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.
Fig. 74.—Twisting of monument at Chhatak. (Oldham.)[ToList]
A curious effect of earthquakes strong enough to damage buildings is that pillars, monuments, etc., may be fractured and the upper part rotated over the lower without being overthrown. Even in Hereford and the surrounding villages, several pinnacles and chimney-stacks were twisted by the earthquake of 1896. The interest of the phenomenon, which has been known, since 1755,[74] is mainly historical, for the endeavour to discover its cause was the origin of Mallet's views on the dynamics of earthquakes. Partly, also, it lies in the difficulty of finding a satisfactory explanation, or rather in deciding which of three or four possible explanations is the true one in any particular case.
The Indian earthquake offered exceptional opportunities for studying the phenomenon in the large number of examples observed and the variety of objects rotated. None could be more striking than the twisted monument to George Inglis, represented in outline in Fig. 74. Chhatak, where this is situated, lies close to the southern boundary of the epicentral area. The monument is an obelisk, built of broad flat bricks or tiles on a base of 12 feet square, and originally more than 60 feet high. It was split by the earthquake into four portions. The two upper, about six and nine feet long, were thrown down; while the third, 22 feet high, remains standing, but is twisted through an angle of 30° with respect to the lowest part, which is unmoved. The upper of these two parts had evidently rocked on the lower, as the corners and edges were splintered, and below the fracture a slice of masonry about 15 inches thick, which was not bonded into the main mass, was split off by the pressure on its upper end. The plan of the parts still standing is shown in the lower part of Fig. 74.
The possible explanations of the phenomenon are at least three in number. According to the first, which was given by Mallet in 1846, the adhesion of the twisted portion to its base is not uniform, and the resultant resistance to motion is not in the same vertical plane as the wave-movement.[75] Some years later, Mallet offered another explanation. The body, he imagined, might be tilted on one edge by the earthquake, and, while still rocking, a second shock oblique to the first might twist it about that edge.[76] In 1880, Professor T. Gray suggested that the column might be tilted on one corner and then twisted round it by later vibrations of the same shock.[77]
None of these theories, Mr. Oldham argues, can give by itself a complete explanation of the phenomena observed in the central district of the Indian earthquake; and he therefore favours an extension of the second theory, which, though first proposed in 1882,[78] was thought out independently and in greater detail by himself. When the focus is of considerable dimensions, the shock at neighbouring places is constantly varying in direction, owing to the arrival of vibrations from different parts of the focus. Thus, instead of the two separate shocks required by Mallet's second explanation, we have a number of closely successive impulses frequently changing in direction and giving rise to what is known in the South of Europe as a vorticose shock. And, instead of a single twist of the pillars about one centre only, we have a series of small twists round a number of different centres, accompanied in consequence by a much smaller displacement of the centre of gravity than would have occurred had the same rotation been accomplished in one operation.
The theory, it will be seen, accounts for the twisting of the pillar without overthrow, and for the splintering of the edges during the rocking of the column. It explains why in any district a number of similarly placed objects are generally twisted in the same direction. Moreover, a low column rocks to and fro more rapidly than a tall one similar in form and position, so that, at the instant when a later impulse comes from a different direction, two such columns might happen to be tilted on opposite edges, and would then be twisted in opposite directions. In certain cases, then, as occurred at several places during the Indian earthquake, an object may rotate in one direction, while others, similar in every respect but size, may be twisted in the opposite direction.