Part 22 (1/2)
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 gla.s.s 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 s.h.i.+llong, 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 s.h.i.+llong 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 s.h.i.+llong) 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 s.h.i.+llong 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 s.h.i.+llong), 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, s.h.i.+llong, 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 pa.s.s 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 a.s.sign the Indian earthquake to a small cla.s.s 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 j.a.panese 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 a.s.sam 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 ma.s.s. 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.
[Ill.u.s.tration: 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 princ.i.p.al 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.