Fig. 77.—Re-triangulation of Khasi hills. (Oldham.)[ToList]
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 assumption 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, Rangsanobo, 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 meridianal direction.
For a similar reason, the slight general upheaval of the hills indicated by the repeated calculations, must be regarded as doubtful, for it depends on the assumed fixity of the station of Rangsanobo, whereas it is more probable that it was the height of Taramun Tila that remained unchanged. Reducing the calculated heights of all the other stations by six feet (the assumed rise of the latter), it follows that, on the whole, the height of the Khasi hills underwent but little change, except at Mautherrican and Landau Modo, and the secondary stations of Mairang and Kollong Rock, near Maonoi. The apparent elevations of 24, 17, 11, and 15 feet at these places exceed the probable error of the observations; and it is worthy of notice that all four stations lie close to the edge of fault-scarps, while Landau Modo is not far from two of the pools formed by distortion of the surface unaccompanied by faulting.
If, then, the revised triangulation of the Khasi hills has failed to provide absolute measures of the displacements in the epicentral area, it has, nevertheless, proved that important movements, both horizontal and vertical, have taken place.
Distribution of the Structural Changes.—The boundary of the epicentral area, as drawn in Figs. 68 and 75, lays no claim to great accuracy; but its departure from the true line is probably in no place considerable. It must evidently include all the districts where marked structural changes occurred, and must therefore extend east of Maophlang and west of Tura. Towards the north, these changes have been traced to the foot of the Garo hills, and there is some, though not very certain, evidence of alterations of level along the course of the Brahmaputra. The very large number of after-shocks recorded at Borpeta and Bijni also points to an extension of the epicentral area beyond these places. To the east, the course of the boundary becomes doubtful, but it must pass close to Gauhati and east of Shillong, and probably ends a short distance beyond Jaintiapur. The southern boundary must coincide nearly with the north edge of the alluvial plains of Sylhet, for there is no evidence of its intrusion into the plains. On the west side, the epicentral area includes the Garo hills and part of the alluvial plain to the west; and, from the large number of after-shocks felt at Rangpur and Kaunia, and the great violence of the shock at the former, we may infer that both places lie within the boundary-line. If, then, there is no great error in the mapping of this line, it follows that the epicentre was about 200 miles long from east to west, not less than 50, and possibly as much as 100, miles in maximum width, and contained an area of at least 6000 square miles.
Near the boundary, the permanent displacements must have been comparatively small; but they were certainly marked in the northern part of the Assam hills for a distance of 100 miles from east to west. At the limits of the latter area, as Mr. Oldham remarks, "the evidence points to the changes being of the nature of long, low rolls, the change of slope being insufficient to cause any appreciable change in the drainage channels. Then comes a zone in which the surface changes are more abrupt, the slopes of the stream beds have been altered so as to cause conspicuous changes in the nature of the streams, but any fracture or faulting which may have taken place has died out before the surface was reached. And north of this, close to the edge of the hills, the rocks have been fractured and faulted right up to the surface."