A Study of Recent Earthquakes Part 21
[Ill.u.s.tration: 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 pa.s.ses 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 cla.s.s 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 ma.s.ses 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 pa.s.sed, 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 Stra.s.sburg 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.
[Ill.u.s.tration: 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.
[Ill.u.s.tration: 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 pa.s.sed over a quarter-circ.u.mference 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 pa.s.sing 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 pa.s.sage 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.
[Ill.u.s.tration: 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 pa.s.sage 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 a.s.sam-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 a.s.sam, 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 cla.s.s. 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 quant.i.ties 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 a.s.serted their superiority. Among the more important may be mentioned the const.i.tution 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 Mahadeo, 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 Balpakram 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 canons of Colorado. At the bottom of the valley was a piled-up heap of _debris_ 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 acc.u.mulated 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.
[Ill.u.s.tration: 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 ma.s.s, 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.