A Study of Recent Earthquakes

by Charles Davison

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."