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A Study of Recent Earthquakes
by Charles Davison
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We may imagine, then, that the slip within the seismic focus would be greatest in a central region, and that it would die outwards in all directions towards the edges. The friction arising from the slipping in the central region would produce chiefly the comparatively large oscillations that formed the perceptible shock; the evanescent creep within the marginal regions would produce the small and rapid vibrations that were sensible only as sound.

ORIGIN OF THE EARTHQUAKE.

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, and the 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.



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.

THE AFTER-SHOCKS AND THEIR ORIGIN.

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 on September 18th between 3.56 and 9 A.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.

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 9 A.M., and on September 30th at 3.39 A.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 3.39 A.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) is very 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 4.24 P.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.

SYMPATHETIC EARTHQUAKES.

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 and Inverness. 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.

CONCLUSION.

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. With the 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.

REFERENCES.

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.

FOOTNOTES:

[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.



CHAPTER IX.

THE INDIAN EARTHQUAKE OF JUNE 12TH, 1897.

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 AND DISTURBED AREA.

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 next curve 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 little idea 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.

NATURE OF THE SHOCK.

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 distinct sensation 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 with an 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 earthquake of 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 easily followed 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."

ELEMENTS OF THE WAVE-MOTION.

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.



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 side parallel 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 of the 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 6-1/2 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 1-1/2 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, and indeed 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 formul 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 1-1/2 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 per second 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.

SOUND-PHENOMENA.

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 no sound 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.

VELOCITY OF THE EARTH-WAVES.

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 arithmetic mean 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 Madras time-signals through frequent repetition along the line.



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 4 P.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 to a 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 4.26 P.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 between 4h. 25m. 45s., and 4h. 26m. 30s. P.M., Madras time, or between 11h. 4m. 45s. and 11h. 5m. 30s. A.M., Greenwich mean time.

THE UNFELT EARTHQUAKE.

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, horizontal pendulums 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 their records 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.



In the more complete records, and especially in those given by the Italian apparatus, Mr. Oldham distinguishes three phases of motion. The first consists of rapid and nearly horizontal movements of the ground. In Italy, it begins at about 11.17 A.M.—that is, about 12-1/2 minutes after the commencement of the shock at the epicentre (Fig. 71, a). Without any break in the movement, and after a further interval of about 8-1/2 minutes, the second phase begins; the vibrations are similar to the preceding, but they are larger and more open, and are accompanied by an unmistakable tilting of the surface of the ground (Fig. 71, b). Lastly, after the lapse of about twenty minutes more, the second phase gives place, without interruption, to the third (Fig. 71, c),[72] consisting of well-marked slow undulations, which have been aptly compared by Professor Milne to the movements caused by an ocean-swell. As they travelled across Europe, the surface of the ground was thrown into a series of flat waves, 34 miles in length, and 20 inches in maximum height, the complete period of each wave being 22 seconds. This phase is by far the longest of the three; in the more sensitive instruments, two or three hours elapsed before their traces ceased to show any sign of movement.



Knowing the distances of the different observatories from the epicentre, and the times taken by each phase to reach them, we can form some idea of the rates at which they travelled. If the early tremors moved in straight lines, their mean velocity for the first phase was 9.0, and for the second 5.3, kilometres per second; but, if they moved along curved paths through the body of the earth, their mean velocities must have exceeded these amounts. For the first undulations of the third phase, the velocity would be 2.9 kilometres per second if they travelled along straight lines, and 3.0 kilometres per second if they were confined to the surface of the earth.

The existence of the second phase was noticed for the first time by Mr. Oldham in the records of the Indian earthquake, but he has since detected it in those of other shocks. He believes, in common with most seismologists, that the first phase corresponds to waves of elastic compression, or longitudinal waves, travelling through the body of the earth; and the second phase he attributes to waves of elastic distortion, or transversal waves, travelling in the same way, in which the particles move at right angles to the direction in which the wave travels, thus causing a slight tilting of the surface. It is probable that the waves of both phases move along curved, rather than straight, lines through the earth, that the curves are concave towards the surface, and that the velocity of the waves increases with the depth of their path below the surface.

On the other hand, the surface-velocity of the first undulations of the third phase is practically constant for all distances from the epicentre, and, in the case of the Indian earthquake, it agrees almost exactly with that obtained for the velocity within the disturbed area, and as far as Bombay. It is therefore difficult to resist the conclusion that the third phase consists of undulations which travel along the surface of the earth. Diverging in two dimensions only, they fade away much more slowly than the vibrations of the other two phases.

We may thus imagine these surface-undulations speeding outwards from the epicentre in ever-widening circles until they have passed over a quarter-circumference of the earth, when they should begin to converge towards the antipodes. Here they should cross each other, and again spread out as circular waves, once more in their course passing the same observatories where they were first recorded, but in the opposite order. It has been reserved for the most violent earthquake of modern times to verify this interesting conclusion. Faint, but decided, are the traces of the second crossing. At Edinburgh, they occur at 2.6 P.M., at about the same time at Shide, at Leghorn 2.10, Catania 2.12-3/4, while at Ischia there are several movements between 2 and 3 P.M. At Rocca di Papa, near Rome, the time is slightly earlier, but the undulations, like those at the first crossing, have a complete period of about 20 seconds. The distances traversed by the waves are more than 20,000, instead of less than 5000 miles; but the mean velocity with which they travelled is almost exactly the same as at first—namely, 2.95 kilometres per second.

EARTH-FISSURES, SAND-VENTS, ETC.

Earth-Fissures.—Among the superficial effects of the earthquake, none take a more important place than the fissures formed in alluvial plains. Not only were they remarkably abundant, more so than in any other known earthquake, but they occurred over an unusually wide area. Wherever the necessary conditions prevailed, they were found to be numerous over a district bounded approximately by the isoseismal 1 (Fig. 68), and measuring about 400 miles from east to west, and about 300 miles from north to south; and they were present, though in smaller numbers, over an area nearly 600 miles long in an east-north-east and west-south-west direction. They were naturally more frequent near river-channels and reservoirs, on account of the absence of lateral support, and as a rule were parallel to the edge of the bank, a few hundred yards in length, and in width varying from some inches to four or five feet.

Fissures in such positions are formed with every violent earthquake, and even with some of those more moderate shocks that visit the British Islands (see p. 247). But an interesting point established by the Indian earthquake is that they also occurred at a distance from any water-channel or excavation, often running parallel to, and along either side of, a road or embankment. In other situations, they showed a distinct tendency to range themselves parallel to one another; and, in these cases, it is possible that their formation was connected with the passage of the visible surface-waves. In an account already quoted (p. 247), it is stated that these waves came from opposite directions and that, as they separated after meeting, the ground opened slightly.

Among the Khasi and Garo hills (see Fig. 75), wherever the alluvium of the plains runs up to the foot of the hills, another form of fissure, represented in Fig. 73, was constantly noticed. Close to the junction, there was a sudden drop, as at a, of from one to five feet, the vertical face having the appearance of a fault, but distinguished from one by following the windings of the hills. Then came a depressed band b, from ten to twenty feet wide, and outside this a low rounded ridge c raised above its former level, and merging beyond at d into the undisturbed plain. When Mr. Oldham visited the district in March 1898, the natives had flooded the rice-fields, and the features described were clearly depicted by the gathering of the water in the depression and the isolation of the ridge.



The explanation of these peculiarities is evidently that given by Mr. Oldham. During the passage of repeated waves of compression, the thrust of the hill and plain against one another caused the heaping up of the alluvium in the ridge c; while the return movements resulted in the tearing of the alluvium away from the hillside, leaving the scarp a and the depression b.

Displacements of Alluvium.—Many other remarkable evidences of compression were observed. Telegraph posts, originally set up in a straight line, were displaced, occasionally as much as ten or fifteen feet; sometimes without any apparent connection with neighbouring river-channels. In one part of the Assam-Bengal Railway, for nearly half a mile, the whole embankment, including borrow-pits and trees on either side, was shifted laterally without any sign of wrenching from the adjoining ground, the maximum distance amounting to 6-3/4 feet. As the displacement took place parallel to the only river-course in the neighbourhood, Mr. Oldham attributes it to the sliding of the surface-layers over some yielding bed beneath. Again, throughout large areas of Northern Bengal, Lower Assam, and Maimansingh, rice-fields, which had been carefully levelled so that they might be uniformly flooded, were thrown into gentle undulations, the crests of which were occasionally two or three feet above the hollows. The piers of bridges were also moved parallel to, as well as towards, the streams, showing that the displacements extended to the depth of the foundations.

The buckling of railway lines was often violent and took place over a large area. In the Charleston earthquake, every such bend was accompanied by a corresponding extension elsewhere (p. 113); but, in the Baluchistan earthquake of 1892, the neighbouring fish-joints were jammed up tight.[73] In the one case, there was merely local compression; in the other, a permanent displacement of the earth's crust. The distortion of the Indian lines seems to belong to the former class. Repairs were of course generally made without delay; but all the information that could be obtained on this point showed that the compression causing the crumpling of the lines was accompanied by a compensating expansion, generally at a distance of about 300 yards.

Sand-Vents.—Shortly after the earthquake, large quantities of water and sand issued from fissures in the ground. At Dhubri, "innumerable jets of water, like fountains playing, spouted up to heights varying from 18 inches to quite 3-1/2 or 4 feet. Wherever this had occurred, the land was afterwards seen to occupy a sandy circle with a depression in its centre. These circles ranged from 2 to 6 and 8 feet in diameter, and were to be seen all over the country. In some places, several were quite close together; in others they were at a distance of several yards." Near Maimansingh, they seem to have been almost as numerous, fifty-two, of four feet and less in diameter, being counted within an area 100 yards long and about 20 feet wide.

The sand and water were ejected from the vents with some force. A few observers estimated the height of the spouts at about 12 feet, but this probably refers to stray splashes. It is clear, however, that the sand and water were forced not only up to the surface, but even in a continuous stream to heights of from two to ten feet above it. In many districts, trunks of trees or lumps of coal and fossil resin were washed up with the water, and even, in one or two cases, pebbles of hard rock weighing as much as half-a-pound.

The origin of the sand-vents is to be sought in the presence of a water-bearing bed situated not far below the surface. In the central area, where there was a marked vertical component in the motion, this bed during the earthquake was compressed between those above and below it, and the resulting pressure was in places sufficient to force the water and sand, through the fissures formed by the earthquake, up to and beyond the surface. The gradual settling of the upper layer, cut up by the fissures, into the underlying quicksand, prolonged the process for some time after the shock was over; and, when the pressure was at last relieved, some of the water was sucked back and so produced the crateriform hollows.

Rise of River-Beds, etc.—Over a large area, river-channels, tanks, wells, etc., were filled up, partly by the outpouring of the sand from vents, but chiefly, as shown by the forcing up of the central piers of bridges, by the elevation of the beds of the excavations. In the lowlands which lie between the Garo hills and the Brahmaputra, there were numerous channels from 15 to 20 feet in depth, the beds of which were pressed up until they became level with the banks, while a compensating subsidence took place close to the streams on either side. The general tendency of the earthquake was thus to obliterate the surface inequalities, so that, when the rivers rose later on, the district was extensively flooded.

Besides these deferred floods, there occurred immediately after the earthquake a sudden rise in many rivers, amounting to from two to ten feet, followed by a gradual decline to the former state in two or three days. At Gauhati, for instance, the river-gauge showed that, at about three-quarters of an hour after the earthquake, the water stood 7 feet 7 inches higher than on the morning of June 12th; at 7 A.M. on June 13th it had fallen to 5 feet 8 inches, and at the same time on the two following days to 2 feet 7 inches and 6 inches, showing that the water had returned nearly to its original level after the lapse of two and a half days.

In most of the large rivers, the rise of water was due to the formation of partial dams formed by the local elevation of the river-beds described above. As the barriers were composed of loose sand, they were gradually scoured away and the material was spread over the bottom so as to leave the water at a level slightly higher than that which it maintained before the earthquake.

LANDSLIPS.

The distribution of landslips shows that their formation depends almost as much on local conditions as on the violence of the shock. The effect of the latter is manifested by their limitation to a certain central area. To the east of the North Cachar hills, few, if any, were to be seen; but, as far as Kohima, cracks or incipient landslips were formed on the hillsides. The Sylhet valley and a line to the west of Darjiling form the southern and western boundaries of the landslip area, which was therefore not less than 300 miles in length from east to west.

Within this area, however, local conditions asserted their superiority. Among the more important may be mentioned the constitution of the hills and the presence of a thick superficial layer of subsoil or rock with an inner bounding surface of weak cohesion, the slope of the hillsides, and their height from base to crest. Thus, though the epicentral area was situated chiefly to the south of the Brahmaputra valley (Fig. 75), the east and west range of the landslips was more extensive in the Himalayas on the north side than in the Garo and Khasi hills on the south. In many places, the steep sides of the Himalayan valleys exist always in a critical condition of repose, and the effect of the Indian earthquake was such that all along the north side of the Brahmaputra valley, the range is scarred by landslips, even to the east of Tezpur.

Again, along the southern edge of the Garo and Khasi hills, landslips were unusually prevalent. "Viewed from the deck of a steamer sailing up to Sylhet," says Mr. Oldham, "the southern face of these hills presented a striking scene. The high sandstone hills facing the plains of western Sylhet, usually forest-clad from crest to foot, were stripped bare, and the white sandstone shone clear in the sun, in an apparently unbroken stretch of about 20 miles in length from east to west." At Cherrapunji, also, the deep valleys were so scored that, from a distance, there appeared to be more landslip than untouched hillside.

But in no part, probably, were landslips more strikingly developed than in the small valley of the Mahdeo, which forms an amphitheatre about four miles long from east to west, and a mile and a half across, lying to the south of the Blpakrm and Pundengru hills. "Here," remarks Mr. Oldham, "everything combined to favour the formation of landslips. The hills were composed of soft sandstone, they were steep-sided, high, and narrow from side to side, and consequently were doubtless thrown into actual oscillation as a whole; while the range of motion of the wave particle was not less than eight inches near the edge of the precipices. The result ... has been to produce an indescribable scene of desolation. Everywhere the hillsides facing the valley have been stripped bare from crest to base, and the seams of coal and partings of shale could be seen running in and out of the irregularities of the cliffs with a sharpness and distinctness which recalled the pictures of the caons of Colorado. At the bottom of the valley was a piled-up heap of dbris and broken trees, while the old stream had been obliterated and the stream could be seen flowing over a sandy bed, which must have been raised many feet above the level of the old watercourse."

In the sandstone districts of the area here considered, the landslips had some important secondary effects. Along the southern edge of the Garo and Khasi hills, great sand-fans spread over the fields, and the exposure of the hillsides formerly protected by forest left free scope for future denudation. Every stream of any size has in this way devastated many square miles of country. Among the hills themselves, more sand was brought down than the streams could carry away, and everywhere their beds were raised. "Ordinarily, the beds of these rivers, which are raging torrents when in flood, consist of a succession of deep pools separated by rocky rapids. After the rains of 1897, it was found that the pools had been filled up, and the rapids obliterated by a great deposit of sand, over which the rivers flowed in a broad and shallow stream."

A few valleys were for a short time barred across by landslips. In one, on the northern foot of the Garo hills, a landslip crossed the drainage channel and formed a shallow pond, which was not filled up by sand until the end of January 1898. Near Sinya, in the northern Khasi hills, an unusually large landslip formed a barrier, of which the remains are more than 200 feet above the level of the river-bed. Behind this, the water accumulated in a great lake until the beginning of September 1897, when the barrier burst and a flood of water rushed down the valley.

ROTATION OF PILLARS, ETC.

A curious effect of earthquakes strong enough to damage buildings is that pillars, monuments, etc., may be fractured and the upper part rotated over the lower without being overthrown. Even in Hereford and the surrounding villages, several pinnacles and chimney-stacks were twisted by the earthquake of 1896. The interest of the phenomenon, which has been known, since 1755,[74] is mainly historical, for the endeavour to discover its cause was the origin of Mallet's views on the dynamics of earthquakes. Partly, also, it lies in the difficulty of finding a satisfactory explanation, or rather in deciding which of three or four possible explanations is the true one in any particular case.



The Indian earthquake offered exceptional opportunities for studying the phenomenon in the large number of examples observed and the variety of objects rotated. None could be more striking than the twisted monument to George Inglis, represented in outline in Fig. 74. Chhatak, where this is situated, lies close to the southern boundary of the epicentral area. The monument is an obelisk, built of broad flat bricks or tiles on a base of 12 feet square, and originally more than 60 feet high. It was split by the earthquake into four portions. The two upper, about six and nine feet long, were thrown down; while the third, 22 feet high, remains standing, but is twisted through an angle of 30 with respect to the lowest part, which is unmoved. The upper of these two parts had evidently rocked on the lower, as the corners and edges were splintered, and below the fracture a slice of masonry about 15 inches thick, which was not bonded into the main mass, was split off by the pressure on its upper end. The plan of the parts still standing is shown in the lower part of Fig. 74.

The possible explanations of the phenomenon are at least three in number. According to the first, which was given by Mallet in 1846, the adhesion of the twisted portion to its base is not uniform, and the resultant resistance to motion is not in the same vertical plane as the wave-movement.[75] Some years later, Mallet offered another explanation. The body, he imagined, might be tilted on one edge by the earthquake, and, while still rocking, a second shock oblique to the first might twist it about that edge.[76] In 1880, Professor T. Gray suggested that the column might be tilted on one corner and then twisted round it by later vibrations of the same shock.[77]

None of these theories, Mr. Oldham argues, can give by itself a complete explanation of the phenomena observed in the central district of the Indian earthquake; and he therefore favours an extension of the second theory, which, though first proposed in 1882,[78] was thought out independently and in greater detail by himself. When the focus is of considerable dimensions, the shock at neighbouring places is constantly varying in direction, owing to the arrival of vibrations from different parts of the focus. Thus, instead of the two separate shocks required by Mallet's second explanation, we have a number of closely successive impulses frequently changing in direction and giving rise to what is known in the South of Europe as a vorticose shock. And, instead of a single twist of the pillars about one centre only, we have a series of small twists round a number of different centres, accompanied in consequence by a much smaller displacement of the centre of gravity than would have occurred had the same rotation been accomplished in one operation.

The theory, it will be seen, accounts for the twisting of the pillar without overthrow, and for the splintering of the edges during the rocking of the column. It explains why in any district a number of similarly placed objects are generally twisted in the same direction. Moreover, a low column rocks to and fro more rapidly than a tall one similar in form and position, so that, at the instant when a later impulse comes from a different direction, two such columns might happen to be tilted on opposite edges, and would then be twisted in opposite directions. In certain cases, then, as occurred at several places during the Indian earthquake, an object may rotate in one direction, while others, similar in every respect but size, may be twisted in the opposite direction.

AFTER-SHOCKS.

Frequency of After-Shocks.—For some days after the great earthquake, the after-shocks by their very frequency and by their wide distribution baffled close inquiry. During the first 24 hours, hundreds were felt at all points of the epicentral area; indeed, it is not too much to say that for several days the ground was never actually at rest. At the Bordwar tea-estate, which is traversed by one of the great fractures to be described in the next section, the surface of a glass of water on a table was for a whole week in a constant state of tremor; and at Tura a hanging lamp was kept continually swinging for the first three or four days.

Most of these shocks were, of course, very slight; but, interspersed among them, were others of greater strength, and a few of considerable violence. One, on June 13th, about eight hours after the earthquake, was sensible beyond Allahabad—that is, for more than 520 miles from the epicentre; and another on the same day was felt in Calcutta, distant 255 miles. On June 14th, 22nd, and 29th, and again on August 2nd and October 9th, shocks were noticed in that city; but, after the latter date, the disturbed area of no shock reached to so great a distance.

To form any estimate of the total number of after-shocks is impossible, even for any one station. At first, lists were kept at isolated places, such as Shillong, Maimansingh, Dhubri, and a few others. Then, from July 15th, through Mr. Oldham's efforts, the records became more numerous until the end of the year, after which interest in the subject declined. Mr. Oldham's catalogue closes with the year 1898; but the register of a roughly-constructed seismograph, erected at Shillong in July 1897, continues to the present day.

Imperfect as all non-instrumental registers must be, they nevertheless furnish some idea of the frequency of the after-shocks. Thus, until the end of June, 679 shocks were recorded at Rangmahal (North Gauhati), 135 at Maimansingh, 89 at Kuch Bihar, and 83 at Kaunia (omitting those on June 12th). Again, from August 1st to 15th, 182 were felt at Goalpara, 151 at Darangiri, 124 at Tura, 105 at Bijni, 94 at Lakhipur, 94 at Krishnai, 48 at Dhubri, 28 at Rangpur, and 12 at Kuch Bihar; while at Borpeta, 113 shocks were reported during the first nine days of August. Turning to the registers of longer duration, we find that at Maophlang (near Shillong) 1,194 shocks were felt by one observer from September 12th, 1897, to October 7th, 1898; at the neighbouring station of Mairang, 1,065 from September 7th, 1897, to December 31st, 1898; and at Tura, in the Garo hills, 1,145 shocks from July 21st, 1897, to December 31st, 1898. The total number of earthquakes registered by the seismograph at Shillong from August 1897 to the end of 1901 amounts to 1,274, and all of these were probably strong enough to arouse the observer from sleep. Outside the epicentral area, Mr. Oldham's list includes 88 shocks from June 12th to July 15th, about 950 from July 16th to December 31st (the period when the after-shocks were most carefully observed), and 296 shocks during the year 1898.

Geographical Distribution of After-Shocks.—When we endeavour to compare the lists of after-shocks at different places, we are at once met by two serious difficulties,—the imperfection of the records and the approximate character of the times of occurrence. Making every allowance, however, for these deficiencies, it is evident that very few of the shocks felt at any one station were perceptible at its neighbours; in other words, that the shocks originated in a large number of foci scattered over a very wide area.

For instance, two of the most carefully kept registers of after-shocks are those compiled at Maophlang (near Shillong), and at Mairang, only 11 miles to the north-west. Now, between September 12th and September 28th, 1897 (both dates inclusive), 92 shocks were felt at Maophlang and 83 at Mairang. Of the former, 37 were described as smart, 45 slight, and 10 feeble; of the latter, 6 as smart, 9 slight, 65 feeble, and 3 very feeble. But, of the total number, only 20 were felt at both places at recorded times that were not more than fifteen minutes apart; 13 being described as smart—one at both places, one at Mairang alone, and the remaining 11 at Maophlang alone. When shocks occur so frequently, as in these cases, it is inevitable that, even if all were independent, some should coincide approximately in time of occurrence. It is therefore probable that only one in every eight shocks, and possibly only one in every twelve, was felt at both places.

The actual numbers of shocks felt within stated periods at different places are perhaps hardly comparable, owing to the obvious imperfection of the records and the probably varying standards adopted by the reporters. But there can be little doubt that certain districts were more subject to after-shocks than others, especially such places as North Guahati, Shillong, and neighbouring villages, Tura, Darangiri, Goalpara, Bijni, Borpeta, Kaunia, and Rangpur. On the other hand, they seem to have been unusually scarce at Dhubri and in the district to the north-west, and they became rare at Gauhati long before they ceased to be frequent at Borpeta. In the plain to the south of the Garo and Khasi hills, they were also uncommon, the combined records for Sylhet and Sonamganj for August 1-15 giving only 20 shocks, and, neither to the east nor to the west of these places, is there any sign of greater frequency.

Sound-Phenomena of After-Shocks.—Many of the after-shocks were accompanied by sound, or else consisted of sound-vibrations only; and Mr. Oldham notices that such sounds were equally frequent both on the rocky ground of the hills and on alluvial plains nearly all the shocks that originated under the Borpeta plain being attended by distinctly audible rumblings.

During his tour in the epicentral area in the winter of 1897-98, Mr. Oldham had many opportunities for observing these earth-sounds. They were, he says, close to the lower limit of audibility, less a note than a rumble, and very like distant thunder, though sometimes they consisted of a rapid succession of short sounds, such as is caused by a cart when driven rapidly over a rough pavement. "As a rule, they began as a low, almost inaudible rumble, gradually increasing in loudness, though to a very varying degree, and then gradually dying out after having lasted anything from 5 to 50 seconds. It cannot be said that there was any connection between the duration and the loudness of the sounds, some of the most prolonged never becoming loud, and some of those which lasted a shorter period being as loud as ordinary thunder at a distance of two or three miles."

Mr. Oldham records an interesting fact in connection with the distribution of the earth-sounds. At Naphak, in the Garo hills and about five miles south of Samin, 48 distinct rumbles were heard during 23 hours on January 21-23, 1898, only seven of them being accompanied by a perceptible shock. At Samin, which was visited next, they were much less frequent, not more than 8 or 10 a day, and most of them attended by tremors. At Damra, a few miles to the north-east, they again became frequent; while, in the Chedrang valley, very few were heard, and only a small proportion of them were unaccompanied by sensible shocks. In the next section, it will be seen that the most conspicuous fault-scarps known in the epicentral area pass close by Samin and along the Chedrang valley. Thus, though the statement perhaps requires further confirmation, it would appear that earth-sounds were more common where the surface of the ground had been merely bent than where fractures extended right up to the surface.

STRUCTURAL CHANGES IN THE EPICENTRAL AREA.

We come now to the important features which assign the Indian earthquake to a small class apart from nearly every other shock. Most earthquakes are due to movements that are entirely deep-seated. If strong enough, they may precipitate landslips or fissure the alluvial soil near river-channels. In the Neapolitan, Andalusian, and Charleston earthquakes, there were many such effects of the shock within the meizoseismal areas. In all three, however, the disturbances produced were superficial; no structural change, no fissuring that did not die out rapidly downwards, was in any place perceptible. In the Riviera earthquake, the seismic sea-waves point to a small displacement of the ocean-bed; but it is only in the long fault-scarp of the central Japanese plain that we find a rival of the mountain-making movements that gave rise to the Indian earthquake.

The boundary of the epicentral area, to the growth of which these distortions contributed, is represented by the curve marked A in Fig. 68, and on a larger scale by the continuous line A in Fig. 75. A great part of the district is occupied by a group of hills known by various names locally, but which are conveniently included under the general term of the Assam range. To avoid the confusion of hill-shading, only the boundary of the range is indicated (by the broken line) in the map in Fig. 75. The Garo hills form the western part, and the Khasi and Jaintia hills the central and western parts, of the range as there depicted. They are formed chiefly of crystalline gneissic and granitic rocks and some metamorphic schists and quarzite, with cretaceous and tertiary rocks of varying thickness along its southern edge.

Three stages have been distinguished in the history of the range. During the earliest, an old land-surface was worn down by rain and rivers till they were almost incapable of producing any further change. Traces of this surface are still visible in the plateau character of the mass. It was then elevated, not uniformly, but along a series of faults, so that it now consists of a succession of ranges, the face of each range being a fault-scarp, and its crest the edge of an adjoining plateau sloping away from the summit. With this elevation began the third and last stage. The streams were able to work again, and deep gorges were cut out of the range, so that in parts its original character was nearly effaced. But the retention of that character in other districts is of course evidence of the comparatively recent date of the final elevation.



Owing to the great size of the epicentre and to the thickness of the forests which cover so much of its area, a comparatively small part of it could be traversed by Mr. Oldham during his tour in the winter of 1897-98. The positions of the more important structural changes are indicated in Fig. 75. Of these, the fault-scarps are represented by continuous straight lines, the Bordwar fracture by the dotted straight line, pools and lakes not due to faulting by black ovals, reported changes in the aspects of the hills by circles, and the principal stations of the revised trigonometrical survey by crosses.

Fault-Scarps.—The most important fault-scarp is that called by Mr. Oldham the Chedrang fault, after the stream which coincides roughly with a great part of its course. The longer straight line in Fig. 75 represents its position and general direction, and the sketch-map in Fig. 76 gives the plan of its southern half. From these, it will be seen that the fault follows on the whole a nearly straight path from south-south-east to north-north-west for not less than twelve miles, and that its throw, as indicated by the numbers to the right in Fig. 76; is very variable, being zero in some places, and in one as much as 35 feet or more. The upthrow is uniformly on the eastern side of the fault.

At its southern end, as mapped in Fig. 76, there is no perceptible throw at the surface, but various marks of violence are manifested in the fissuring of the hillside and the snapping of small trees. About a quarter of a mile from this point, the fault crosses a tributary stream, where the throw amounts to two feet, and the same distance farther on it meets the Chedrang river, the bed of which it crosses many times in its short course.



Mr. Oldham describes the fault in detail, as observed by him in February 1898. Here, it will be sufficient to refer to its more important features, and to its effects on the superficial drainage of the district. At the spot marked a (Fig. 76) the river, after running on the west or down-throw side of the fault for nearly half a mile, meets the scarp, and is ponded back by it for about a quarter of a mile upstream. For the next half-mile, the river keeps to the upthrow side of the fault, the scarp of which blocks the tributary streams from the west, forming a number of small pools. At the last of these, the total throw is not less than 25 feet. A little farther on, the fault crosses the Chedrang and causes the waterfall at b, the height of which, owing to the fall of dislodged fragments, does not exceed nine feet. The fault then runs along the old and now dry bed of the river, while the stream itself flows in a depression on the down-throw side. About a quarter of a mile below the waterfall, the fault crosses the river, and soon after enters a large sheet of water at c, half a mile long, from 300 to 400 yards wide, and with a maximum depth of 18 feet. At first, the pool spreads on both sides of the fault, but the inequalities due to the scarp are evidenced by soundings. At the point where the fault leaves the pool, its throw is reduced to nothing, and it is just here that the water attains its greatest depth. To the north the throw increases rather rapidly, to 25 feet in a quarter of a mile. But the peculiarity of this pool is that it is not, like the others mentioned above, dammed back by the fault-scarp. There is no barrier at its northern end, where the river escapes, except that formed by the gradually increasing throw of the fault. The pool is simply due to the reversal of the natural slope of the river-bed, caused by the formation of a roll or undulation in the ground on the upthrow side of the fault. Its recent origin is evident from the number of dead trees and bamboo clumps still standing in the water.

For a mile after the fault leaves the pool, its throw varies considerably. It rises, as already mentioned, from zero to 25 feet. A little farther on, the fault runs up the side of a spur, the throw increasing to 31 feet; and, in this part, the violence of the shock was shown by the dislodgment of blocks of granite as much as 20 feet in diameter, and by the overthrow or destruction of many trees. After crossing the spur, the fault returns to the neighbourhood of the river, and crosses its bed four times, forming pools (e, g) or waterfalls (d, f) according as the scarp occurs on the downstream or upstream side. The throw of the fault then changes considerably within little more than half a mile, from 18 feet to zero and again to 20 feet, the undulation so formed producing a large pool (h) entirely on the upthrow side of the fault.

At the point marked i on the map, the river once more crosses the fault; but the bottom of the valley is filled with alluvium, and, instead of a waterfall, a large sandy delta spreads down the stream. The scarp is, however, readily traced on the east side of the river, a throw of 32 feet being measured. After this, the alluvium becomes of considerable thickness, and the continuation of the fault is marked by a short slope, which tilts over the trees when it traverses forest-land. Leaving the valley of the Chedrang, the fault crosses an open plain, and is followed with some difficulty to the neighbourhood of Jhira, where, owing to the thick bed of alluvium, it forms a gentle roll or undulation of the surface, crossing the main channel of the Krishnai to the north-east of Jhira. On the west side of this barrier a large sheet of water, a mile and a half in length, three-quarters of a mile wide, and 12 feet in depth, gathered over the village of Jhira. "On the east side of the Jhira lake," says Mr. Oldham, "there is ample evidence of change of level, for part of the dry land was formerly ... perpetually under water, and at one place the remains of an old irrigation channel can be seen.... At the northern end of the lake the drainage now makes its escape in a broad and shallow sheet of water over what was once high land covered with sal forest."

This is the last marked feature due to the Chedrang fault. Beyond the north of Jhira the throw rapidly diminishes, and perhaps dies out altogether before reaching the low hills lying to the north of that village.

In several ways, this fault-scarp differs from that formed with the Japanese earthquake of 1891. Throughout its course the down-throw, wherever it is perceptible, is invariably to the west; in no place could any trace of horizontal shifting be detected; and the plane of the fault, when it traversed rock, is practically vertical.

Whether the scarp was formed by the elevation of the rock to the east of the fault, or by the depression of that to the west, or by both such movements at once, there is no decisive evidence; but there are very good reasons for believing the first alternative to be the true one. The undulations in the ground which gave rise to the large pools at c and h (Fig. 76) occur on the east side of the fault. Also, between the outlet of the lake at Jhira and the point where the Krishnai rejoins its original channel, the gradient of the river approaches that of a mountain stream, although the new bed consists of alluvium, and not of rock. Now, the alluvial plain of this district is raised so slightly above the sea-level that no subsidence great enough to have caused the existing gradient could have occurred without the depressed area being flooded with water. Though some movements may have taken place on the west side of the fault, it seems clear, then, that elevation of the rock on the east side was the predominant, if not the sole, cause of the fault-scarp.

As the Chedrang fault has been described somewhat fully, a brief reference to the rest will be sufficient The only other known scarp of any consequence lies about ten miles to the south of the Chedrang fault, and runs by the village of Samin, with an average course from E. 30 S. to W. 30 N. Its total length does not exceed 2-1/2 miles. The down-throw is uniformly to the north, and the throw, which amounts to ten feet near its centre, gradually diminishes to zero at either end. Several pools are formed along the course of the fault-scarp by the blocking of small streams.

The Bordwar Fracture.—In the map of the epicentral area (Fig. 75), this remarkable fracture is represented by a dotted straight line. It is apparently an incipient fault. Though traceable for a distance of about seven miles, at no point is there any decisive evidence of either vertical or horizontal displacement; and, even if some doubtful indications of a change of level should be real, the throw must certainly be less than one foot. Yet, in the immediate neighbourhood of the fracture, the violence of the shock was extreme. "Trees have been overthrown or killed as they stood; a huge mass of rock, dislodged from near the crest of the hills, has rolled down the slope, scoring the side of the hill. On the opposite side an equally large block has been dislodged, and in its downward course cleared a straight track down the hill; and on the summit a gap has been cleared by the overthrow of trees along the line of fracture." Being only a few inches in width where it has rent the solid rock, the fracture was difficult to follow in many parts of its course. But, through forest-clad land, its track was marked by "a well-defined band of about half a mile broad, in which overturned trees are much more abundant than on either side, and towards the centre of this band the overturned trees are not only more numerous, but many of the smaller ones, up to six inches in diameter, have been snapped across by the violence of the shock."

Lakes and Pools not due to Faulting.—A few miles to the south of the Chedrang and Samin faults, and also of the Bordwar fracture, occurs a group of lakes or pools, represented on the map of the epicentral area (Fig. 75) by small black ovals. In the gradual increase in depth from either end, they resemble the two large sheets of water along the course of the Chedrang fault (c and h, Fig. 76), but they differ from them in having no direct connection with any apparent fault.

One of these pools lies in the valley of the Rongtham river, to the south of the Samin fault. It seemed, at first sight, to be nothing more than an ordinary pool, such as may be seen on any mountain stream. On the bottom, and close to the outlet, however, are coarse, partially rounded boulders, exactly resembling those farther down the river; and, as the old bed was followed up, these became coated with a slight deposit of sand and mud, pointing clearly to a change in the conditions under which they were formed. The water gradually deepened, until trees were met standing in the water, but killed by the recent submergence of their roots. The pool is nearly a quarter of a mile long, and its greatest depth (12 feet) occurs near the middle, just where the former stream, with an average depth of about a foot, was crossed by the track from Darangiri. Towards the upper end, the water shallows as gradually as it deepens at the other, and ends in a delta of boulders brought down by the stream above. As no fault could be discovered in the neighbourhood of the pool, it is evident that its formation was due to a bend of the river-bed, the maximum change of level, taking into account the river-slope, being not less than 24 feet.

Similar features characterise the other pools that were examined, some of which are smaller, and others larger, than that described above. One, higher up the valley of the Rongtham, has a length of about 1-1/2 mile and a maximum depth of 18 feet. Others of the same type, but of smaller size, were observed among the Khasi hills, about fifteen miles south of the Bordwar fissure; and it is probable that many others would have been found in the intermediate district, which Mr. Oldham was unable to visit.

Changes in the Aspects of the Hills.—There are, again, other facts of considerable interest which point to changes of level over a wide area; the places where they were noticed being indicated by small circles in Fig. 75. For instance, from Maophlang, near Shillong, a road leads to the neighbouring station of Mairang. Before the earthquake, only a short stretch of this road could be seen from the former place, as it rounded a spur about three miles away. Now, a much longer stretch is visible, and it can also be seen passing round the next, and previously hidden, spur. In this district the movements seem to have continued with the after-shocks; for, before the earthquake, the crest only of a ridge about a mile and a half to the west was visible; while, after it, a considerable portion could be seen, and much more some months later than immediately after the shock.

Again, from a spot near the southern end of the Chedrang fault, it used to be only just possible to see the Brahmaputra over an intervening hill; whereas, now, the whole width of the river has come into view.

Lastly, at Tura, which is 95 miles west of Maophlang, a battalion of military police were accustomed to signal by heliograph with another station, Rowmari, 15 miles farther to the west. This, formerly, could just be done by means of a ray which grazed a hill between the two places; it can now be done quite easily, and, in addition, a broad stretch of the plains east of the Brahmaputra is visible from the same spot.

Revision of the Trigonometrical Survey.—The movements described in the preceding pages are of course referred to points which may themselves have been displaced, and only a revision of the trigonometrical survey of the epicentral area and of part of the surrounding district could determine their absolute magnitude. During the cold weather of 1897-98, some of the triangles were re-measured by a member of the trigonometrical survey; but, as the time at his disposal was short, they were confined to the eastern part of the epicentral area, as the focus at that time was supposed to lie under the Khasi hills. The positions of some of these stations are indicated by crosses in Fig. 75; and in Fig. 77 the more important triangles are shown. In the revised work, all tower stations, consisting of brick towers built on alluvium, were omitted, as it could not be assumed that they had been undisturbed by displacements of the superficial beds.

In re-calculating the lengths of the sides, the side Rangsanobo-Taramun Tila was adopted as the initial base, and the height of Rangsanobo as the initial height; a choice which later experience showed to be unfortunate, for Taramun Tila probably lies just outside, and Rangsanobo within, the epicentral area. Of the 16 sides, whose old and new lengths were compared, only one was found to be apparently unchanged, two were shortened by an inch or two, while the others were all lengthened by amounts varying from one to eight or nine feet, the numbers affixed to the sides in Fig. 77 denoting the calculated increases in feet. Assuming the new base-line to be unaltered by the earthquake movements, these changes imply the following displacements of the principal stations:—Thanjinath 6 feet, Mun 4, and Laidera 2, feet to the north; Mopen 5, Dinghei 9, Landau Modo 12, and Umter 11, feet to the north-west; and Mosingi 3, and Mautherrican 5, feet to the west. At the same time, the height of most of the stations was found to be increased with reference to that of Rangsanobo: Mun by 2 feet, Thanjinath and Umter by 3, Mosingi by 4, Taramun Tila and Laidera by 6, Dinghei by 7, Landau Modo by 17, and Mautherrican by 24, feet; while the height of Mopen seems to have been diminished by 4 feet. Thus, at first sight, these calculations appear to indicate "a general elevation and extension of the hills, such as might follow on a bulging upwards of the surface due to the extension of a large mass of molten matter underground."



Unfortunately, as Mr. Oldham shows, a very different, and more probable, interpretation may be given of these results; for all the calculated changes are rendered uncertain by the choice of the two stations which form the ends of the new base-line. One at least may have been displaced by the structural movements within the epicentral area; and, moreover, the line joining them runs nearly north and south. As compression in this direction is to be expected, it is probable that this line was shortened; and the assumption that its length was unchanged would therefore lead to an apparent expansion of all the other sides.

The calculated changes seem to favour this explanation to a great extent. The sides joining Mopen, Rangsanobo, and Thanjinath run nearly east and west, and are apparently lengthened by 4.9 and 3.4 feet respectively; while, of the four sides joining these stations to Mosingi and Mun, lying next to the north, two are nearly or quite unchanged, and the others increased by 2.3 and 3.2 feet. Again, the estimated increase of the Mosingi-Mun line is 4.4 feet; while the four sides joining these stations to the next northerly group are increased by small amounts—namely, 1.2, 2.6,-0.3, and 2.4 feet. Thus, the apparent expansion that should have occurred in these more or less northerly sides is lessened, or roughly compensated, probably by a compression of the whole region in a meridianal direction.

For a similar reason, the slight general upheaval of the hills indicated by the repeated calculations, must be regarded as doubtful, for it depends on the assumed fixity of the station of Rangsanobo, whereas it is more probable that it was the height of Taramun Tila that remained unchanged. Reducing the calculated heights of all the other stations by six feet (the assumed rise of the latter), it follows that, on the whole, the height of the Khasi hills underwent but little change, except at Mautherrican and Landau Modo, and the secondary stations of Mairang and Kollong Rock, near Maonoi. The apparent elevations of 24, 17, 11, and 15 feet at these places exceed the probable error of the observations; and it is worthy of notice that all four stations lie close to the edge of fault-scarps, while Landau Modo is not far from two of the pools formed by distortion of the surface unaccompanied by faulting.

If, then, the revised triangulation of the Khasi hills has failed to provide absolute measures of the displacements in the epicentral area, it has, nevertheless, proved that important movements, both horizontal and vertical, have taken place.

Distribution of the Structural Changes.—The boundary of the epicentral area, as drawn in Figs. 68 and 75, lays no claim to great accuracy; but its departure from the true line is probably in no place considerable. It must evidently include all the districts where marked structural changes occurred, and must therefore extend east of Maophlang and west of Tura. Towards the north, these changes have been traced to the foot of the Garo hills, and there is some, though not very certain, evidence of alterations of level along the course of the Brahmaputra. The very large number of after-shocks recorded at Borpeta and Bijni also points to an extension of the epicentral area beyond these places. To the east, the course of the boundary becomes doubtful, but it must pass close to Gauhati and east of Shillong, and probably ends a short distance beyond Jaintiapur. The southern boundary must coincide nearly with the north edge of the alluvial plains of Sylhet, for there is no evidence of its intrusion into the plains. On the west side, the epicentral area includes the Garo hills and part of the alluvial plain to the west; and, from the large number of after-shocks felt at Rangpur and Kaunia, and the great violence of the shock at the former, we may infer that both places lie within the boundary-line. If, then, there is no great error in the mapping of this line, it follows that the epicentre was about 200 miles long from east to west, not less than 50, and possibly as much as 100, miles in maximum width, and contained an area of at least 6000 square miles.

Near the boundary, the permanent displacements must have been comparatively small; but they were certainly marked in the northern part of the Assam hills for a distance of 100 miles from east to west. At the limits of the latter area, as Mr. Oldham remarks, "the evidence points to the changes being of the nature of long, low rolls, the change of slope being insufficient to cause any appreciable change in the drainage channels. Then comes a zone in which the surface changes are more abrupt, the slopes of the stream beds have been altered so as to cause conspicuous changes in the nature of the streams, but any fracture or faulting which may have taken place has died out before the surface was reached. And north of this, close to the edge of the hills, the rocks have been fractured and faulted right up to the surface."

ORIGIN OF THE EARTHQUAKE.

Almost every feature of the great earthquake points to an origin very different from that of the others described in this volume. The suddenness with which the shock began, its unusual duration, and the occurrence of many maxima of intensity, are inconsistent with a simple fault-displacement. Again, the excessive velocities of projection at Rambrai and elsewhere, the existence of isolated fault-scarps and fractures, the local changes of level, the compression indicated by the revised trigonometrical survey, the wide area over which these structural changes took place, and the numerous distinct centres of subsequent activity, all these phenomena demonstrate the intense and complex character of the initial disturbances, as well as the widespread bodily displacement of the earth's crust within the epicentral area. There may, it is conceivable, have been a number of foci, nearly or quite detached from one another, and giving rise to a group of nearly concurrent shocks. Or—and this is a far more probable supposition—there may have been one vast deep-seated centre, from which off-shoots ran up towards the surface, each partaking to a greater or less degree in the movement within the parent focus.

As Mr. Oldham points out, we have recently become acquainted with a structure exactly corresponding to that which is here inferred. The great thrust-planes, so typically developed in the Scottish Highlands, are only reversed faults which are nearly horizontal instead of being highly inclined; and they are accompanied by a number of ordinary reversed faults running upwards to the surface. In Fig. 78, the main features of a section drawn by the Geological Survey of Scotland are reproduced; T, T, representing thrust planes, and t, t, minor thrusts or faults. A great movement along one of the main thrust-planes would carry with it dependent slips along many of the secondary planes. Direct effects of the former might be invisible at the surface, except in the horizontal displacements that would be rendered manifest by a renewed trigonometrical survey; whereas the latter might or might not reach the surface, giving rise in the one case to fissures and fault-scarps, in the other to local changes of level, and in both to regions of instability resulting in numerous after-shocks.



The enormous dimensions of the parent focus will be obvious from the phenomena that have been described above. Mr. Oldham has traced the probable form of the epicentre. It may in reality be neither so simple nor so symmetrical as is represented in Fig. 75, but there are good reasons for thinking that it does not differ sensibly either in size or form from that laid down. The part of the thrust-plane over which movement took place must therefore have been about 200 miles long, not less than 50 miles wide, and between 6000 and 7000 square miles in area. With regard to its depth, we have no decisive knowledge. It may have been about five miles or less; it can hardly have been much greater.

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