229.On the morning of the 31st December 1881 an earthquake occurred in the Bay of Bengal, which operated with considerable violence in the neighbourhood of the Andaman and Nicobar Islands, and with more or less violence along the entire length of the west coast of the Bay, from Ceylon to Calcutta, and was also felt, though comparatively slightly, at various points on the east coast. In addition to the ordinary shocks produced by the waves of force acting through the ground, the surface of the ocean was greatly disturbed, and waves were formed which continued to roll against the coast lines for several hours after the cessation of the earth-waves, which lasted for only a few seconds. The clerk in charge of the tidal observatory at Port Blair reported a great disturbance of the surface of the sea to have taken place there, which had violently agitated the pencil of the self-registering tide gauge, causing it to oscillate in the course of a few minutes through spaces nearly equal to the entire normal semi-diurnal oscillation, and after a time to tear the paper of the diagram. This had alarmed him so much that he stopped the clock and did not restart it for some hours, when there was less agitation of the sea-surface. He then found by the diagram that the earthquake waves were still.

*Major Rogers reports:-"Mr. Belcham inspected the Indian Tidal Station and worked in all respects satisfactorily.

"Mr. Connor had charge of the Computing Branch, and to his zeal and hard work the very successful outturn of computations is greatly due."

existing, and were following one another with great regularity; and they continued to do so for about twenty­five hours after the first shock of the earthquake was felt, when they died away. The diagrams at all the other tidal stations, for the same day, were then examined, and evidence of a succession of ocean-waves caused by the great earth-wave was unmistakeable at all the stations on the west coast of the Bay and at Dublat station-at the south end of Saugor Island-as well as at Port Blair. There was evidence of slight disturbance at Diamond Harbour, 38 miles up the Hooghly beyond Saugor Island; but there appeared to have been no disturbance whatever either of river-surface at Rangoon and Moulmein, or of ocean-surface at Amherst, and these are the only points on or near the east coast of the Bay at which tidal registrations were being taken.

230. Diagrams of the disturbed tidal curves, reduced from the original records, are here given to indicate what actually took place at each spot and at the same moment of time. For the latter purpose, all the hour lines of the diagrams have reference to local mean time at Port Blair. The curves from midnight of the 30th December up to the times when the sea-waves began to reach each station­ which fall between 8 A.M. at Port Blair and 1 P.M. at Dublat, and possibly was as late as 3 P.M. at Diamond Harbour-are normal in every instance; and thus by comparing them with the curves for the remainder of the twenty-four hours, the influence of the earthquake in disturbing the normal tides is readily seen. For Port Blair and Negapatam the normal curves are drawn below the actual curves. At the former place the diagram was torn by the pencil, and the record is not continuous; at Negapatam the curve from midnight up to the commencement of the sea­waves is vibratory, and not firm, as at all the other stations; but there the curves are normally vibratory, probably because the piping, connecting the well of the gauge with the sea, ends in shallow water and has not been carried out far enough into the sea. The longitudes of the several tidal stations west of Port Blair, in time, are given in the margin.

231. Both the officers in charge of the tidal operations, first Major Hill and afterwards Major Rogers, have taken much pains to ascertain an the facts of the primary 'Great earth-wave' and the subsequent 'Sea­waves.' It so happened that at the time of the occurrence of the earth-wave Major Rogers was measuring angles with one of the great theodolites of this Survey at a station on the Island of Kisseraing, below Tenasserim, on the east coast of the Bay, as a part of the operations which are described in paragraph 34 of this report. He writes that he "saw the earthquake before feeling it," as he was at the moment observing a signal-distant some 15 miles­which appeared to rise and fall in the field of the telescope. On looking at the levels of his instrument, he found that they were violently agitated. He immediately recorded the time at which the phenomenon occurred. Subsequently he ascertained that the earthquake had been felt, at almost the same moment, at Madras and False Point, on the opposite coast. Thus then Major Rogers, assuming the great earth-wave to have travelled with equal velocity in all directions from the origin or centre of impulse, considers that the origin must have been situated at some point in the Bay nearly equidistant from Madras, False Point, and Kisseraing, not in the centre of the triangle joining the three places, but more to the south, towards the line joining Port Blair and Negapatam, which was probably the line of greatest disturbance, as at, those places the sea-waves were greatest.

It is remarkable that there should be no indication of any sea-wave at either of the tidal stations at Rangoon, Elephant Point, Moulmein or Amherst. This may be due to the circumstance that the belt of islands and shoals which extends from Cape Negrais down to the Island of Sumatra forms a barrier to waves issuing from an origin near the centre of the Bay; the sea-waves were propelled with great violence against these islands on all sides and over the surrounding shallows, but they seem to have died away very rapidly in the deep sea beyond. Moreover, the great earth-wave must have operated with far greater force towards the west than towards the east of the centre of impulse; for violent shocks were felt all along the west coast of the Bay, and to a considerable distance inland, whereas on the east coast the shocks were very slight and barely perceptible.

232. The accompanying Chart of the Bay of Bengal shows the positions of all the tidal stations on both coasts, the trigonometrical station at Kisseraing, and Major Rogers' assumed centre of impulse. It also gives the values of all the soundings in the Bay, which are believed to have yet been taken. Major Rogers' report is given in extenso in the appendix, and will be found to contain much additional matter of interest, including estimates of the respective velocities of the earth-wave and the primary sea-waves. It is believed that so full an account and such precise details of the phenomena of an earthquake have rarely been acquired hitherto. That they have been obtained in the present instance is mainly due to the existence of the many tidal stations, which have been established on Indian coasts.

Memorandum by Major M. W. Rogers, R.E., Deputy Superintendent, 4th grade, on the earthquake of the 31st December 1881 and the great sea-waves resulting therefrom, as shown on the diagrams of the tidal observatories in the Bay of Bengal.

PORT BLAIR mean time is used throughout. Latitude 11°40'30" N., longitude 92°45' E.

The tide-wave can be traced clearly on the diagrams at seven tidal stations, viz. Port Blair, Paumben, Madras, Negapatam, Vizagapatam, False Point, Dublat, and may be suspected on an eighth, viz. Diamond Harbour, on the Hooghly.

At Port Blair, in the Andaman Islands, the first indication of the shock is at 7h. 42m. A.M., and this, I am inclined to think, is due to the earth-wave, or rather to the forced sea-wave, which is formed when the earth-wave gets into shallow water; for the tidal curve goes on undisturbed for some 30 minutes afterwards, and it is not until 8h. 10m. A.M that the first wave is recorded, followed by others in succession at about 15 minutes' interval, with a height of about 3 feet from crest to hollow. The diagram is unfortunately incomplete, for the pencil of the gauge, in its violent oscillations, caught in and tore the paper of the diagram, and the clerk, being frightened, stopped the driving clock, which was not started again until 1 P.M. There is evidence that the waves continued to follow one another with great regularity until about 3 P.M., when they became of a much smaller size, but are traceable until 9 P.M. Small shocks were felt on Ross Island all that day, and the violence of the great shock, which damaged the barracks and did other injury, seems to indicate that the centre of impulse could not have been far from the Andamans.

At Madras there is a trustworthy time for the advent of the earth-wave; it is obtained from the Telegraph Office, where the shock affected the recording instruments. It occurred at 7h. 56m. A.M (7h. 5m. 45s. Madras time), whilst the great sea-wave reached at l0h. 10m. A.M., and continued until 7 PM., with intervals of about an hour from crest to crest, and the influence of the disturbance can be traced until 10 P.M.

At Negapatam the first and largest wave came in at l0h. 10m. A.M., with a height of nearly four feet from crest to hollow, and it was succeeded by a series at about half-hour intervals, which continued until midnight. Judging from the diagrams, the sea at this port seems to have been more affected by the earthquake than at any other.

At Paumben the first wave was registered at 11h. 35m. A.M., and was followed until midnight by a succession of waves with about two hours' interval between them.

At Vizagapatam the first wave was recorded at 10h. 48m. A.M., and from that time there was a succession of small waves at irregular intervals until past midnight.

At False Point the diagram shows the passage of the earth-wave or forced sea-wave at 7h. 54m. A.M. The pencil seems to have been moved rapidly up and down a small quantity for some minutes, and the clerk notes that the building was shaken by an earthquake. The sea-wave here is hardly indicated. Its first appearance is at 11h. 12m. A.M., and there is a second one at 1 P.M.

At Dublat the wave appears to have arrived at 1 P.M., and a second, one hour afterwards; there is also an indication of a third at 3 P.M.

At Diamond Harbour the indications of the wave are untrustworthy, and very slight. If felt at all, it was at about 3 P.M.

Looking over the data at our disposal, I find that the shock, i.e. the earth-wave, was recorded as felt at Madras, Coconada, Vizagapatam, Gopalpur, False Point, Calcutta. Port Blair, and Kisseraing, an island in the Mergui Archipelago in latitude 11°39' N., longitude 90°31' E., and also on board a ship, The Commonwealth, in latitude 5°55' N., longitude 92°49' E.

Of these the times of the shock at Madras, False Point, and Kisseraing were probably recorded correctly within a minute. Madras was recorded in the Telegraph Office, and given to the nearest second; False Point was recorded on the tidal diagram, and also by Mr. Rendell, of the Survey, who was leveling a few miles from False Point, and whose recorded notice of the time at which he felt the shock agrees to the minute with the tide gauge record.

At Kisseraing I was observing at the trigonometrical station there with a 24" theodolite, and saw the earthquake before feeling it; the heliotrope (distant some 15 miles) to which 1 was observing appearing to rise and fall in the field of the telescope, and the levels of the instrument being violently agitated. The motion was, to my feeling, barely perceptible, but the recorder and other men with me said that they felt it distinctly. It was not, however, felt by several of the officers of the Indian Marine, who were on the island that morning, thus proving that it was not a severe shock, though plainly noticeable by instrumental means.

At Madras, False Point, and Kisseraing, the earth­wave was felt at about the same Minute-7h. 55m. A.M. On the hypothesis that the strata between them and the centre of impulse is homogeneous, this centre was equidistant from them, and would be at a spot in the Bay of Bengal in latitude 11°55' N. and longitude 89°33' E.

There is no reliable evidence on the subject of the velocity of earth-waves; it varies with the nature of the strata through which it passes and the violence of the initial shock, and also on the depth of the locus of the centre of impulse.

If we assume that the centre of impluse in this case was at the point mentioned, it will be found that it, and Port Blair and Kisseraing, are almost in a straight line. The distance from Port Blair to Kisseraing is 400 miles; and if we assume that the mark on the diagram at the former place at 7h. 42m. was due to the earth-wave, it took 13 minutes to travel 400 miles, which gives a rate of 30 miles per minute-a velocity which I find mentioned in books on the subject as probable under favourable circumstances. With this velocity the central shock should have taken place at 7h. 35m, the distance to Madras, &c., being a little over 600 miles. The distance from this assumed centre of impulse to Port Blair is 218 miles, which would take seven minutes in transit and cause the shock to be felt there at 7h. 42m.

This fixing of the locality of the centre is of course merely hypothetical: the whole of the region is volcanic. Narcondam and Barren Island, to the east of the Andamans, are volcanoes, the latter having been in eruption as late as 1792; the only thing certain is that the centre must have been not far from Port Blair and Car­ Nicobar, and about equidistant from the whole of the east coast of the Bay of Bengal, and also it must have been subaqueous in order to have caused such distinct tidal waves.

All the times of the earth-wave reaching places on the west side of the Bay agree very fairly; but in all the cases except those mentioned, the times are not likely to be sufficiently accurate to aid in the investigation.

The force of the earthquake was great at Port Blair, where it did damage to the barracks, &c., and at the Island of Car-Nicobar it was felt severely, the huts of the natives and many of their palm-trees being thrown down.

Several slight shocks were felt at Port Blair on the same and the two succeeding days, and The Commonwealth, which, as mentioned, felt the shock of the 31 st, felt three shocks again on the 1st off the Island of Car-Nicobar. All this points to there having been considerable subterranean disturbances in those regions at that time.

I can find no trustworthy indication of the direction of the motion as felt at the various places. At Madras there are three estimations of it: one north to south, and two others north-east to south-west; whilst the clerk of the tidal observatory says that there were two shocks-the first north to south and the second east to west. Mr. Rendell, at False Point, states that the direction appeared to him to be from north-west to south-east whilst a person at Calcutta says that it appeared to go from west to east. At Kisseraing Island the motion was so slight that I could not decide on any direction. My first Impression was that it came from the west, but after careful consideration. I could not decide sufficiently satisfactorily to place it on record.

The great tide wave, of which we have full evidence on the tidal diagrams, was felt, as was to be expected, a considerable time after the shock, varying with the distance and other causes, such as wind and its velocity of translation, which again varies with the depth of the water at any given point.

The wave reached Port Blair first at 8h. 10m., or, if we assume the foregeing idea of the locality of the centre of disturbance and time of the original shock, in 35 minutes, with an average rate of 6.2 miles per minute. It reached Madras and Negapatam at l0h. 10m., two hours later than Port Blair. These places are 614 and 640 miles from the assumed centre, and would give a velocity of 4 miles per minute.

At Paumben the first wave came in at 11h. 35m., or more than one hour later than Negapatam; but owing to the intervening land and straits, I do not think any estimate of velocity can be made. At Vizagapatam the wave reached at l0h. 48m., about 40 minutes later than Madras, giving a velocity of 2.9 miles per minute. At False Point the wave reached at 11h. 12m.,or 24 minutes later than at Vizagapatam, giving the same velocity of 2.9 miles per minute. The wave reached Dublat at 1 P.M., giving a velocity of a little over two miles per minute.

The direction of the wind all day was N.N.E., which would tend to reduce the velocity of the wave on its road to the northern ports.