THE INDIAN EARTHQUAKE OF JUNE 12TH, 1897.
Very different from the shocks of Britain was the earthquake that overwhelmed so large a part of its great dependency on June 12th, 1897—an earthquake which, if it is not without a rival, is certainly one of the most disastrous and most widely-felt of which we possess any record. That it was of the first magnitude was evident at once in Calcutta from the extensive injury to buildings, and its investigation was undertaken without delay by the members of the Geological Survey of India. The four officers who were at the headquarters in Calcutta were despatched to the area of greatest damage, letters and circulars were distributed as widely as possible, a large number of observers were induced to co-operate by keeping records of the after-shocks, and, later on, during the cold weather of 1897-98, Mr. R.D. Oldham, one of the superintendents of the Survey, made a tour through the epicentral district. To him, moreover, fell the much harder task of discussing the very numerous observations collected by himself and others; and the least that can be said of the valuable report prepared by him is that it is worthy of a great subject. Professor Omori also spent several months in studying the earthquake on behalf of the Japanese Government; but the account, which is written in his own language, unfortunately remains a sealed book to western seismologists.
Fig. 68.—Isoseismal Lines of Indian Earthquake. (Oldham.)[ToList]
ISOSEISMAL LINES AND DISTURBED AREA.
In Fig. 68, which shows the area disturbed by the earthquake, Mr. Oldham has drawn two series of curves. In the absence of detailed records of the intensity—records that could not have been obtained from some parts of the disturbed area, and would have been difficult to procure in sufficient number from others—he has represented by the dotted curves a group of isoseismals in the form which he believes they would have assumed had the earth-waves been propagated in a homogeneous medium. The first includes all places, such as Shillong and Goalpara, where the destruction of brick and stone buildings was practically universal; the second, those, like Darjiling, in which damage to buildings was universal and often serious; the third, places, like Calcutta, where the earthquake was strong enough to injure all or nearly all brick buildings. Inside the fourth isoseismal, the shock was strong enough to disturb furniture and loose objects, but not to cause more than slight damage; within the fifth, it was generally noticed; and, beyond this, and as far as the sixth isoseismal, the earthquake was perceived only by a small number of sensitive persons at rest. The approximation of the curves towards the east and south-east, Mr. Oldham believes to be partly real, and not due to imperfect information.
The continuous curves represent more closely the actual variation of intensity. The innermost curve A indicates the probable boundary of the epicentral tract, which is about 200 miles in length and more than 6000 square miles in area. This will be referred to afterwards in greater detail. The next curve B bounds the region within which serious damage to brick houses was common. Its irregular course is closely connected with the geological structure of the country, and is due to the fact, of which we have already met with several examples, that earthquakes are more destructive to houses built on alluvial ground than to those founded on rock. The area included within this curve is not less than 145,000 square miles; and, if we include the parts from which reports were not obtainable, it must amount to about 160,000 square miles.
The curve C represents the boundary of the disturbed area, so far as known, for about one-third of the area lies in regions from which no information was procurable, while another third is inhabited by ignorant and illiterate tribes. But, notwithstanding this, the shock is known to have been felt over an area of at least 1,200,000 square miles. If we include the detached region to the west, near Ahmedabad, the portion of the Bay of Bengal in which the shock would have been felt had the sea been replaced by land, and a large part of Thibet or Western China, from which no reports have come, but in which the shock was certainly sensible, this estimate, great as it is, must be raised to about 1,750,000 square miles.[69]