A Study of Recent Earthquakes Part 22
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 Shillong, 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 Shillong 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 Shillong) 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 Shillong 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 Shillong), 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, Shillong, 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.
At its southern end, as mapped in Fig. 76, there is no perceptible throw at the surface, but various marks of violence are manifested in the fissuring of the hillside and the snapping of small trees. About a quarter of a mile from this point, the fault crosses a tributary stream, where the throw amounts to two feet, and the same distance farther on it meets the Chedrang river, the bed of which it crosses many times in its short course.
[Ill.u.s.tration: FIG. 76.--Plan of Chedrang fault. (_Oldham._)]
Mr. Oldham describes the fault in detail, as observed by him in February 1898. Here, it will be sufficient to refer to its more important features, and to its effects on the superficial drainage of the district. At the spot marked _a_ (Fig. 76) the river, after running on the west or down-throw side of the fault for nearly half a mile, meets the scarp, and is ponded back by it for about a quarter of a mile upstream. For the next half-mile, the river keeps to the upthrow side of the fault, the scarp of which blocks the tributary streams from the west, forming a number of small pools. At the last of these, the total throw is not less than 25 feet. A little farther on, the fault crosses the Chedrang and causes the waterfall at _b_, the height of which, owing to the fall of dislodged fragments, does not exceed nine feet. The fault then runs along the old and now dry bed of the river, while the stream itself flows in a depression on the down-throw side. About a quarter of a mile below the waterfall, the fault crosses the river, and soon after enters a large sheet of water at _c_, half a mile long, from 300 to 400 yards wide, and with a maximum depth of 18 feet. At first, the pool spreads on both sides of the fault, but the inequalities due to the scarp are evidenced by soundings. At the point where the fault leaves the pool, its throw is reduced to nothing, and it is just here that the water attains its greatest depth. To the north the throw increases rather rapidly, to 25 feet in a quarter of a mile. But the peculiarity of this pool is that it is not, like the others mentioned above, dammed back by the fault-scarp. There is no barrier at its northern end, where the river escapes, except that formed by the gradually increasing throw of the fault. The pool is simply due to the reversal of the natural slope of the river-bed, caused by the formation of a roll or undulation in the ground on the upthrow side of the fault. Its recent origin is evident from the number of dead trees and bamboo clumps still standing in the water.
For a mile after the fault leaves the pool, its throw varies considerably. It rises, as already mentioned, from zero to 25 feet. A little farther on, the fault runs up the side of a spur, the throw increasing to 31 feet; and, in this part, the violence of the shock was shown by the dislodgment of blocks of granite as much as 20 feet in diameter, and by the overthrow or destruction of many trees. After crossing the spur, the fault returns to the neighbourhood of the river, and crosses its bed four times, forming pools (_e_, _g_) or waterfalls (_d_, _f_) according as the scarp occurs on the downstream or upstream side. The throw of the fault then changes considerably within little more than half a mile, from 18 feet to zero and again to 20 feet, the undulation so formed producing a large pool (_h_) entirely on the upthrow side of the fault.
At the point marked _i_ on the map, the river once more crosses the fault; but the bottom of the valley is filled with alluvium, and, instead of a waterfall, a large sandy delta spreads down the stream.
The scarp is, however, readily traced on the east side of the river, a throw of 32 feet being measured. After this, the alluvium becomes of considerable thickness, and the continuation of the fault is marked by a short slope, which tilts over the trees when it traverses forest-land. Leaving the valley of the Chedrang, the fault crosses an open plain, and is followed with some difficulty to the neighbourhood of Jhira, where, owing to the thick bed of alluvium, it forms a gentle roll or undulation of the surface, crossing the main channel of the Krishnai to the north-east of Jhira. On the west side of this barrier a large sheet of water, a mile and a half in length, three-quarters of a mile wide, and 12 feet in depth, gathered over the village of Jhira.
"On the east side of the Jhira lake," says Mr. Oldham, "there is ample evidence of change of level, for part of the dry land was formerly ...
perpetually under water, and at one place the remains of an old irrigation channel can be seen.... At the northern end of the lake the drainage now makes its escape in a broad and shallow sheet of water over what was once high land covered with _sal_ forest."
This is the last marked feature due to the Chedrang fault. Beyond the north of Jhira the throw rapidly diminishes, and perhaps dies out altogether before reaching the low hills lying to the north of that village.
In several ways, this fault-scarp differs from that formed with the j.a.panese earthquake of 1891. Throughout its course the down-throw, wherever it is perceptible, is invariably to the west; in no place could any trace of horizontal shifting be detected; and the plane of the fault, when it traversed rock, is practically vertical.
Whether the scarp was formed by the elevation of the rock to the east of the fault, or by the depression of that to the west, or by both such movements at once, there is no decisive evidence; but there are very good reasons for believing the first alternative to be the true one. The undulations in the ground which gave rise to the large pools at _c_ and _h_ (Fig. 76) occur on the east side of the fault. Also, between the outlet of the lake at Jhira and the point where the Krishnai rejoins its original channel, the gradient of the river approaches that of a mountain stream, although the new bed consists of alluvium, and not of rock. Now, the alluvial plain of this district is raised so slightly above the sea-level that no subsidence great enough to have caused the existing gradient could have occurred without the depressed area being flooded with water. Though some movements may have taken place on the west side of the fault, it seems clear, then, that elevation of the rock on the east side was the predominant, if not the sole, cause of the fault-scarp.
As the Chedrang fault has been described somewhat fully, a brief reference to the rest will be sufficient The only other known scarp of any consequence lies about ten miles to the south of the Chedrang fault, and runs by the village of Samin, with an average course from E. 30 S. to W. 30 N. Its total length does not exceed 2-1/2 miles.
The down-throw is uniformly to the north, and the throw, which amounts to ten feet near its centre, gradually diminishes to zero at either end. Several pools are formed along the course of the fault-scarp by the blocking of small streams.
_The Bordwar Fracture._--In the map of the epicentral area (Fig. 75), this remarkable fracture is represented by a dotted straight line. It is apparently an incipient fault. Though traceable for a distance of about seven miles, at no point is there any decisive evidence of either vertical or horizontal displacement; and, even if some doubtful indications of a change of level should be real, the throw must certainly be less than one foot. Yet, in the immediate neighbourhood of the fracture, the violence of the shock was extreme. "Trees have been overthrown or killed as they stood; a huge ma.s.s of rock, dislodged from near the crest of the hills, has rolled down the slope, scoring the side of the hill. On the opposite side an equally large block has been dislodged, and in its downward course cleared a straight track down the hill; and on the summit a gap has been cleared by the overthrow of trees along the line of fracture." Being only a few inches in width where it has rent the solid rock, the fracture was difficult to follow in many parts of its course. But, through forest-clad land, its track was marked by "a well-defined band of about half a mile broad, in which overturned trees are much more abundant than on either side, and towards the centre of this band the overturned trees are not only more numerous, but many of the smaller ones, up to six inches in diameter, have been snapped across by the violence of the shock."
_Lakes and Pools not due to Faulting._--A few miles to the south of the Chedrang and Samin faults, and also of the Bordwar fracture, occurs a group of lakes or pools, represented on the map of the epicentral area (Fig. 75) by small black ovals. In the gradual increase in depth from either end, they resemble the two large sheets of water along the course of the Chedrang fault (_c_ and _h_, Fig.
76), but they differ from them in having no direct connection with any apparent fault.
One of these pools lies in the valley of the Rongtham river, to the south of the Samin fault. It seemed, at first sight, to be nothing more than an ordinary pool, such as may be seen on any mountain stream. On the bottom, and close to the outlet, however, are coa.r.s.e, partially rounded boulders, exactly resembling those farther down the river; and, as the old bed was followed up, these became coated with a slight deposit of sand and mud, pointing clearly to a change in the conditions under which they were formed. The water gradually deepened, until trees were met standing in the water, but killed by the recent submergence of their roots. The pool is nearly a quarter of a mile long, and its greatest depth (12 feet) occurs near the middle, just where the former stream, with an average depth of about a foot, was crossed by the track from Darangiri. Towards the upper end, the water shallows as gradually as it deepens at the other, and ends in a delta of boulders brought down by the stream above. As no fault could be discovered in the neighbourhood of the pool, it is evident that its formation was due to a bend of the river-bed, the maximum change of level, taking into account the river-slope, being not less than 24 feet.
Similar features characterise the other pools that were examined, some of which are smaller, and others larger, than that described above.
One, higher up the valley of the Rongtham, has a length of about 1-1/2 mile and a maximum depth of 18 feet. Others of the same type, but of smaller size, were observed among the Khasi hills, about fifteen miles south of the Bordwar fissure; and it is probable that many others would have been found in the intermediate district, which Mr. Oldham was unable to visit.
_Changes in the Aspects of the Hills._--There are, again, other facts of considerable interest which point to changes of level over a wide area; the places where they were noticed being indicated by small circles in Fig. 75. For instance, from Maophlang, near Shillong, a road leads to the neighbouring station of Mairang. Before the earthquake, only a short stretch of this road could be seen from the former place, as it rounded a spur about three miles away. Now, a much longer stretch is visible, and it can also be seen pa.s.sing round the next, and previously hidden, spur. In this district the movements seem to have continued with the after-shocks; for, before the earthquake, the crest only of a ridge about a mile and a half to the west was visible; while, after it, a considerable portion could be seen, and much more some months later than immediately after the shock.
Again, from a spot near the southern end of the Chedrang fault, it used to be only just possible to see the Brahmaputra over an intervening hill; whereas, now, the whole width of the river has come into view.
Lastly, at Tura, which is 95 miles west of Maophlang, a battalion of military police were accustomed to signal by heliograph with another station, Rowmari, 15 miles farther to the west. This, formerly, could just be done by means of a ray which grazed a hill between the two places; it can now be done quite easily, and, in addition, a broad stretch of the plains east of the Brahmaputra is visible from the same spot.
_Revision of the Trigonometrical Survey._--The movements described in the preceding pages are of course referred to points which may themselves have been displaced, and only a revision of the trigonometrical survey of the epicentral area and of part of the surrounding district could determine their absolute magnitude. During the cold weather of 1897-98, some of the triangles were re-measured by a member of the trigonometrical survey; but, as the time at his disposal was short, they were confined to the eastern part of the epicentral area, as the focus at that time was supposed to lie under the Khasi hills. The positions of some of these stations are indicated by crosses in Fig. 75; and in Fig. 77 the more important triangles are shown. In the revised work, all tower stations, consisting of brick towers built on alluvium, were omitted, as it could not be a.s.sumed that they had been undisturbed by displacements of the superficial beds.
In re-calculating the lengths of the sides, the side Rangsan.o.bo-Taramun Tila was adopted as the initial base, and the height of Rangsan.o.bo as the initial height; a choice which later experience showed to be unfortunate, for Taramun Tila probably lies just outside, and Rangsan.o.bo within, the epicentral area. Of the 16 sides, whose old and new lengths were compared, only one was found to be apparently unchanged, two were shortened by an inch or two, while the others were all lengthened by amounts varying from one to eight or nine feet, the numbers affixed to the sides in Fig. 77 denoting the calculated increases in feet. a.s.suming the new base-line to be unaltered by the earthquake movements, these changes imply the following displacements of the princ.i.p.al stations:--Thanjinath 6 feet, Mun 4, and Laidera 2, feet to the north; Mopen 5, Dinghei 9, Landau Modo 12, and Umter 11, feet to the north-west; and Mosingi 3, and Mautherrican 5, feet to the west. At the same time, the height of most of the stations was found to be increased with reference to that of Rangsan.o.bo: Mun by 2 feet, Thanjinath and Umter by 3, Mosingi by 4, Taramun Tila and Laidera by 6, Dinghei by 7, Landau Modo by 17, and Mautherrican by 24, feet; while the height of Mopen seems to have been diminished by 4 feet. Thus, at first sight, these calculations appear to indicate "a general elevation and extension of the hills, such as might follow on a bulging upwards of the surface due to the extension of a large ma.s.s of molten matter underground."
[Ill.u.s.tration: FIG. 77.--Re-triangulation of Khasi hills.
(_Oldham._)]
Unfortunately, as Mr. Oldham shows, a very different, and more probable, interpretation may be given of these results; for all the calculated changes are rendered uncertain by the choice of the two stations which form the ends of the new base-line. One at least may have been displaced by the structural movements within the epicentral area; and, moreover, the line joining them runs nearly north and south. As compression in this direction is to be expected, it is probable that this line was shortened; and the a.s.sumption that its length was unchanged would therefore lead to an apparent expansion of all the other sides.
The calculated changes seem to favour this explanation to a great extent. The sides joining Mopen, Rangsan.o.bo, and Thanjinath run nearly east and west, and are apparently lengthened by 4.9 and 3.4 feet respectively; while, of the four sides joining these stations to Mosingi and Mun, lying next to the north, two are nearly or quite unchanged, and the others increased by 2.3 and 3.2 feet. Again, the estimated increase of the Mosingi-Mun line is 4.4 feet; while the four sides joining these stations to the next northerly group are increased by small amounts--namely, 1.2, 2.6,-0.3, and 2.4 feet. Thus, the apparent expansion that should have occurred in these more or less northerly sides is lessened, or roughly compensated, probably by a compression of the whole region in a meridia.n.a.l direction.