also a detached portion of the area lying to the east of Lake Biwa, but it is uncertain whether the exceptional intensity there was due to the nature of the ground or to the occurrence of a secondary or sympathetic earthquake in its immediate neighbourhood.

General Plan of Geological Structure of Meizoseismal Area.Fig.42.—General Plan of Geological Structure of Meizoseismal Area. (Koto.)ToList

Fig.42.—General Plan of Geological Structure of Meizoseismal Area. (Koto.)ToList

The general plan of the geological structure of the central district is represented in Fig. 42. The thick line, partly continuous and partly broken, shows the course of the great fault, to the growth of which the earthquake chiefly owed its origin; while the thin continuous lines represent the changing direction of strike of the Palæozoic rocks which surround the Mino-Owari plain, and the arrowheads the direction of the dip. It will be seen that the direction of the strike forms an S-shaped curve, and it is clear thatthe present torsion-structure of the district could not have been produced without the formation of many fractures at right angles and parallel to the lines of strike. Professor Koto points out that the regular and parallel valleys of the rivers Tokuno-yama, Neo, Mugi, and Itatori, indicated by broken lines in Fig. 42, have probably been excavated along a series of transverse fractures running from north-west to south-east; while fractures which are parallel to the line of strike may be responsible for the zigzag course of the valleys.

The great earthquake occurred at 6.37A.M., practically without warning, and in a few seconds thousands of houses were levelled with the ground. Within the whole meizoseismal area there was hardly a building left undamaged. The road from Nagoya to Gifu, more than twenty miles in length, and formerly bordered by an almost continuous succession of villages, was converted into a narrow lane between two long drawn-out banks ofdébris. "In some streets," says Professor Milne, "it appeared as if the houses had been pushed down from the end, and they had fallen like a row of cards." Or, again, a mass of heaped-up rubbish might be passed, "where sticks and earth and tiles were so thoroughly mixed that traces of streets or indications of building had been entirely lost." At Gifu, Ogaki, Kasamatsu, and other towns, fires broke out after the earthquake. In Kasamatsu the destruction was absolutely complete; nothing was left but a heap of plaster, mud, tiles, and charred timbers. At Ogaki, not more than thirtyout of 8000 houses remained standing, and these were all much damaged. Within the whole district, according to the official returns, 197,530 buildings were entirely destroyed, 78,296 half destroyed, and 5,934 shattered and burnt; while 7,279 persons were killed, and 17,393 were wounded.

Next to buildings, the embankments which border the rivers and canals suffered the most serious damage, no less than 317 miles of such works having to be repaired. Railway-lines were twisted or bent in many places, the total length demolished being more than ten miles. In cuttings, twenty feet or more in depth, both rails and sleepers were unmoved; it was on the plains that the effects of the earthquake were most marked. The ground appeared as if piled up into bolster-like ridges between the sleepers, and in many places the sleepers had moved end-ways. When the line crossed a small depression in the general level of the plain, the whole of the track was bowed, as if the ground were permanently compressed at such places. "Effects of compression," says Professor Milne, "were most marked on some of the embankments, which gradually raise the line to the level of the bridges. On some of these, the track was bent in and out until it resembled a serpent wriggling up a slope.... Close to the bridges the embankments had generally disappeared, and the rails and sleepers were hanging in the air in huge catenaries."

The land area disturbed by the earthquake and the different isoseismal lines are shown in Fig. 41. The "most severely shaken" district, that in whichthe destruction of buildings and engineering works was nearly complete, contains an area of 4,286 square miles, or about two-thirds that of Yorkshire. This is indicated on the map by the black portion. Outside this lies the "very severely shaken" district, 17,325 square miles in area, extending from Kobe on the west to Shizuoka on the east, in which ordinary buildings were destroyed, walls fractured, embankments and roads damaged, and bridges broken down. The third or "severely shaken" district contains 20,183 square miles; and in this some walls were cracked, pendulum clocks stopped, and furniture, crockery, etc., overthrown. Tokio and Yokohama lie just within this area. In the fourth region the shock was "weak," the motion being distinctly felt, but not causing people to run out-of-doors; and in the fifth it was "slight," or just sufficient to be felt. These two regions together include an area of 51,976 square miles.

Thus, the land area disturbed amounts altogether to 93,770 square miles—i.e., to a little more than the area of Great Britain. According to Professor Omori, the mean radius of propagation was about 323 miles, and the total disturbed area must therefore have been about 330,000 square miles, or nearly four times the area of Great Britain. Considering the extraordinary intensity of the shock in the central district, this can hardly be regarded as an over-estimate.

The isoseismal lines shown in Fig. 41 are not to be regarded as drawn with great accuracy; for there is no marked separation between the tests corresponding to the different degrees of the scale of intensity. The seismographs at Gifu and Nagoyawere thrown down within the first few seconds, and failed to record the principal motion. But a great number of well-formed stone lanterns and tombstones were overturned, and, from the dimensions of these, Professor Omori calculated the maximum horizontal acceleration necessary for overturning them at fifty-nine places within the meizoseismal area.[55]At five of these it exceeded 4000 millimetres per second per second, an acceleration equal to about five-twelfths of that due to gravity. Making use of these observations, Professor Omori has drawn two isoseismal lines within the central district, which are shown in Fig. 44. At every point of the curve marked 2, the maximum acceleration was 2000 millimetres per second per second, and of that marked 1, 800 millimetres per second per second. The dotted line within the curve marked 2 represents the boundary of the meizoseismal area, which, it will be observed, differs slightly from that given by Professor Koto (see Fig. 45). The difference, however, is apparently due to the standard of intensity adopted, Professor Koto's boundary agreeing rather closely with the curve marked 2 in Fig. 44.

Little has yet been made known with regard to the nature of the shock, and the published records of the accompanying sound are so rare that it seems as a rule to have passed unheard. The seismographsat Gifu and Nagoya registered the first half-dozen vibrations, and were then buried beneath the fallen buildings. In the following table, the data from these two stations are therefore incomplete:—

Gifu.Nagoya.Osaka.Tokio (Imp. Univ.).Maximum horizontal motion> 18 mm.> 26 mm.30 mm.> 35 mm.Period of ditto2.0 secs.1.3 sec.1.0 sec.2.0 secs.Maximum vertical motion> 11.3 mm.6.2 mm.8 mm.9.5 mm.Period of ditto0.9 sec.1.5 sec.1.0 sec.2.4 secs.

If the period of the principal vibrations were known, the observations of Professor Omori on the overturning of bodies would enable us to determine the range of motion at different places. For instance, the maximum acceleration at Nagoya was found by these observations to be 2,600 millimetres per second per second, and if we take the period of the greatest horizontal motion to be the same as that of the initial vibrations—namely, 1.3 second, the total range (or double amplitude) would be 223 millimetres, or 8.8 inches. With the same period, and the maximum acceleration observed (at Iwakura and Konaki) of more than 4,300 millimetres per second per second, the total range would be greater than 14.5 inches.[56]

In the meizoseismal area, many persons saw waves crossing the surface of the ground. At Akasaka, according to one witness, the waves came down the streets in lines, their height being perhaps one foot, and their length between ten and thirty feet. To the north of the same area, we are told that "the shoreline rose and fell, and with this rising and falling the waters receded and advanced." Even at Tokio, which is about 175 miles from the epicentre, the tilting of the ground was very noticeable. After watching his seismographs for about two minutes, Professor Milne next observed the water in an adjoining tank, 80 feet long and 28 feet wide, with nearly vertical sides. "At the time it was holding about 17 feet of water, which was running across its breadth, rising first on one side and then on the other to a height of about two feet." Still clearer is the evidence of the seismographs in the same city. Instead of a number of irregular waves, all the records show a series of clean-cut curves. The heavy masses in the horizontal pendulums were tilted instead of remaining as steady points. They were not simply swinging, for the period of the undulations differed from that of the seismograph when set swinging, and also varied in successive undulations. It was ascertained afterwards, by measurement with a level, that to produce these deflections, the seismograph must have been tilted through an angle of about one-third of a degree.

Direction of the Shock.—Shortly after the earthquake, Professor Omori travelled over the meizoseismal area and made a large number of observations on the directions in which bodies were overturned, taking care to include only those in which thedirection of falling would not be influenced by the form of the base, such as the cylindrical stone lanterns so frequently found in Japanese gardens. At some places these bodies fell in various directions, at others with considerable uniformity in one direction. For instance, at Nagoya, out of 200 stone lanterns with cylindrical stems, 119 fell between west and south, and 36 between east and north; the numbers falling within successive angles of 15° being represented in Fig. 43. The mean direction of fall is W. 30° S., coinciding with that in which the majority of the lanterns were overturned. Similar observations were made at forty-two other places within and near the meizoseismal area, and the resulting mean direction for each such place in the Mino-Owari district is shown by short lines in Fig. 44, the arrow indicating the direction towards which the majority of bodies at a given place were overturned. It will be seen from this map that the direction of the earthquake motion was generally at right angles, or nearly so, to that ofthe neighbouring part of the meizoseismal zone, and that on both sides of it, the majority of overturned bodies at each place fell towards this zone.

Plan of Directions of Fall of Overturned Bodies at Nagoya.Fig.43.—Plan of Directions of Fall of Overturned Bodies at Nagoya.ToList

Fig.43.—Plan of Directions of Fall of Overturned Bodies at Nagoya.ToList

Map of Mean Directions of Shock and Isoseismal Lines in Central District.Fig.44.—Map of Mean Directions of Shock and Isoseismal Lines in Central District. (Omori.)ToList

Fig.44.—Map of Mean Directions of Shock and Isoseismal Lines in Central District. (Omori.)ToList

The times of the great earthquake and of sixteen minor shocks on October 28th and 29th and November 6th were determined at the Central MeteorologicalObservatory at Tokio, and at either two or three of the observatories of Gifu, Nagoya, and Osaka, each of which is provided with a seismograph and chronometer. The after-shocks referred to originated near a point about 6 miles west of Gifu, and the difference between the distances of Tokio and Osaka from this point is 89½ miles, of Tokio and Nagoya 147 miles, and of Tokio and Gifu 165 miles. The mean time-intervals between these three pairs of places were 67, 111, and 128 seconds respectively; and these give for the mean velocity for each interval 2.1 kilometres (or 1.3 mile) per second. Thus there appears in these cases to be no sensible variation in the velocity with the distance from the origin.

As might be expected, an earthquake of such severity was recorded by magnetometers at several distant observatories. Disturbances on the registers of Zikawei (China), Mauritius, Utrecht, and Greenwich have been attributed to the Japanese earthquake, but the times at which they commenced are too indefinite to allow of any determination of the surface-velocity of the earth-waves to great distances from the origin.

As in all disastrous earthquakes, the surface of the ground was scarred and rent by the shock. From the hillsides great landslips descended, filling the valleys withdébris; and slopes which were formerly green with forest, after the earthquake looked as if they had been painted yellowish-white. Innumerable fissures cut up the plains, the general appearance of the ground, according to Professor Milne, being "as if gigantic ploughs, each cutting a trench from 3 to 12 feet deep, had been dragged up and down the river-banks." But by far the most remarkable feature ofthe earthquake was a great rent or fault, which, unlike the fissures just referred to, pursued its course regardless of valley, plain, or mountain. Although at first sight quite insignificant in many places, and some time hardly visible to the untrained eye, Professor Koto has succeeded in tracing this fault along thesurface for a distance of forty miles, and he gives good reasons for believing that its total length must be not less than seventy miles.

Map of Meizoseismal Area.Fig.45.—Map of Meizoseismal Area. (Koto.)ToList

Fig.45.—Map of Meizoseismal Area. (Koto.)ToList

Ploughshare Appearance of the Fault near Fujitani.Fig.46.—Ploughshare Appearance of the Fault near Fujitani. (Koto.)ToList

Fig.46.—Ploughshare Appearance of the Fault near Fujitani. (Koto.)ToList

The Fault-scarp at Midori.Fig.47.—The Fault-scarp at Midori. (Koto.)ToList

Fig.47.—The Fault-scarp at Midori. (Koto.)ToList

The general character of the fault-scarp changes with the surface features. On flat ground, where the throw is small, it cuts up the soft earth into enormous clods, or makes a rounded ridge from one to two feet high, so that it resembles, more than anything else,the pathway of a gigantic mole (Fig. 46). When the throw is considerable—and in one place it reaches from 18 to 20 feet—the fault-scarp forms a terrace, which from a distance has the appearance of a railway embankment (Fig. 47). Or, again, where the rent traverses a mountain ridge or a spur of hills, "it caused extensive landslips, one side of it descending considerably in level, carrying the forest with it, but with the trees complicatedly interlocked or prostrate on the ground."

Displacement of Field Divisions by the Fault near Nishi-Katabira.Fig.48.—Displacement of Field Divisions by the Fault near Nishi-Katabira. (Koto.)ToList

Fig.48.—Displacement of Field Divisions by the Fault near Nishi-Katabira. (Koto.)ToList

At its southern end, the fault was seen for the first time crossing a field near the village of Katabira. The field was broken into clods of earth, and swollen up to a height of 5½ yards, while a great landslip had descended into it from an adjoining hill. A little farther to the north-west, the ground was sharply cut by the fault, the north-east side having slightly subsided and at the same time been shifted horizontally through a distance of 3¼ to 4 feet to the north-west Adjoining fields were formerly separated by straight mounds or ridges running north and south and east and west, and these mounds were cut through by the fault and displaced, as shown in Fig. 48. From this point the fault runs in a general north-westerly direction, the north-east side being always slightly lowered with respect to the other and shifted to the north-west. Near Seki it takes a more westerly direction, and continues so to a short distance east ofTakatomi, where the north side is lowered by five feet, and moved about 1¼ feet to the west. At the north end of Takatomi, a village in which every house was levelled with the ground, the fault is double, and the continuous lowering towards the north has converted a once level field into sloping ground. At this point, the small river Toba, flowing south, is partially blocked by the fault-scarp, and an area of about three-quarters of a square mile, on which two villages stand, was converted into a deep swamp (Fig. 49), so that, as the earthquake occurred at the time of the rice-harvest, the farmers were obliged to cut the grain from boats. After passing Takatomi, the fault again turns to the west-north-west, but, the throw being small, it resembles here the track of an enormousmole. At Uméhara it crosses a garden between two persimmon trees, appearing on the hard face of the ground as a mere line; but the trees, which were before in an east-and-west line, now stand in one running north and south, without being in the least affected by the movement (Fig. 50). From here to Kimbara, where the fault enters the Neo valley, the north side is always depressed and shifted westwards by about 6½ feet.

Map of Swamp formed by stoppage of River Toba by Fault-scarp.Fig.49.—Map of Swamp formed by stoppage of River Toba by Fault-scarp. (Koto.)ToList

Fig.49.—Map of Swamp formed by stoppage of River Toba by Fault-scarp. (Koto.)ToList

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