July 2, 1900.] 



KNOWLEDGE. 



149 



Nature of the Shock. 



'• I was out for a walk at the time," says Mr. F. H. 

 Smith, of the Geological Survey of India, " and was 

 standing on the road which passes the foot of the filter- 

 ing tank of tlie Shillong waterworks, near the school. 

 At 5.15 (according to the ordinai-y Shillong time) a deep 

 rumbling sound, like near thunder, commenced. The 

 rumbling preceded the shock by about two seconds, and 

 the shock reached its maximum violence almost at once. 

 The ground began to rock violently, and in a few 

 seconds it was impossible to stand upright, and I had 

 to sit down suddenly on the road. The shock was of 

 considerable dui'ation, and maintained roughly the same 

 amount of violence from the beginning to the end. It 

 produced a very distinct sensation of searsickness. The 

 earth-movement was exceedingly sudden and violent. 

 The feeling was as if the ground was being violently 

 jerked backwards and forwards vei-y rapidly, every 

 third or fourth jerk being of greater scope than the 

 intermediate ones. The surface of the ground vibrated 

 visibly in eveiy direction, as if it was made of soft 

 jelly; and long cracks appeai-ed at once along the road. 

 The sloping earth-bank round the water tank, which 

 was some 10 feet high, began to shake down, and at 

 one point cracked and opened out bodily. The road 

 is bounded here and there by low banks of eai-th, about 

 2 feet high, and these were all shaken down quite flat. 

 The school building, which was in sight, began to shake 

 at the first shock, and large slabs of plaster fell from 

 the walls at once. A few moments aiterwai-ds the whole 

 building was lying flat, the walls collapsed, and the 

 corrugated iron roof lying bent and broken on the 

 ground. A pink cloud of plaster and dust was seen 

 hanging over every house in Shillong at the end of the 

 shock. "+ 



Many other observers within and near the epicentral 

 district noticed a marked undulation of the ground. 

 According to one at Shillong, the surface of 

 the earth presented " the aspect of a stonn-tossed sea, 

 with this difference that the undulations were infinitely 

 more rapid than any seen at sea." Mr. Oldham thinks 

 that, on an average, the waves were about 30 feet long 

 and one foot in height, though some may have been 

 both shorter and higher. They could be seen following 

 each other at intervals, and tho rate at which they 

 travelled, as one witness states, " though decidedly 

 faster than a man could walk, was not so fast as he could 

 run." 



In the epicentral area there was a considerable vertical 

 component in the motion, for loose stones on the roads 

 were tossed in the air " like peas on a drum." At the same 

 time there was a still more marked horizontal move- 

 ment, the range of which must have been at least 8 or 

 9 inches, and during which people felt as if they were 

 being shaken like a rat by a terrier. As they left 

 the epicentral region, the waves lengthened out, so that, 

 at a distance, the shock no longer consisted of short 

 jerks but became a gentle rocking motion, occasionally 

 giving rise to a sensation of nausea. 



Sound-Phenomena. 

 According to an observer at Shillong, the crash of 

 houses falling within thirty yards was completely 

 drowned by the roar of the earthquake. The sounds 

 are generally described as resembling distant thunder, 

 the passage of a train or cart, etc. There was the usual 

 conflict in the evidence of different observers due to the 



+ I have abridged this account slightly, without iuclicatin<; the 

 passages that are omitted. 



depth of the sound. J In Calcutta, which lies well 

 within the sound-area, some persons asserted that they 

 heard a rumbling noise; others were positive that the 

 only noise was that caused by falling buildings and 

 furniture. Some, again, noticecl that the shock was pre- 

 ceded by a loud roar; while others were certain that 

 there was no sound of any kind until the earthquake 

 had become severe. 



Leaving possibly doubtful records out of account, the 

 sound was heard for a distance of 330 miles to the west 

 and south-west, and 290 miles to the ea.st of the epi- 

 central ai-ea ; that is, allowing for the dimensions of 

 that area, it must have been perceptible over a district 

 measuring not less than 800 miles from east to west. 



Besides these sounds, several observers in different 

 parts of the disturbed area heard after the shock 

 was over three or more loud and short explosive 

 sounds, like the booms of cannons fired a few miles away. 

 Though, as Mr. Oldham remarks, the sounds were 

 evidently connected with the earthquake, they were 

 separated from it by an interval too great for them to 

 be due to the passage of the sound-wave through the 

 air. 



Aftkr-Shocks. 



However scanty the preparation for it may have 

 been, a great earthquake is always followed by an 

 attendant crowd of after-shocks, which, for months or 

 even years, do not altogether cease. Near the centre, 

 they are so numerous as to baffle all inquiry. For 

 several days, it may be, the ground is hardly ever 

 still. At Tura, in the epicentral area of the Indian 

 eai'thquake, several hundred shocks were at first felt 

 evei-y day, and for three or four days a hanging lamp 

 was kept constantly on the swing ; while, at another 

 place within tho same area, the surface of a glass of 

 water standing on a table was for a week in a constant 

 state of tremor. On June 13, tho day after the great 

 earthquake, there were two shocks which would cer- 

 tainly have caused considerable destruction in the 

 central area if any houses had been left standing ; 

 while a third shock, later in the day, was felt as far 

 as Calcutta. 



Velocity of the Earth-Waves. 



Among the minor problems which a great earthquake 

 presents for solution, one of the most important is to 

 determine the velocity with which the earth-waves were 

 propagated along tho surface. The best determina- 

 tions of the time are those which were obtained 

 from a few self-recording instruments, from the 

 more busy telegraph-oifices, from the larger railway 

 stations, and especially from those on the main lines, 

 and in some cases fi'om private individuals. The 

 average of the observations at Calcutta (including that 

 from the tide-gauge) gives I6h. 27m. 49s. (Madras 

 time, which is 5h. 20m. 59.2s. in advance of Greenwich 

 time) for tho beginning of tho shock. Bombay lies 

 outside the disturbed area, but the initial time there, 

 as determined from the diagrams of three magneto- 

 graphs and a barograph, is 16h. 35Jm. These two 

 records are probably the most accurate of the series. 

 In calculating the surface-velocity of the earth-waves, 

 Mr. Oldham assumes that they started from a point 

 in lat. 25° 45' N. and long. 90° 15' E. From this 

 point, Calcutta is 255.5 miles distant, and Bombay 

 1208.3 miles. The average velocity for the intervening 

 distance is therefore 119 miles a minute or 3 km. a 



t See KsowLBDOE, Vol. XXIII., 1900, pp. 83-85. 



