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.

[Illustration: FIG. 68.--Isoseismal Lines of Indian Earthquake.

(_Oldham._)]

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.

[Illustration: FIG. 69.--Section of Tombs in the Cemetery at

Cherrapunji. (_Oldham._)]

_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 formulæ to the quantities obtained at Gauhati--namely, 8

feet per second for the maximum velocity and 12 feet per second per

second for the maximum acceleration, it follows that the amplitude

would be 5 feet and the period 4 seconds--values, which are evidently

inadmissible. Or, taking the maximum vertical component at 32 feet per

second per second, the corresponding values would be 2 feet and 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.

[Illustration: FIG. 70.--Time-curve of Indian earthquake.

(_Oldham._)]

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.

[Illustration: FIG. 71.--Seismographic Record of Indian

Earthquake at Rocca di Papa. (_Cancani._)]

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.

[Illustration: FIG. 72.--Seismographic Record of Indian

Earthquake at Edinburgh. (_Heath._)]

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.

[Illustration: FIG. 73.--Displacement of alluvium at foot of a

hill. (_Oldham._)]

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 Mahádeo, 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 Bálpakrám 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 cañons of Colorado. At the bottom of the

valley was a piled-up heap of _débris_ 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.

[Illustration: FIG. 74.--Twisting of monument at Chhatak.

(_Oldham._)]

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.

[Illustration: FIG. 75.--Epicentral Area of Indian Earthquake.

(_Oldham._)]

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.

[Illustration: FIG. 76.--Plan of Chedrang fault. (_Oldham._)]

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."

[Illustration: FIG. 77.--Re-triangulation of Khasi hills.

(_Oldham._)]

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.

[Illustration: FIG. 78.--Diagram of Thrust-planes.]

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.

It is a strain on the imagination to try and picture the displacement

of so huge a mass. We may think, if we will, of a slice of rock three

or four miles in thickness and large enough to reach from Dover to

Exeter in one direction and from London to Brighton in the other; not

slipping intermittently in different places, but giving way almost

instantaneously throughout its whole extent; crushing all before it,

both solid rock and earthy ground alike; and, whether by the sudden

spring of the entire mass or by the jar of its hurtling fragments,

shattering the strongest work of human hands as easily as the

frailest. Such a thrust might well be sensible over half a continent,

and give rise to undulations which, unseen and unfelt, might wend

their way around the globe.

REFERENCES.

1. AGAMENNONE, G.--"Notizie sui terremoti osservati in Italia

durante l'anno 1897 (Terremoto dell' India poco dopo il

mezzogiorno del 12 giugno)." _Ital. Sismol. Soc. Boll._, vol.

iii., pte. ii., 1897, pp. 249-293.

2. ---- "Il terremoto dell' India del 12 giugno 1897." _Ibid._, vol.

iv., 1898, pp. 33-40.

3. ---- "Eco in Europa del terremoto indiano del 12 giugno 1897."

_Ibid._, vol. iv., 1898, pp. 41-67. (See also the same volume,

pp. 167-172.)

4. BARATTA, M.--"Il grande terremoto indiano del 12 giugno 1897."

_Ital. Soc. Geogr. Boll._, vol. x., 1897, fasc. viii.

5. CANCANI, A.--"I pendoli orizzontali del R. Osservatorio

geodinamico di Rocca di Papa, ed il terremoto indiano del 12

giugno 1897." _Ital. Sismol. Soc. Boll._, vol. iii., 1897, pp.

235-240.

6. HEATH, T.--"Note on the Calcutta Earthquake (June 12th, 1897) as

recorded by the bifilar pendulum at the Edinburgh Royal

Observatory." _Edinb. Roy. Soc. Proc._, 1897, pp. 481-488.

7. OLDHAM, R.D.--"Report on the Great Earthquake of 12th June 1897."

_Mems. Geol. Surv. of India_, vol. xxix., 1899, pp. i.-xxx.,

1-379, with 44 plates and 3 maps.

8. ---- "List of After-shocks of the Great Earthquake of 12th June

1897." _Ibid._, vol. xxx., pt. i., 1900, pp. 1-102.

9. ---- "On Tidal Periodicity in the Earthquakes of Assam." _Journ.

Asiat. Soc._, vol. lxxi., 1902, pp. 139-153.

FOOTNOTES:

[69] According to some reports, the earthquake was felt in Italy. At

Livorno, the first movements were registered by seismographs at 11.17

A.M. (G.M.T.), and tremors were noticed by some persons at rest at

about 11.15 A.M. At Spinea, a sensible undulatory shock from

south-east to north-west, and lasting about four seconds, was felt at

the moment when all the seismographs were set in motion by the Indian

earthquake. In spite of the great distance, the perception of the

earthquake in Italy is not impossible, but the records seem to me to

refer to local tremors rather than to the very slow evanescent

oscillations of a very distant earthquake.

[70] All the times in this section are referred to Madras mean time,

which is 5h. 20m. 59.2s. in advance of Greenwich mean time. In the

next section it will be found convenient to use the latter standard.

[71] It may be useful to give references to works in English in which

the principal instruments for registering distant earthquakes are

described. For Cancani's vertical pendulum, see _Brit. Assoc. Rep._,

1896, pp. 46-47; Darwin's bifilar pendulum, _Brit. Assoc. Rep._, 1893,

pp. 291-303, and _Nature_, vol. 1., 1894, pp. 246-249; Milne's

horizontal pendulum, _Seismology_, pp. 58-61; Rebeur-Paschwitz's

horizontal pendulum, _Brit. Assoc. Rep._, 1893, pp. 303-308.

[72] The beginnings of the second and third phases are shown more

clearly in the record of the vertical pendulum at Catania, a record,

however, that will not bear the reduction necessary for these pages.

[73] _Geol. Mag._, vol. x., 1893, pp. 356-360.

[74] _Irish Acad. Trans._, vol. xxi, 1848, p. 52.

[75] _Irish Acad. Trans._, vol. xxi., 1848, pp. 55-57.

[76] _Neapolitan Earthquake of 1857_, vol. i., 1862, pp. 376-378.

[77] _Japan Seismol. Soc. Trans._, vol. i., pt. II., 1880, pp. 33-35.

[78] _Geol. Mag._, vol. ix., 1882, pp. 257-265.