CHAPTER IX.ToC

While the seismic evidence enables us to determine the surface-position and the horizontal dimensions of the seismic focus, it unfortunately throws no light whatever on a point of some importance—namely, the direction of the movement which caused the earthquake. We cannot infer from it whether it was the rock on the south-east or north-west side of the fault that slipped or whether both sides slipped at once; nor, if that point had been settled, do we know if the movement of the displaced side was upward or downward. In the formation of the fault, however, it is clear that either the south-east side has been depressed or the north-west side elevated; and, as the bed of Loch Ness is below the level of the sea, that the former movement has predominated. If the displacements which gave rise to the earthquake were merely a continuation of the original series of movements—and this is, to say the least, a very probable view to take—then we may imagine that, for a distance of five or six miles, and at a depth of about a mile or less, there was a sudden sag downwards of the rock on the south-east side of the fault through a distance which perhaps in no part exceeded a fraction of an inch.

Fig. 66 is an attempt to represent roughly the displacement which caused the principal earthquake. The diagram makes no pretence to accuracy, andthe scale in the vertical direction is enormously greater, perhaps a hundred thousand times greater, than that in the horizontal direction. The straight line is supposed to represent a straight line drawn before the earthquake on the surface of the rock adjoining the fault on the south-east side and at a depth of about a mile, and the curve the form of the same line after the earthquake.

Diagram to illustrate supposed fault-displacement causing Inverness earthquake.Fig.66.—Diagram to illustrate supposed fault-displacement causing Inverness earthquake.ToList

Fig.66.—Diagram to illustrate supposed fault-displacement causing Inverness earthquake.ToList

The effect of this great slip would obviously be to relieve the stress in the central region A, and to increase it suddenly in the parts denoted by the letters B and C. It is, therefore, in these parts especially that we should expect future slips to occur. Each slip would of course give rise to an after-shock, and would in like manner result in an increase of stress in its own terminal regions, though chiefly on the side remote from the centre A.

It is difficult to form any estimate of the total number of after-shocks. The list, compiled from the records of careful observers only, includes forty-six shocks and ten earth-sounds, the last of all occurring on November 21st. But the list is certainly incomplete. It contains, for instance, only one entry onSeptember 18th between 3.56 and 9A.M.; whereas, during the same interval, no fewer than eighteen slight shocks were felt by one observer at Dochgarroch, while another near Aldourie estimates the number of shocks up to October 23rd at about seventy. The total number probably did not fall short of one hundred.

Map of epicentres of after-shocks of Inverness earthquakes.Fig.67.—Map of epicentres of after-shocks of Inverness earthquakes. (Davison.)ToList

Fig.67.—Map of epicentres of after-shocks of Inverness earthquakes. (Davison.)ToList

The majority were certainly very slight, and, at another time, would hardly have attracted any notice. There were, however, three of much greater importance than the rest. These occurred on September 18th at 3.56 and 9A.M., and on September 30th at 3.39A.M.The isoseismal lines of all three are elongated ovals, their longer axes are parallel to the fault, and their centres lie on the south-east side of the fault-line. The shocks were therefore evidently due to slips several miles in length along the fault. At present, we are concerned more with the position of their epicentres. These are indicated by the dots lettered B, C, D in Fig. 67; the dot marked A denoting the centre of the principal earthquake, and the continuous line the path of the fault.

Thus, within two and a half hours, the great slip was followed by one with its centre at B, near the south-west margin of the principal focus. About five hours later, the scene of action was suddenly transferred to a region with its centre at C on the north-east margin. Both slips affected a portion of the fault-surface several miles in length, and must therefore have increased the area of displacement, slightly towards the north-east and considerably towards the south-west. Only small movements occurred during the next twelve days until 3.39A.M.on September 30th, when another long slip took place, with its centre at D, still farther to the south-west, and therefore again extending the area and amount of displacement in this direction.

Turning now to the weaker after-shocks and earth-sounds, we find them affecting chiefly three regions of the fault. One of these is close to Dochgarroch, another near Inverness, and the third between Aldourie and Drumnadrochit; the effects of the slips in the last two districts being, as before, to extend the area of displacement a short distance (perhaps half a mile) to the north-east and not less than six miles to the south-west underneath Loch Ness.

The unequal division of the after-shocks between the two sides of the principal centre (A, Fig. 67) isvery marked. The positions of the epicentres of forty-four shocks and earth-sounds can be determined with more or less accuracy, and, of these, only ten lie to the north-east of the principal centre, while thirty-four lie to the south-west, six or seven of the latter being beneath Loch Ness.

One other point may be referred to before leaving these minor shocks. So far as regards the stronger shocks, there was a continual decrease in the depths of the seismic foci. This is shown by the progressive approach of their epicentres towards the fault-line; the distances in the three chief after-shocks being 1.7, 1.0, and 0.5 miles respectively; and in one of the latest shocks (that of October 13th at 4.24P.M., E, Fig. 67) the distance is no more than one-tenth of a mile. The focus of this shock must, indeed, have been quite close to the surface near Dochgarroch. This constant diminution in the depth of the foci shows that the great slip was followed by a sudden increase of stress upwards as well as laterally, and explains why that slip did not leave any perceptible trace, either as fault-scarp or fissure, at the surface.

It is remarkable that, of the 56 recorded after-shocks, at least six were felt or heard only at Dalarossie and other places in the valley of the Findhorn, a valley which lies about 13 or 14 miles to the south-east of the great fault. That they had no connection with that fault is certain, for two of them were so strong that, if they were so connected, they could not have escaped the notice of one or more of the watchful observers between Drumnadrochit andInverness. The probable explanation of these after-shocks is that they were due to slips of a fault running along the Findhorn valley;[68]and that the great displacement near Inverness on September 18th led to a sudden increase of stress within the rocks for many miles around, which, at and near Dalarossie, was sufficient to precipitate the slips referred to.

At first sight, two earthquakes could hardly be more unlike than the Japanese earthquake of 1891 and the Inverness earthquake of 1901. In the rice-fields of central Japan, as we have seen, the roads for many leagues were edged with ruins, the fault-slip was prolonged up to the surface and visible as a scarp forty, if not seventy, miles in length, plots of ground were compressed and their boundaries altered, the hillsides were scored by landslips, places can now be seen from one another that formerly were hidden by a mountain ridge, and the total number of after-shocks within little more than two years amounted to above three thousand. On the other hand, when we examine the distribution of the after-shocks in space, we find that, though no part of the fault was exempt from slips, they favoured three regions in particular—one, the most important, a central region, yet not coincident with that in which the principal shock was most intense; and the other two surrounding the extremities of the fault. Withthe lapse of time, the after-shocks on the whole became weaker and occurred less frequently, and the average depth of the foci gradually diminished. Moreover, in two districts distant forty-five and fifty-five miles from the fault, the frequency of the shocks during the month succeeding the earthquake was suddenly increased to ten and sixteen times the normal rate.

It is interesting to notice so close a similarity in character, subsisting with so vast a difference in the scale of intensity. The identity of the powers at work in shaping the structure of both islands Is manifest. In Japan, we see the mountain-making forces acting with violence and producing effects that are only too apparent to the eye. In Scotland, whatever may have happened in former geological epochs, the changes in surface-structure are now taking place with almost infinite slowness, and hundreds or thousands of years must elapse before Loch Ness makes any visible progress in its march towards the sea.

1.Davison, C.—The Hereford Earthquake of December 17, 1896.(Birmingham, 1899.)2. —— "The Inverness Earthquake of Sept. 18, 1901, and its accessory shocks."Quart. Journ. Geol. Soc., vol. lviii., 1902, pp. 377-397.

1.Davison, C.—The Hereford Earthquake of December 17, 1896.(Birmingham, 1899.)

2. —— "The Inverness Earthquake of Sept. 18, 1901, and its accessory shocks."Quart. Journ. Geol. Soc., vol. lviii., 1902, pp. 377-397.

[61]The study of the Hereford earthquake is based on 2,902 records, coming from 1,943 places; that of the Inverness earthquake on 710 records from 381 places.

[62]The disturbed area of the Hereford earthquake of 1896 was probably greater than that of any other British earthquake of the nineteenth century; that of the Pembroke earthquake of 1892 being more than 56,000 square miles, of the Pembroke earthquake of 1893 about 63,600 square miles, while that of the Essex earthquake of 1884 (a far stronger shock in the meizoseismal area) is estimated at about 50,000 square miles.

[63]The approximate circularity of the two outer isoseismals is due to the fact that the vibrations propagated to such great distances are those which start from the comparatively small central region of the focus.

[64]The above statement summarises the evidence of the majority of the observers in each portion of the disturbed area. In this, as in other similar cases, discrepancies in the observations are unavoidable; but it is important to notice that they are least frequent in the observations evidently made with the greatest care.

[65]Except in the case of Yorkshire, where the three Ridings are regarded as separate counties.

[66]The Derby earthquake of March 24th, 1903, was also a twin earthquake. The centres of the two foci were situated near Ashbourne and Wirksworth, above eight or nine miles apart, along a line running N. 33° E. and S. 33° W. The two parts of the shock coalesced along a rectilineal band about five miles wide running centrally across the lower isoseismals in a direction at right angles to their longer axes. The isacoustic lines are also elongated in the direction of this band. In this case, the impulses at the two foci must have taken place at the same instant. (Quart. Journ. Geo. Soc., vol. lx., 1904, pp. 215-232.)

[67]If the foci of the two impulses had been detached, there would, with so small an interval between the two parts, have been a variation in the nature of the shock like that observed during the Hereford earthquake.

[68]This part of Inverness-shire has not yet been mapped by the Geological Survey, but a fault is known to exist in the Findhorn valley near Drysachan Lodge, which lies about eleven miles down the valley from Dalarossie.

Very different from the shocks of Britain was the earthquake that overwhelmed so large a part of its great dependency on June 12th, 1897—an earthquake which, if it is not without a rival, is certainly one of the most disastrous and most widely-felt of which we possess any record. That it was of the first magnitude was evident at once in Calcutta from the extensive injury to buildings, and its investigation was undertaken without delay by the members of the Geological Survey of India. The four officers who were at the headquarters in Calcutta were despatched to the area of greatest damage, letters and circulars were distributed as widely as possible, a large number of observers were induced to co-operate by keeping records of the after-shocks, and, later on, during the cold weather of 1897-98, Mr. R.D. Oldham, one of the superintendents of the Survey, made a tour through the epicentral district. To him, moreover, fell the much harder task of discussing the very numerous observations collected by himself and others; and the least that can be said of the valuable report prepared by him is that it is worthy of a great subject. Professor Omori also spent several months in studying the earthquake on behalf of the Japanese Government; but the account,which is written in his own language, unfortunately remains a sealed book to western seismologists.

Isoseismal Lines of Indian Earthquake.Fig.68.—Isoseismal Lines of Indian Earthquake. (Oldham.)ToList

Fig.68.—Isoseismal Lines of Indian Earthquake. (Oldham.)ToList

In Fig. 68, which shows the area disturbed by the earthquake, Mr. Oldham has drawn two series of curves. In the absence of detailed records of the intensity—records that could not have been obtained from some parts of the disturbed area, and would have been difficult to procure in sufficient number from others—he has represented by the dotted curves a group of isoseismals in the form which he believes they would have assumed had the earth-waves been propagated in a homogeneous medium. The first includes all places, such as Shillong and Goalpara, where the destruction of brick and stone buildings was practically universal; the second, those, like Darjiling, in which damage to buildings was universal and often serious; the third, places, like Calcutta, where the earthquake was strong enough to injure all or nearly all brick buildings. Inside the fourth isoseismal, the shock was strong enough to disturb furniture and loose objects, but not to cause more than slight damage; within the fifth, it was generally noticed; and, beyond this, and as far as the sixth isoseismal, the earthquake was perceived only by a small number of sensitive persons at rest. The approximation of the curves towards the east and south-east, Mr. Oldham believes to be partly real, and not due to imperfect information.

The continuous curves represent more closely the actual variation of intensity. The innermost curve A indicates the probable boundary of the epicentral tract, which is about 200 miles in length and more than 6000 square miles in area. This will be referred to afterwards in greater detail. The nextcurve B bounds the region within which serious damage to brick houses was common. Its irregular course is closely connected with the geological structure of the country, and is due to the fact, of which we have already met with several examples, that earthquakes are more destructive to houses built on alluvial ground than to those founded on rock. The area included within this curve is not less than 145,000 square miles; and, if we include the parts from which reports were not obtainable, it must amount to about 160,000 square miles.

The curve C represents the boundary of the disturbed area, so far as known, for about one-third of the area lies in regions from which no information was procurable, while another third is inhabited by ignorant and illiterate tribes. But, notwithstanding this, the shock is known to have been felt over an area of at least 1,200,000 square miles. If we include the detached region to the west, near Ahmedabad, the portion of the Bay of Bengal in which the shock would have been felt had the sea been replaced by land, and a large part of Thibet or Western China, from which no reports have come, but in which the shock was certainly sensible, this estimate, great as it is, must be raised to about 1,750,000 square miles.[69]

Figures, such as those given above, convey but littleidea of the vastness of the area concerned. Transferring them to countries with which we are more familiar, we may say that the disturbed area was only a little less than half the size of Europe; the region in which serious damage occurred to masonry was more than twice as large as the whole of Great Britain; while, if the centre of the epicentral tract had been in Birmingham, nearly every brick and stone building between York and Exeter would have been levelled with the ground.

Few and slight were the forerunners of the greatest of modern earthquakes. Early in June, faint tremors were felt by sensitive persons at Shillong. Others at the same place heard a rumbling sound for ten or fifteen seconds before the shock began, and at Silchar birds were seen to rise suddenly from trees before the movement became sensible to man. Except for these almost imperceptible warnings, the earthquake broke abruptly over the whole district.

"At 5.15," writes one observer at Shillong, "a deep rumbling sound, like near thunder commenced, apparently coming from the south or south-west.... The rumbling preceded the shock by about two seconds ... and the shock reached its maximum violence almost at once, in the course of the first two or three seconds. The ground began to rock violently, and in a few seconds it was impossible to stand upright, and I had to sit down suddenly on the road. The shock was of considerable duration, and maintained roughly the same amount of violence from the beginning to the end. It produced a very distinctsensation of sea-sickness.... The feeling was as if the ground was being violently jerked backwards and forwards very rapidly, every third or fourth jerk being of greater scope than the intermediate ones. The surface of the ground vibrated visibly in every direction, as if it was made of soft jelly; and long cracks appeared at once along the road.... The road is bounded here and there by low banks of earth, about two feet high, and these were all shaken down quite flat. The school building, which was in sight, began to shake at the first shock, and large slabs of plaster fell from the walls at once. A few moments afterwards the whole building was lying flat, the walls collapsed, and the corrugated iron roof lying bent and broken on the ground. A pink cloud of plaster and dust was seen hanging over every house in Shillong at the end of the shock.... My impression at the end of the shock was that its duration was certainly under one minute, and that it had travelled from south to north.... The violence of the shock may be imagined when it is stated that the whole of the damage done was completed in the first ten or fifteen seconds of the shock."

Other estimates of the duration are generally higher than that given above, ranging from three to five or even more minutes at Tura, Dhubri, Silchar, Calcutta, and other places. In some cases, it is possible that the immediately succeeding tremors were included as part of the great shock; but, in the central area, it is probable that the average duration of the shock did not differ much from three or four minutes.

In this district, the movement was most complicated. Changes of direction were frequently noticed. At Silchar, for instance, the earthquake began withan undulatory movement from north to south, like the swinging of a suspension bridge; it closed with a motion like that of a boat tossed in a choppy sea, or by the crossing of great waves which, whatever their dominant direction may have been, certainly did not travel from north to south. The vertical component of the motion must have been considerable; for, at Shillong, loose stones lying on the roads were tossed in the air "like peas on a drum," But this was even less pronounced than the horizontal movement, the range of which was at least eight or nine inches, and during which people felt as if they were being shaken like a rat by a terrier. The period of these vibrations was estimated at about a second.

As they left the central region, the period of the waves lengthened, so that, at a distance, the shock no longer consisted of short jerks, but became a gentle rocking motion, causing in some people a sensation of nausea. At Calcutta, the undulations were regular and resembled the rolling of a mighty ship, the period being between one and two seconds. At Balasor, the motion was a long rolling one, such as would be felt on the deck of a ship in a fairly heavy sea; and, farther to the south as far as the limit of the disturbed area, the same undulatory movements were observed, gradually decreasing in intensity, and usually compared to the easy motion of a ship in a gentle sea.

Visible Earth-Waves.—A few examples have already been given of the observation of visible waves on the surface of the ground. They were seen at Charleston during the earthquake of 1886 (p. 110), and at Akasaka and other places in the meizoseismal area during the Japanese earthquakeof 1891 (p. 186). But they were more than usually prominent in the Indian earthquake; indeed, much of the difficulty experienced in standing during the shock seems to have been due to the passage of these surface-waves.

At Shillong, according to an observer quoted above (p. 266), the surface of the ground vibrated visibly in every direction, as if it were made of soft jelly. Another describes it as presenting "the aspect of a storm-tossed sea, with this difference that the undulations were infinitely more rapid than any seen at sea." Near Maimansingh, earth-waves were watched approaching, exactly like rollers on the sea-coast, and, as they passed, the observers had a difficulty in standing. At Nalbari, the rice in the fields could be seen rising and falling at intervals during the transit of the waves. In the Assam valley, near Mangaldai, there were seen "waves coming from opposite directions and meeting in a great heap and then falling back; each time the waves seemed to fall back the ground opened slightly, and each time they met, water and sand were thrown up to a height of about 18 inches or so." Even as far as Midnapur, the ground was "distinctly billowy," and at Allahabad a series of waves was observed to cross the ground from south-south-west to north-north-east.

It is obviously difficult to judge in any case of the magnitude of such waves. In the epicentral area, Mr. Oldham believes that, on an average, they were probably about thirty feet long and one foot in height, though some may have been both shorter and higher. These movements must have been comparatively slow, for their progress could be easilyfollowed by the eye; indeed, their rate, as one witness remarks, "though decidedly faster than a man could walk, was not so fast as he could run."

In his study of the Neapolitan earthquake, Mallet showed how the amplitude and maximum velocity of the vibrations could be determined roughly from the displacement, projection, or overthrow of various bodies by the earthquake. Somewhat similar methods were employed by Mr. Oldham in the absence of seismographs from the epicentral area. His results are of course only approximate, but they lead nevertheless to a conclusion of great value and interest.

Section of Tombs in the Cemetery at Cherrapunji.Fig.69.—Section of Tombs in the Cemetery at Cherrapunji. (Oldham.)ToList

Fig.69.—Section of Tombs in the Cemetery at Cherrapunji. (Oldham.)ToList

Amplitude.—The best measure of the amplitude was obtained at the cemetery at Cherrapunji, situated near the southern margin of the epicentral area. Here were two oblong masonry tombs (Fig. 69), standing close together with their longer axes pointing north and south. Their inner sides were partially destroyed. "On the outer sides, they are almost intact, but the tombs have been driven bodily down into the ground, and on either side to east and west, there is a depression with a vertical sideparallel to the outer surface of the tomb and a smooth flat bottom over which the base of the tomb has slid.... The edge of the western depression has the grass growing undisturbed up to the edge of it, and along the edge small fragments of lime and plaster show that this was originally in contact with the edge of the tomb, which has now moved away to a distance of 18 inches. On the east the edge of the depression is raised and the grass and earth forced upwards by the thrust of the tomb against it; the breadth of this depression is 10 inches."

During the movement of the ground, the tombs, owing to their inertia, remained comparatively stationary, and the depressions were formed by the backward and forward movement of the ground against them. The movement on the east side was clearly arrested in some manner, and the range therefore cannot have been less than 10 inches. It may have been as much as 18 inches, and was probably, in Mr. Oldham's opinion, the mean of these two amounts—namely, 14 inches. This would give an amplitude of about 7 inches, a value which may be in excess of the average amount elsewhere in the district, as the cemetery is situated near the edge of a high sandstone scarp.

At Tura, also within the epicentral area, a range of not less than 10 inches was given by the sliding of a wooden house over the posts on which it rested. Six months after the shock, Mr. Oldham frequently noticed vacant spaces four or five inches across by the side of large boulders scattered over the Khasi hills, and he infers that "throughout the whole tract lying west of Shillong and Gauhati, as far as the hills extend, and probably over a large area ofthe plains besides, the amplitude of the wave-motion was nowhere less than 3 inches, while in many places it was over 6 inches."

Maximum Velocity.—The most trustworthy measure of the maximum velocity are those obtained from the projection of bodies. Mr. Oldham selects the following as most deserving of notice:—At Goalpara, an obelisk surmounting a tomb was broken off and thrown to one side, giving a maximum velocity of not less than 11 feet per second. At Gauhati, the coping of a small gate-pillar was shot off and fell at a distance of 4 feet 4 inches from the centre of the pillar; in this case the maximum velocity must have exceeded 8 feet per second. The highest velocity, of more than 16 feet per second, was measured at Rambrai, where a small group of monoliths were shot out of the ground, one of them to a distance of 6½ feet. Lastly, at Silchar, a bullet was projected from the corner of a wooden post, acting as a rough form of seismometer, from which a maximum velocity of at least 1½ feet per second was deduced.

Maximum Acceleration.—Estimates of the maximum horizontal acceleration were made from 28 overthrown pillars by means of Professor West's formula (p. 184, footnote). The measures obtained at the same place show some variation, but Mr. Oldham considers as fair average values those of 14 feet per second per second at Goalpara, 12 at Gauhati, Shillong, and Sylhet, 10 at Cherrapunji, 9 at Dhubri, and 4 feet per second per second at Silchar.

Of the vertical component of the acceleration, not even the roughest numerical estimate can be formed. We know, however, that at Shillong, Gauhati, andindeed throughout the epicentral area, stones were projected upwards, and this is evidence that the vertical component was greater than that of gravity—namely, 32 feet per second per second.

Violent as the shock was at the places just mentioned, it must have been still greater in certain parts of the epicentral area. At Dilma, in the Garo hills, the shock seems to have been strong enough to disable men; and, in the neighbourhood of the faults that will be described in a later section, forest trees were snapped in two. Fortunately, as Mr. Oldham remarks, there were in these districts no towns or populous settlements to feel the full power of the earthquake to destroy.

Anomalies in the above Measurements.—If the movements of the ground followed the law of simple harmonic motion, any two of the four elements (period, amplitude, maximum velocity, and maximum acceleration) would suffice to determine the others (p. 4). Applying the usual formulæ to the quantities obtained at Gauhati—namely, 8 feet per second for the maximum velocity and 12 feet per second per second for the maximum acceleration, it follows that the amplitude would be 5 feet and the period 4 seconds—values, which are evidently inadmissible. Or, taking the maximum vertical component at 32 feet per second per second, the corresponding values would be 2 feet and 1½ seconds, that of the amplitude being still too great. Again, at Rambrai, the maximum velocity was found to exceed 16 feet per second. The other elements are unknown, but, if the amplitude were one foot, Mr. Oldham shows that the maximum acceleration would be 256 feet persecond per second; or, taking the amplitude at the impossible amount of two feet, that the maximum acceleration would be 128 feet per second per second.

It follows, therefore, that only part of the high velocities at Rambrai and elsewhere can be due to the elastic waves provoked by the initial disturbances. The remaining portion must be attributed to a bodily displacement of the earth's crust within the epicentral area—a displacement of which the fault-scarps and other distortions of that region furnish ample evidence.

In the epicentral area, the sound that accompanied the earthquake was remarkable for its extraordinary loudness. At Shillong, the crash of houses falling within thirty yards was completely drowned by the roar of the earthquake.

The sound was generally compared to distant thunder, the passage of a train or cart, etc.; but, whatever the type may be, it always implies a sound of deep pitch, close to the lower limit of audibility—a continuous rumbling or rattling noise, as a rule gradually becoming louder and then dying away. There was the usual conflict in the evidence of different observers due to the depth of the sound. In Calcutta, which lies well within the sound-area, some persons asserted that they heard a rumbling noise; others were positive that the only noise was that caused by falling buildings and furniture. Some, again, noticed that the shock was preceded by a loud roar; while others were certain that there was nosound of any kind until the earthquake had become severe.

As in the case of the disturbed area, it is impossible to define the boundary of the region over which the sound was heard. Like the shock, also, it seems to have been observed farther to the west than towards the east. Leaving out of account records that are possibly doubtful, the sound was heard for a distance of 330 miles to the west and south-west, and 290 miles to the east of the epicentral area—that is, allowing for the dimensions of that area, it must have been perceptible over a region measuring not less than 800 miles from east to west.

It is somewhat doubtful whether a more accurate estimate of the velocity is to be obtained from a violent earthquake or from one of moderate intensity. In the former case, the vast distances to which the shock is noticed lessen the effects of errors in the time-determinations, but this advantage is to a great extent compensated by the considerable duration of the shock and the consequent uncertainty whether all observers have timed the same phase of the movement. Also, in the Indian earthquake, there are further sources of error in the variety of standard times employed throughout the country and in the magnitude of the epicentral area.

Of the numerous time-records collected by Mr. Oldham, the best are those which were obtained from a few self-recording instruments, from the more busy telegraph offices, from the larger railway stations, and in some cases from private individuals.All records were in the first place subjected to a rigid process of selection; a large number were rejected on various grounds, and those only were retained which bore internal evidence of accuracy, due either to the conditions of the reporter's occupation or to the care taken by him to ensure exactness. To guard against any unconscious bias in making the selection, this process was carried out before the distances were calculated, and even before the position of the epicentral area was known.

The boundary of this area is shown by the continuous line A in Fig. 68. Its greatest length being about 200 miles from east to west, it is necessary in the first place to fix upon an equivalent centre within it, which may be regarded for this special purpose as the point of departure of the earth-waves. The more natural course perhaps would be to assume this point to coincide with the centre of the area. But, as the rate at which the initial movement spread over that area would probably differ little from the velocity of the earth-wave, and as all the time-stations lie towards the west, Mr. Oldham regards a point near the western boundary of the area (in lat. 25° 45' N. and long. 90° 15' E.) as a sufficiently exact approximation to the position of the equivalent centre.

The nearest place at which good time-observations were made is Calcutta, distant 255.5 miles from the assumed centre. One is indicated on the recording tide-gauge by a sudden rise of the water, while the others were obtained from the central telegraph office, the terminal railway stations, and from two careful readings by interested observers. They vary from 4h. 27m. 0s. to 4h. 28m. 37s.P.M., all being liable to an error of half-a-minute. The arithmeticmean for the beginning of the shock is 4h. 27m. 49s., and this is probably as accurate an estimate as the conditions allow.[70]

Bombay lies outside the disturbed area, 1208.3 miles from the equivalent centre; and, for the time of arrival in that city, we have to depend on the records of the barograph and the three magnetographs. The horizontal force magnet was set in motion two and a half minutes before the others, no doubt by the advance tremors. The times given by the barograph and the vertical force-instrument differ by only one minute, and the best result seems to be that obtained by taking their mean—namely, 4h. 35m. 43s., which is probably accurate to within a minute.

Assuming, then, that the time-interval between Calcutta and Bombay does not err by more than half-a-minute, it follows that the intervening velocity must lie between 2.8 and 3.2 kilometres per second, its probable value being 3 kilometres, or 2 miles, per second.

The remaining records, which are of less value than those obtained in these cities, fall into two groups, the first consisting of a number of stations along a line running north and south between Calcutta and Darjiling or within a hundred miles on either side of the same, and the second a long series of stations crossing Northern India in a nearly westerly direction. The observations made at the Burmese stations were unfortunately affected by an error arising from the retardation of the Madrastime-signals through frequent repetition along the line.

Time-curve of Indian earthquake.Fig.70.—Time-curve of Indian earthquake. (Oldham.)ToList

Fig.70.—Time-curve of Indian earthquake. (Oldham.)ToList

Individually, these records are not exact enough to be used in determining the velocity, but they may be employed collectively for the construction of the time-curve in Fig. 70. In this diagram, distances in hundreds of miles from the equivalent centre are represented along the horizontal line, and the time of occurrence in minutes past 4P.M.along the perpendicular line. The small circles represent the observations at Calcutta and Bombay, the dots those at places lying nearly west of the origin, and the crosses those at places situated to the south or north-west. The continuous curve passes in an average manner through the series of points, and probably does not differ much from the true curve of the time of arrival of the shock at different places. The curve, it will be noticed, is at first concave, and afterwards convex, upwards; indicating that the times required to traverse successive equal distances at first increased, and then decreased. Thus, if the curve is an accurate representation of the facts, it would follow that the surface-velocity was subject toa continual decrease outwards from the centre, until it was a minimum at a distance of about 280 miles, after which it increased.

The deviation of the curve from a straight line is, however, so slight that we cannot feel much confidence in this conclusion. If we join the points corresponding to Calcutta and Bombay by a straight line (drawn dotted in Fig. 70), it does not in any part vary from the continuous line by a distance equivalent to more than half-a-minute. Indeed, if a very few discordant records are excluded, and if less weight is given to those times which are multiples of five minutes, the straight line represents the mean quite as fairly as the curved line does; and that this is the more probable interpretation will appear from the observations on the unfelt earthquake described in the next section. We may therefore conclude that the earth-waves travelled along the surface at an approximately uniform rate of 3 kilometres per second, or about 120 miles a minute—a result which Mr. Oldham considers may be accepted as accurate to within five per cent.

If the two time-curves in Fig. 70 are continued to the right until they meet the time-scale, it will be seen that they intersect it near the point corresponding to 4.26P.M., implying that this would be approximately the time at which the shock was felt within the epicentral area. This agrees closely with the observed times of about 4.25 at Parbatipur and Kuch Bihar, 4.26 at Siliguri, and 4.27 at Shillong and Goalpara; and it is probable that the error is not more than a quarter of a minute in defect or half-a-minute in excess. Thus, the time of arrival of the first sensible waves at the surface would lie between4h. 25m. 45s., and 4h. 26m. 30s.P.M., Madras time, or between 11h. 4m. 45s. and 11h. 5m. 30s.A.M., Greenwich mean time.

Of the crowd of vibrations that agitate the ground during an earthquake, part only combine to form the perceptible shock. Some are insensible owing to their small amplitude, others to the slowness of the motion. An interesting observation belonging to the latter class was made by an engineer near Midnapur, a place which lies just within the area of damage. At the time of the earthquake, he was taking levels on a railway bank, and was about to take a reading when he noticed the bubble of the level oscillating. In five or ten seconds the shaking began and appeared to last three or four minutes; but, for more than five minutes after it had apparently ceased, the level showed that the ground continued to rock.

Again, in Burmah, at a place nineteen miles east of Tagaung and close to the border of the disturbed area, the water in a shallow tank, about 300 yards in length, was seen lapping up against the side in a manner that was at first attributed to elephants bathing. No shock was felt, but the shaking of the trees at the same time showed that the disturbance was due to the earthquake.

Far beyond the limits of the disturbed area, however, the earthquake was recorded by many of the delicate instruments which have been employed during the last few years for the registration of distant shocks. Among the more important of these instruments are long vertical pendulums, horizontalpendulums of various forms, and magnetographs. In the vertical, and some of the horizontal, pendulums, especially in those used in the Italian observatories, the masses carried are heavy, and the movements of the ground are magnified by lightly-balanced levers ending in points which trace their records on bands of smoked paper driven by clockwork. In the other horizontal pendulums and in the magnetographs, the method of registration is photographic. The paper required for the mechanical records being inexpensive, a high velocity (half-an-inch or more per minute) can be given to it, and the resulting diagrams are open and detailed. The Italian instruments also respond more readily than the others to the earlier and slighter tremors: while the apparatus in which photographic methods are used are sometimes so violently disturbed by the later undulations that the spot of light fails to leave any trace on the photographic paper. It is therefore from the Italian observatories that the more interesting records come. One of these, given by a horizontal pendulum at Rocca di Papa near Rome, is reproduced in Fig. 71; while the curve of the bifilar pendulum at Edinburgh (Fig. 72) is a good example of those obtained by the photographic method of registration.[71]

All over Italy, from Ischia and Catania in the south to Pavia in the north, the different instruments employed began, one after the other, to write theirrecords of the movement as the unfelt earth-waves sped outwards from the centre. Italy passed, the tale was taken up by magnetographs at Potsdam and Wilhelmshaven, Pawlovsk (near St. Petersburg), Copenhagen, Utrecht, and Parc St. Maur (near Paris); by horizontal pendulums at Strassburg and Shide (in the Isle of Wight), and by a bifilar pendulum at Edinburgh. Shide is 4,891 miles from the centre of disturbance, but, as we shall see, the movement could be traced for a distance greater even than this.

Back to Index Next