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.
Fig. 73.—Displacement of alluvium at foot of a hill. (Oldham.)[ToList]
Among the Khasi and Garo hills (see Fig. 75), wherever the alluvium of the plains runs up to the foot of the hills, another form of fissure, represented in Fig. 73, was constantly noticed. Close to the junction, there was a sudden drop, as at a, of from one to five feet, the vertical face having the appearance of a fault, but distinguished from one by following the windings of the hills. Then came a depressed band b, from ten to twenty feet wide, and outside this a low rounded ridge c raised above its former level, and merging beyond at d into the undisturbed plain. When Mr. Oldham visited the district in March 1898, the natives had flooded the rice-fields, and the features described were clearly depicted by the gathering of the water in the depression and the isolation of the ridge.
The explanation of these peculiarities is evidently that given by Mr. Oldham. During the passage of repeated waves of compression, the thrust of the hill and plain against one another caused the heaping up of the alluvium in the ridge c; while the return movements resulted in the tearing of the alluvium away from the hillside, leaving the scarp a and the depression b.
Displacements of Alluvium.—Many other remarkable evidences of compression were observed. Telegraph posts, originally set up in a straight line, were displaced, occasionally as much as ten or fifteen feet; sometimes without any apparent connection with neighbouring river-channels. In one part of the Assam-Bengal Railway, for nearly half a mile, the whole embankment, including borrow-pits and trees on either side, was shifted laterally without any sign of wrenching from the adjoining ground, the maximum distance amounting to 6¾ 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 3½ 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.