CHAPTER VIII.ToC

Shifting of Trees by fault at Uméhara.Fig.50.—Shifting of Trees by fault at Uméhara. (Koto.)ToList

Fig.50.—Shifting of Trees by fault at Uméhara. (Koto.)ToList

It was in the Neo valley that the supreme efforts of the earthquake were manifested. Landslips were so numerous that the greater part of the mountain slopes had descended into the valley, the whole appearance of which had changed. "Unfamiliar obstacles," remarks Professor Koto, "made themselves apparent, and small hills covered with forest had come into sight which had not been seen before." But the ground was not only lowered and shifted by the fault; it was permanently compressed, plots originally 48 feet in length afterwards measuring only 30 feet. In fact, "it appears," in the words of Professor Milne, "as if the whole Neo valley had become narrower."

A few miles after entering the Neo valley, the throw of the fault reaches its maximum at Midori. But instead of the relative depression of the east side, which prevails throughout the rest of the line, that side is here about 20 feet higher than the other. It is, however, shifted as usual towards the north, by about 13 feet; and this displacement is renderedespecially evident by the abrupt break in the line of a new road to Gifu (Fig. 47). That the east side has really risen is clear, for, a little higher up, the river has changed from a shallow rapid stream 30 yards wide into a small lake of more than twice the width, and so deep that a boatman's pole could not reach the bottom. At Itasho, about a mile north of Midori, both sides are nearly on the same level, the fault appearing like a mole's track; and seven miles farther, at Nagoshima, the east side is relatively depressed by more than a yard, and at the same time shifted about 6½ feet to the north.

Daily frequency of after-shocks at Gifu and Nagoya.Fig.51.—Daily frequency of after-shocks at Gifu and Nagoya.ToList

Fig.51.—Daily frequency of after-shocks at Gifu and Nagoya.ToList

At Nogo, the main Neo valley turns off at right angles to the east, and the fault continues its course up a side valley, the east side, with respect to the other, being continually depressed and shifted towards the north. It was traced by Professor Koto through Fujitani (Fig. 46), where there were many unmistakable evidences of the violence of the shock, as far as the eastern shoulder of Haku-san; and here, after following the fault for 40 miles, the lateness of the season compelled him to return. There can be no doubt, however, that it runs as far as Minomata; and it is probable, from thelinear extension of the meizoseismal area, that it does not entirely die out before reaching the city of Fukui, 70 miles from its starting-point at Katabira.

For some hours after the earthquake, shocks were so frequent in the meizoseismal area that the ground in places hardly ever ceased from trembling. Without instrumental aid, detailed record was of course impossible; but fortunately the buried seismographs at Gifu and Nagoya were uninjured, and in about seven hours both were once more in working order. To the energy by which this result was accomplished, we owe our most valuable registers of the after-shocks of a great earthquake.

Monthly frequency of after-shocks at Gifu.Fig.52.—Monthly frequency of after-shocks at Gifu. (Omori.)ToList

Fig.52.—Monthly frequency of after-shocks at Gifu. (Omori.)ToList

Until the end of 1893—that is, in little more than two years—the total number of shocks recorded at Gifu was 3,365, and at Nagoya 1,298. None of these approached the principal earthquake in severity. Nevertheless, of the Gifu series, 10 were described as violent and 97 strong; while of the remainder, 1,808 were weak, 1,041 feeble, and 409 were sounds alone without any accompanying shock. The slight intensity of most of the shocks is also evidentfrom the inequality in the numbers recorded at Gifu and Nagoya, from which it appears that nearly two-thirds were imperceptible more than about 25 miles from the chief origin of the shocks. Only 70 of the after-shocks during the first two years were registered at Osaka, and not more than 30 at Tokio.

Distribution of After-shocks in Time.—The decline in frequency of the after-shocks was at first extremely rapid, the numbers recorded at Gifu during the six days after the earthquake being 303, 147, 116, 99, 92, and 81, and at Nagoya 185, 93, 79, 56, 30, and 31; in fact, half of the shocks up to the end of 1893 occurred by November 23rd at Gifu, and by November 6th at Nagoya. The daily numbers at these two places are represented in Fig. 51, in which the crosses correspond to the numbers at Gifu, and the dots to those at Nagoya; and the curves drawn through or near the marks represent the average daily number of shocks from October 29th to November 20th. It will be seen that these curves are hyperbolic in form, the change from very rapid to very gradual decline in frequency taking place from five to ten days after the great earthquake. Fig. 52 illustrates the distribution in time of the after-shocks at Gifu to the end of 1893, the ordinates in these cases representing the number of shocks during successive months.[57]

A similar rapid and then gradual decline in frequency characterises the strong and weak shocks recorded at Gifu. Of the ten violent shocks, only one occurred after the beginning of January 1892; and of the 97 strong shocks, only three after April 1892. But at the commencement of the series, feeble shocks (i.e., shocks that could just be felt) and earth-sounds without any accompanying movement were comparatively rare, and did not become really prominent until two months had elapsed. Of the 308 after-shocks recorded in 1893, none could be described as strong, only 10 were weak, while 263 were feeble shocks and 35 merely earth-sounds.

The last two diagrams show at a glance that the decline in frequency of after-shocks is very far from being uniform. Some of the fluctuations are due to the occurrence of exceptionally strong shocks, each of which is followed by its own minor train of after-shocks.[58]Others seem to be periodic, and possibly owe their origin to external causes unconnected with the earthquake.[59]

Method of representing the Distribution of After-shocks in Space.—The maps in Figs. 54-57 show the distribution of the after-shocks in space during four successive intervals of two months each. They are founded on Professor Milne's great catalogue of Japanese earthquakes, which give, among other data, the time of occurrence and the position of the epicentre for every shock until the end of 1892. For the latter purpose, the whole country is divided by north-south and east-west lines into numbered rectangles, each one-sixth of a degree in length and breadth; and the position of an epicentre is denoted by the number of the rectangle in which it occurs. The area included within the maps is bounded by the parallels 34° 40' and 36° 20' lat. N., and by the meridians 2° 10' and 3° 50' long. W. of Tokio, so that ten rectangles adjoin each side of the map. The number of epicentres lying within each rectangle having been counted, curves are then drawn through the centres of all rectangles containing the same number of epicentres, or through points which divide the line joining the centres of two rectangles in the proper proportion. Taking, for example, the curve marked 5, if the numbers in two consecutive rectangles are 3 and 7, the curve bisects the line joining their centres; if the numbers are 1 and 6, the line joining their centres is divided into five equal parts, and the curve passes through the first point of division reckoned from the centre of the rectangle in which six epicentres are found. Thus the meaning of the curve marked, say,5 may be stated as follows:—If any point in the curve be imagined as the centre of a rectangle whose sides are directed north-south and east-west, and are respectively one-sixth of a degree of latitude and longitude in length; then the number of epicentres within this rectangle is at the rate of 5 for the time considered.

Preparation for the Great Earthquake.—At first sight, there appears to have been but little direct preparation for the great earthquake. Except for a rather strong shock on October 25th, at 9.14P.M., it occurred without the warning of any preliminary tremors. But a closer examination of the evidence shows, as we should indeed expect, that there was a distinct increase in activity for many months beforehand. The region had become "seismically sensitive." Of the hundred rectangles included in the maps in Figs. 53-57, there are thirteen lying along the meizoseismal area of the earthquake of 1891, in which nearly all the after-shocks originated. During the five years 1885-89, 53 out of 125 earthquakes (or 42 per cent.) had their epicentres lying within the thirteen rectangles; or, in other words, the average frequency in one of the rectangles of the meizoseismal area was five times as great as in one of those outside it. In 1890 and 1891 (until October 27th), the percentage in the thirteen rectangles rose to 61, and the average frequency in one of them to ten times that of one of the exterior rectangles.

The curves in Fig. 53 illustrate the distribution of epicentres during the latter interval. It will be seen that they follow roughly the course of the meizoseismal area southwards to the Sea of Isé, and that to the south-east they continue for several miles the shortbranch of the meizoseismal area which surrounds the southern end of the fault-scarp.

Distribution of preliminary Shocks in Space.Fig.53.—Distribution of preliminary Shocks in Space. (Davison.)ToList

Fig.53.—Distribution of preliminary Shocks in Space. (Davison.)ToList

Thus, the preparation for the great earthquake is shown, first, by the increased frequency of earthquakes originating within its meizoseismal area; and, secondly, by the uniformity in the distribution of epicentres throughout the same region, the markedconcentration of effort which characterises the after-shocks being hardly perceptible during the years 1890-91.

Distribution of After-shocks in Space (November-December 1891).Fig.54.—Distribution of After-shocks in Space (November-December 1891). (Davison).ToList

Fig.54.—Distribution of After-shocks in Space (November-December 1891). (Davison).ToList

Distribution of After-shocks in Space.—We have seen that the after-shocks were subject to a fluctuating decline in frequency, rapid at first, and more gradual afterwards. It is evident, from Figs. 54-57, that asimilar law governs the area within which the after-shocks originated. During the first two months, epicentres occur over nearly the whole of the meizoseismal area, but afterwards they are confined to a smaller district, which slowly, though not continually, decreases in size.

Distribution of After-shocks in Space (January-February, 1892).Fig.55.—Distribution of After-shocks in Space (January-February, 1892). (Davison.)ToList

Fig.55.—Distribution of After-shocks in Space (January-February, 1892). (Davison.)ToList

Distribution of After-shocks in Space (March-April).Fig.56.—Distribution of After-shocks in Space (March-April). (Davison.)ToList

Fig.56.—Distribution of After-shocks in Space (March-April). (Davison.)ToList

The most important feature in the distribution of the epicentres is the central region of extraordinary activity; but there are also districts of minor and more short-lived activity near the three extremities of the meizoseismal band. The seat of chief seismic action shifts slightly from one part to another of the epicentral region, especially about the end of 1891, aswill be seen by comparing the innermost curves of Figs. 54 and 55. Thus, with the decline in frequency of the after-shocks and the decrease in their sphere of action, there took place concurrently a gradual but oscillating withdrawal of that action to a more or less central region of the fault.

Distribution of After-shocks in Space (May-June, 1892).Fig.57.—Distribution of After-shocks in Space (May-June, 1892). (Davison.)ToList

Fig.57.—Distribution of After-shocks in Space (May-June, 1892). (Davison.)ToList

Sound Phenomena of After-shocks.—While comparatively few observers seem to have noticed any noise with the principal earthquake, many of the after-shocks were accompanied by sounds. Professor Omori describes them as belonging to two types. They were either rushing feeble noises like that of wind, or loud rumbling noises like those of thunder, the discharge of a gun, or the fall of a heavy body. In the Neo valley, sounds of the second type were most frequent and distinct, but they either occurred without any shock at all, or the attendant tremor was very feeble; while, on the other hand, severe sharp shocks were generally unaccompanied by distinctly audible sounds.

It is remarkable, also, that sounds were less frequently heard with the early than with the later after-shocks. In November 1891, the percentage of audible shocks was 17, and from December to the following April always lay between 10 and 12. In May the percentage suddenly rose to 39, and until the end of 1892 was always greater than 32, while in November 1892, it rose as high as 49. This, of course, agrees with Professor Omori's observation that sounds attended feeble shocks more often than strong ones.

The distribution of the audible after-shocks in space is shown in Fig. 58. These curves are drawn in the same way as those in Figs. 53-57, but they represent the percentages, not the actual numbers, of shocks accompanied by sound. It will be noticed that all three groups of curves lie along the meizoseismal area, or the continuation of the south-east branch; while the axis of the principal group of curves lies to the west of the central regions in which most after-shocks originated.

Distribution of Audible After-shocks in Space (November 1891-December 1892).Fig.58.—Distribution of Audible After-shocks in Space (November 1891-December 1892). (Davison.)ToList

Fig.58.—Distribution of Audible After-shocks in Space (November 1891-December 1892). (Davison.)ToList

The explanation of these peculiarities is no doubt connected with the comparative inability of the Japanese people to perceive the deep sounds which in Europe are always heard with earthquake shocks. The sounds are rarely heard by them more than afew miles from the epicentre.[60]We may therefore conclude that slight after-shocks originated nearer the surface than strong ones, that the mean depth of the foci decreased with the lapse of time, and that the axes of the systems of curves in Fig. 58 mark out approximately the lines of the growing faults. The separation of the two westerly groups of curves appears to show that the main branch of the meizoseismal area is connected with a fault roughly parallel to that traced by Professor Koto, but of which no scarp (if it existed) could be readily distinguished among the superficial fissures produced by the great shock.

So great and sudden a displacement as occurred along the fault-scarp could hardly take place without affecting the stability of adjoining regions of the earth's crust, and we should naturally expect to find a distinct change in their seismic activity shortly after October 28th. In Fig. 59 two such regions are shown, bounded by the straight dotted lines. The district in which the principal earthquake and its after-shocks originated is enclosed within the undulating dotted lines. The continuous lines inside all three districts are the curves corresponding to 10 and 5 epicentres for the years 1885-92. Not far from the axes of the outer groups of curves there areprobably transverse faults, approximately parallel to the great fault-scarp and the main branch of the meizoseismal band, and distant from them about 45 and 55 miles respectively.

Map of Adjoining Regions in which Seismic Activity was affected by the Great Earthquake.Fig.59.—Map of Adjoining Regions in which Seismic Activity was affected by the Great Earthquake. (Davison.)ToList

Fig.59.—Map of Adjoining Regions in which Seismic Activity was affected by the Great Earthquake. (Davison.)ToList

In the district represented in the north-east corner of Fig. 59, 29 earthquakes originated between January 1st, 1885, and October 27th, 1891, and 30 between October 28th, 1891, and December 31st, 1892, 7 of the latter number occurring in November 1891. In the south-west district, the corresponding figures before and after the earthquake are 20 and 36, 8 of the latter occurring in November 1891. Thus, in the north-east district, for every shock in the interval before the earthquake there were six in an equal time afterwards, and at the rate of 10 during November 1891; and in the south-west district, for every shock before the earthquake there were 10 afterwards, and at the rate of 16 during November 1891.

Now, it is unlikely that the gradual increase of stress should be so nearly proportioned everywhere to the prevailing conditions of resistance as to give rise to a marked and practically simultaneous change in seismic activity over a large area; whereas theparoxysmal occurrence of a strong earthquake might alter the surrounding conditions with comparative rapidity, and so induce a state of seismic excitement in the neighbourhood. It therefore seems very probable that the increased activity in the two districts here described was a direct consequence of the occurrence of the great earthquake.

The preponderance of preliminary earthquakes within the meizoseismal area and the outlining of the fault-system by the frequency curves of 1890-91 (Fig. 53) point to the previous existence of the originating fault or faults, and to the earthquake being due, not to the formation of a new fracture, as has been suggested, but to the growth of an old fault.

The last severe earthquake in the Mino-Owari plain occurred in 1859, so that for more than thirty years there had been but little relief to the gradually increasing stresses. Now, the distribution of stress must have been far from uniform throughout the fault-system, and also the resistance to displacement far from proportional to the stresses at different places. At certain points, therefore, the effective stress would be greater than elsewhere, and it would be at these points that fault-slips would first occur. Such slips tend to remove the inequalities in effective stress. Thus, the function of the slight shocks of 1890 and 1891 was, briefly, to equalise the effective stress over the whole fault-system, and so to clear the way for one or more great slips throughout its entire length.

As to which side of the fault moved during thegreat displacement, or whether both sides moved at once, we have no direct evidence but as regards the neighbourhood of Midori, and there the conditions were exceptional. Professor Koto thinks that it was probably the rock on the north-east side that was generally depressed and always shifted to the north-west But the disturbance in reality seems to have been more complicated. That this was the case, that displacement occurred along more than one fault, is probable from the branching of the meizoseismal area, the isolation of the audibility curves of the after-shocks (Fig. 58), and the sudden increase in seismic activity both to the north-east and south-west of the epicentre. The detached portion of the meizoseismal area near Lake Biwa may also point to a separate focus. The whole region, indeed, was evidently subjected to intense stresses, and the depression on the north-east side of the fault-scarp can hardly fail to have been accompanied by other movements, especially along a fault running near the western margin of the main branch of the meizoseismal area.

The later stages of the movements are somewhat clearer. From a study of the after-shocks, we learn that the disturbed masses began at once to settle back towards the position of equilibrium. At first the slips were numerous and took place over the whole fault-system, but chiefly at a considerable depth, where no doubt the initial displacement was greatest. After a few months, stability was nearly restored along the extremities of the faults; slips were confined almost entirely to the central regions, while a much larger proportion of them took place within the superficial portions of the faults.

The official records bring down the history to the end of 1893. Since that time more than one strong shock has been felt in the Mino-Owari plain; but the stage of recovery from the disturbances of 1891 is probably near its end, and we seem rather to be entering on a period in which the forces are once more silently gathering that sooner or later will result in another great catastrophe.

1.Conder, J.—"An Architect's Notes on the Great Earthquake of October 1891."Japan Seismol. Journ., vol. ii., 1893, pp. 1-91.2.Davison, C.—"On the Distribution in Space of the Accessory Shocks of the Great Japanese Earthquake of 1891."Quart. Journ. Geol. Soc., vol. liii., 1897, pp. 1-15.3. —— "On the Effect of the Great Japanese Earthquake of 1891 on the Seismic Activity of the Adjoining Districts."Geol. Mag., vol. iv., 1897, pp. 23-27.4. —— "On the Diurnal Periodicity of Earthquakes."Phil. Mag., vol. xiii., 1896, pp. 463-476, especially pp. 466-468.5. —— "On Earthquake-Sounds."Phil. Mag., vol. xlix., 1900, pp. 31-70—especially pp. 49-53, 60-61.6.Koto, B.—"The Cause of the Great Earthquake in Central Japan, 1891."Journ. Coll. Sci. Imp. Univ. Japan, vol. v., 1893, pp. 295-353.7.Masato, H.—"Report on Earthquake Observations in Japan."Cent. Meteor. Obs. of Japan(Tokio, 1892), pp. 16-18, 41, and map 30.8.Milne, J.—"A Note on the Great Earthquake of October 28th, 1891."Japan Seismol. Journ., vol. i., 1893, pp. 127-151;Brit. Assoc. Rep., 1892, pp. 114-128.9. —— "A Catalogue of 8,331 Earthquakes recorded in Japan between 1885 and 1892."Japan Seismol. Journ., vol. iv., 1895, pp. 1-367—especially pp. 134-234, 303-353.10. —— "On Certain Disturbances in the Records of Magnetometers and the Occurrence of Earthquakes."Brit.Assoc. Rep., 1898, pp. 226-251—especially pp. 227, 232, 234, 241, and 245.11.Milne, J., andW.K. Burton.—"The Great Earthquake in Japan."Journ. Coll. Sci. Imp. Univ. Japan, vol. v., 1893, pp. 295-352.12.Omori, F.—"On the After-shocks of Earthquakes."Journ. Coll. Sci. Imp. Univ. Japan, vol. vii., 1894, pp. 111-200; abstract inJapan Seismol. Journ., vol. iii., 1894, pp. 71-80.13. —— "A Note on the Great Mino-Owari Earthquake of October 28th, 1891."Pub. Earthquakes Inves. Com. in Foreign Languages, No. 4, Tokio, 1900, pp. 13-24.14. —— "Sulla velocità di propagazione e sulla lunghezza delle onde sismiche."Ital. Soc. Sismol. Boll., vol. i., 1895, pp. 52-60—especially pp. 52-57.15. —— "Sull' intensità e sull' ampiezza del movimento nel gran terremoto giapponese del 28 ottobre 1891."Ital. Soc. Sismol. Boll., vol. ii., 1896, pp. 189-200.16. —— "Note on the After-shocks of the Mino-Owari Earthquake of Oct. 28th, 1891."Pub. Earthquakes Inves. Com. in Foreign Languages, No. 7, Tokio, 1902, pp. 27-32.17. —— "Note on the relation between Earthquake Frequency and the Atmospheric Pressure."Tokyo Phys.-Math. Soc. Reports, vol. ii., 1904, No. 8.18.Tanakadate, A., andH. Nagaoka. "The Disturbance of Isomagnetics attending the Mino-Owari Earthquake of 1891."Journ. Coll. Sci. Imp. Univ. Japan, vol. v., 1893, pp. 149-192.

1.Conder, J.—"An Architect's Notes on the Great Earthquake of October 1891."Japan Seismol. Journ., vol. ii., 1893, pp. 1-91.

2.Davison, C.—"On the Distribution in Space of the Accessory Shocks of the Great Japanese Earthquake of 1891."Quart. Journ. Geol. Soc., vol. liii., 1897, pp. 1-15.

3. —— "On the Effect of the Great Japanese Earthquake of 1891 on the Seismic Activity of the Adjoining Districts."Geol. Mag., vol. iv., 1897, pp. 23-27.

4. —— "On the Diurnal Periodicity of Earthquakes."Phil. Mag., vol. xiii., 1896, pp. 463-476, especially pp. 466-468.

5. —— "On Earthquake-Sounds."Phil. Mag., vol. xlix., 1900, pp. 31-70—especially pp. 49-53, 60-61.

6.Koto, B.—"The Cause of the Great Earthquake in Central Japan, 1891."Journ. Coll. Sci. Imp. Univ. Japan, vol. v., 1893, pp. 295-353.

7.Masato, H.—"Report on Earthquake Observations in Japan."Cent. Meteor. Obs. of Japan(Tokio, 1892), pp. 16-18, 41, and map 30.

8.Milne, J.—"A Note on the Great Earthquake of October 28th, 1891."Japan Seismol. Journ., vol. i., 1893, pp. 127-151;Brit. Assoc. Rep., 1892, pp. 114-128.

9. —— "A Catalogue of 8,331 Earthquakes recorded in Japan between 1885 and 1892."Japan Seismol. Journ., vol. iv., 1895, pp. 1-367—especially pp. 134-234, 303-353.

10. —— "On Certain Disturbances in the Records of Magnetometers and the Occurrence of Earthquakes."Brit.Assoc. Rep., 1898, pp. 226-251—especially pp. 227, 232, 234, 241, and 245.

11.Milne, J., andW.K. Burton.—"The Great Earthquake in Japan."Journ. Coll. Sci. Imp. Univ. Japan, vol. v., 1893, pp. 295-352.

12.Omori, F.—"On the After-shocks of Earthquakes."Journ. Coll. Sci. Imp. Univ. Japan, vol. vii., 1894, pp. 111-200; abstract inJapan Seismol. Journ., vol. iii., 1894, pp. 71-80.

13. —— "A Note on the Great Mino-Owari Earthquake of October 28th, 1891."Pub. Earthquakes Inves. Com. in Foreign Languages, No. 4, Tokio, 1900, pp. 13-24.

14. —— "Sulla velocità di propagazione e sulla lunghezza delle onde sismiche."Ital. Soc. Sismol. Boll., vol. i., 1895, pp. 52-60—especially pp. 52-57.

15. —— "Sull' intensità e sull' ampiezza del movimento nel gran terremoto giapponese del 28 ottobre 1891."Ital. Soc. Sismol. Boll., vol. ii., 1896, pp. 189-200.

16. —— "Note on the After-shocks of the Mino-Owari Earthquake of Oct. 28th, 1891."Pub. Earthquakes Inves. Com. in Foreign Languages, No. 7, Tokio, 1902, pp. 27-32.

17. —— "Note on the relation between Earthquake Frequency and the Atmospheric Pressure."Tokyo Phys.-Math. Soc. Reports, vol. ii., 1904, No. 8.

18.Tanakadate, A., andH. Nagaoka. "The Disturbance of Isomagnetics attending the Mino-Owari Earthquake of 1891."Journ. Coll. Sci. Imp. Univ. Japan, vol. v., 1893, pp. 149-192.

[54]I have not referred to the results of this survey, for, though changes in all the magnetic elements (especially in horizontal intensity) have taken place between 1887 and 1891-92, these changes cannot be ascribed with confidence to the earthquake in the absence of a thorough knowledge of the secular variation.

[55]From the formulaa=xg/y, whereais the maximum horizontal acceleration,gthe acceleration due to gravity,ythe height of the centre of gravity, andxits horizontal distance from the edge about which the body was overturned.

[56]These estimates are made, on the supposition of simple harmonic motion, from the formula 2a= αt² / (2π²), where 2ais the total range or double amplitude,athe maximum acceleration, andtthe period of the vibration.

[57]Professor Omori finds that the mean daily number of earthquakesyduring the monthx(reckoned from November 1891) may be approximately represented by the equation—y= 16.9 / (x+ 0.397);or, taking the semi-daily earthquake numbers during the five days between October 29th and November 2nd, 1891, by the equation—y= 440.7 / (x+ 2.314),whereydenotes the number of earthquakes observed during the twelve hours denoted byx, the time being measured from the first half of October 29th. It is interesting to notice that, taking account of the mean annual frequency of earthquakes in ordinary years, the number of shocks observed at Gifu during the two years 1898-99 should, according to the latter formula, be 163; the actual number recorded was 160.

[57]Professor Omori finds that the mean daily number of earthquakesyduring the monthx(reckoned from November 1891) may be approximately represented by the equation—

y= 16.9 / (x+ 0.397);

or, taking the semi-daily earthquake numbers during the five days between October 29th and November 2nd, 1891, by the equation—

y= 440.7 / (x+ 2.314),

whereydenotes the number of earthquakes observed during the twelve hours denoted byx, the time being measured from the first half of October 29th. It is interesting to notice that, taking account of the mean annual frequency of earthquakes in ordinary years, the number of shocks observed at Gifu during the two years 1898-99 should, according to the latter formula, be 163; the actual number recorded was 160.

[58]The last violent shock before the end of 1893 occurred on September 7th, 1892, and its effects on the frequency of after-shocks is shown by the daily numbers recorded at Gifu during the first fortnight in September. These are—2, 2, 2, 3, 5, 5, 28 (on September 7th), 8, 8, 5, 4, 3, 2, 4, 3.

[59]The periodicity of after-shocks is discussed in the papers numbered 4, 12, 16, and 17 at the end of this chapter. In these, the existence of diurnal and other periods is clearly established. Professor Omori also shows that the mean daily barometric pressure is subject to fluctuations with maxima occurring on an average every 5½ days, and that earthquakes are least frequent on the days of the barometric maxima and minima, and more frequent in the days immediately preceding and following them.

[60]Of the Japanese earthquakes of 1885-92 originating beneath the land, twenty-six per cent. were accompanied by a recorded sound; but less than one per cent. of those originating beneath the sea and not more than ten miles from the coast.

Among the earthquakes described in this volume, the Hereford and Inverness earthquakes hold but a minor place. The damage to buildings, though unusual for this country, was slight when compared with that caused by the preceding shocks; there was no loss of life, not a single person was injured by falling masonry. The interest of the earthquakes lies entirely in the detailed study rendered possible by numerous observations of the shock and sound,[61]and in the bearing of this evidence on the general theory of the origin of earthquakes.

The principal earthquake of this series occurred at 5.32A.M.on December 17th, and was preceded by at least nine minor shocks (the first of which was felt at about 11 or 11.30P.M.on December 16th), and followed by two others on the same day, and by a third and last on July 19th, 1897. The accountsof these preliminary movements will be found on a later page, as their bearing will be more fully apparent after the discussion of the principal shock.

Isoseismal and Isacoustic lines of Hereford earthquake.Fig.60.—Isoseismal and Isacoustic lines of Hereford earthquake. (Davison.)ToList

Fig.60.—Isoseismal and Isacoustic lines of Hereford earthquake. (Davison.)ToList

On the map in Fig. 60, the continuous curves represent isoseismal lines corresponding to the degrees 8, 7, 6, 5, and 4 of the Rossi-Forel scale. The isoseismal 8, which is the most accurately drawn of the series, isan elongated oval, 40 miles long, 23 miles broad, and containing an area of 724 square miles. The longer axis is directed W. 44° N. and E. 44° S. Within this curve, there are 73 places where buildings are known to have been damaged, 55 places being in Herefordshire, 17 in Gloucestershire, and one in Worcestershire.

The most important damage occurred in the city of Hereford, which, in 1901, contained 4,565 inhabited houses. Here, no fewer than 218 chimneys had to be repaired or rebuilt. The Cathedral was slightly injured. The finial of a pinnacle of the Lady Chapel was thrown down, a fragment of a stone fell from one of the arches in the south transept, and the three pinnacles of the western front were fractured. Several churches suffered to a similar extent, while, at the Midland Railway Station, all the seven chimney-stacks were shattered. At Dinedor, Fownhope, Dormington, Withington, and a few other villages, the damage was also relatively greater than elsewhere, these places all lying within a small oval about 8½ miles long, which surrounds, not the centre, but rather the north-west focus, of the isoseismal 8.

The isoseismal 7, which includes places where the shock was strong enough to overthrow ornaments, vases, etc., is also very nearly an ellipse, whose axes are 80 and 56 miles in length, and whose area is 3,580 square miles. Its longer axis, running from W. 42° N. to E. 42° S., is practically parallel to that of the inner curve. Next in succession comes the isoseismal 6, surrounding those places where the shock was strong enough to make chandeliers, pictures, etc., swing; but, as most of the observers seem to have slept in darkened rooms, the number ofdetermining points for this curve is less than usual, and its course is therefore laid down with a somewhat inferior degree of accuracy. The error, however, is probably small, and we may therefore regard the isoseismal 6 as another ellipse, 141 miles long, 116 miles broad, and containing an area of 13,000 square miles. Its longer axis is again nearly parallel to those of the preceding isoseismals.

The next two isoseismals are nearly circular in form. It will be noticed that large portions of them, and especially of the isoseismal 4, traverse the sea. In these parts, the paths of the curves are to some extent conjectural. In drawing them, the chief guides are their trend before leaving the land and the known intensity along the neighbouring coastlines. The isoseismal 5 bounds the area within which the shock was perceptible as a sensible displacement and not merely a quiver. Its dimensions are 233 miles from north-west to south-east, and 229 miles from south-west to north-east, and its area 41,160 square miles. The isoseismal 4, which includes places where the shock was strong enough to make doors, windows, etc., rattle, is 356 miles from north-west to south-east, and 357 miles from south-west to north-east, and 98,000 square miles in area; its centre coincides nearly with that of the small oval area in the neighbourhood of Hereford, where the damage to buildings was relatively greater than elsewhere.

Outside the isoseismal 4, the earthquake was observed at several places. The shock was certainly felt at Middlesbrough, 12½ miles from the curve, and probably at Killeshandra (in Ireland), 65 miles distant. Thus, if we consider the boundary of thedisturbed area to coincide with the isoseismal 4, its area would be 98,000 square miles, or 1-2/3 that of England and Wales; if it were a circle concentric with the isoseismal 4, and passing through Middlesbrough, its area would be 115,000 square miles, or nearly twice that of England and Wales; while, if it passed through Killeshandra, its area would be 185,000 square miles, or more than three times the area of England and Wales.[62]

Position of the Originating Fault.—The form, directions, and relative positions of the isoseismal lines furnish important evidence with regard to the originating fault. We conclude in the first place that its mean direction is parallel to the longer axes of the three innermost isoseismal lines—that is, north-west and south-east, or, more accurately, W. 43° N. and E. 43° S.[63]In this case, the elongated forms of the isoseismal lines cannot be attributed to variations in the nature of the surface rocks. The district embraced contains about 13,000 square miles, and it is improbable that the axes of the three isoseismals should retain their parallelism over so large an area, if these variations had any considerable effect. Moreover, in the same district, an earthquake occurred in 1863, whose meizoseismal area was elongated fromnorth-east to south-west, or almost exactly perpendicular to the direction in 1896.

Secondly, it will be noticed (Fig. 60) that the isoseismal lines are not equidistant from one another. On the north-east side, they are separated by distances of 20, 34, 55, and 51 miles; and on the south-west side by distances of 13¼, 25, 60, and 77 miles. It follows from this that the fault-surface must hade or slope towards the north-east; for, near the epicentre, the intensity is greatest and dies out more slowly on the side towards which the fault hades.

If we could ascertain any one place through which the fault passed, its position would thus be completely determined. Unfortunately, there is no decisive evidence on this point. There are, however, several places to the south-west of Hereford where the intensity of the shock was distinctly less than in the surrounding district, and it is possible that this was due to their neighbourhood to the fault-line (see p. 135). If so, the originating fault must have extended from a point about a mile and a half west of Hereford for a distance of about 16 miles to the south-east; and a fault in this position would certainly satisfy all the details of the seismic evidence.

Throughout the disturbed area, considerable variations were observed in the nature of the shock. These changes were due to the mere size of the focus, to its elongated form and, as will be seen, to its discontinuity, and also to the distance of the place of observation from the epicentre.

At places near the epicentre, rapid changes in thedirection of the shock were observed owing to the large angle subtended by the focus; while, at considerable distances, this angle being small, the changes of direction were imperceptible. A further variation with the distance was an increase in the period of the vibrations. Close to the epicentre, the general impression was that of crossing the wake of a steamer in a very short rowing-boat, or of riding in a carriage without springs. At distances of a hundred miles or more, the movement is described as being of a pleasant, gentle, undulating character, like that felt during the rocking of a ship at anchor or in a carriage with well-appointed springs.

The most remarkable feature of the shock, however, was its division into two distinct parts or series of vibrations, separated by an interval, lasting two or three seconds, of absolute rest and quiet. And this was no mere local phenomenon. With the exception of a narrow band that will be referred to presently, records of the double shock come from nearly all parts of the disturbed area, even from districts so remote as the Isle of Man and the east of Ireland. The two parts differed in intensity, in duration, and in the period of their constituent vibrations. For instance, at Oaklands (near Chard), a shivering motion was first felt, and then, after about three or four seconds, a distinct rocking from side to side. At Exeter, there was a sudden tremor lasting about two seconds, followed, after two or three seconds, by another and more severe shaking lasting four or five seconds. Again, at West Cross (near Swansea), an undulatory movement for about four seconds was followed soon after by a tremulous shock. At Liverpool, the durations of the first part, interval, andsecond part were respectively estimated at about six, two, and four seconds.

As a first result of the observations, then, it appears that in the south-east half of the disturbed area, the second part of the shock was the stronger, of greater duration and consisted of longer-period vibrations (as ata, Fig. 61); while, in the north-west half, the same features characterised the first part of the shock (b, Fig. 61). A closer examination of the records shows, however, that the boundary between the two portions of the disturbed area was not a straight line, but slightly curved, the concavity facing the south-east. The broken line on the map (Fig. 60), which is hyperbolic in form, represents roughly the position of this curved boundary.[64]

Nature of shock of Hereford earthquake.Fig.61.—Nature of shock of Hereford earthquake.ToList

Fig.61.—Nature of shock of Hereford earthquake.ToList

Along this hyperbolic boundary-line, or rather within a narrow band of which it is the central line, the shock lost its double character, and was manifested as a single series of vibrations graduallyincreasing in intensity and then dying away. Close to the edges of this band, careful observers were able to distinguish two maxima of intensity connected by a continuous series of tremors (c, Fig. 61). Thus, within the band, the two series of vibrations, which elsewhere were isolated, must have been superposed on one another; while, near the edges of the band, the concluding tremors of the first series overlapped the initial tremors of the second.

Origin of the Double Series of Vibrations.—The Hereford earthquake thus belongs to the same class as the Neapolitan, Andalusian, Charleston, and Riviera earthquakes. As in these cases, the hypothesis of a single focus is inadmissible. The division of the disturbed area into two regions of opposite relative intensity, duration, etc., is sufficient proof that a single series of vibrations was not duplicated by reflection or refraction, or by separation into longitudinal and transverse waves. It is equally conclusive against a repetition of the impulse within the same focus. We must therefore infer that the focus consisted of two nearly or quite detached portions arranged along a north-west and south-east line, and that the impulse at the north-west focus was the stronger of the two. The only question that remains to be decided is whether the impulses at the two foci were simultaneous or not.

Now, if the impulses occurred at the same instant, the waves from the two foci would travel with the same velocity, and would therefore coalesce along a straight band which would bisect at right angles the line joining the two epicentres. But we have already seen that this band is curved, and it thus follows that the two impulses were not simultaneous. Again,since the concavity of the hyperbolic band faces the south-east, the waves from the north-west focus must have travelled farther than those from the south-east focus before the two met along the hyperbolic band; in other words, the impulse at the north-west focus must have occurred two or three seconds before the impulse at the other.

Position and Dimensions of the Two Foci.—There can be little doubt that the impulse at the north-west focus was responsible for the greater damage to buildings at Hereford, Dinedor, Fownhope, etc. The centre of its epicentral area must therefore lie about three miles south-east of Hereford. It is probable, also, that the corresponding centre of the other focus is similarly placed with respect to the south-east portion of the isoseismal 8—that is, about two or three miles north-east of Ross. These two points are eight or nine miles apart. Now, since, as we shall see, the mean surface-velocity of the earth-waves was about 3000 feet per second, and the mean duration of the quiet interval between the two series was 3½ seconds, the nearest ends of the two foci must have been separated by a distance of not less than two miles. Moreover, since the series of vibrations from the north-west or Hereford focus lasted a few seconds longer than that from the south-east or Ross focus, the former must have been about two miles longer than the latter, and we may therefore estimate their lengths at about eight and six miles respectively. Including the undisturbed intermediate portion, this would give a total length of focus of about 16 miles, a result we have already inferred from the dimensions of the isoseismal 8.

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