A Study of Recent Earthquakes

by Charles Davison

Of the vertical component of the acceleration, not even the roughest numerical estimate can be formed. We know, however, that at Shillong, Gauhati, and indeed throughout the epicentral area, stones were projected upwards, and this is evidence that the vertical component was greater than that of gravity—namely, 32 feet per second per second.

Violent as the shock was at the places just mentioned, it must have been still greater in certain parts of the epicentral area. At Dilma, in the Garo hills, the shock seems to have been strong enough to disable men; and, in the neighbourhood of the faults that will be described in a later section, forest trees were snapped in two. Fortunately, as Mr. Oldham remarks, there were in these districts no towns or populous settlements to feel the full power of the earthquake to destroy.

Anomalies in the above Measurements.—If the movements of the ground followed the law of simple harmonic motion, any two of the four elements (period, amplitude, maximum velocity, and maximum acceleration) would suffice to determine the others (p. 4). Applying the usual formulæ to the quantities obtained at Gauhati—namely, 8 feet per second for the maximum velocity and 12 feet per second per second for the maximum acceleration, it follows that the amplitude would be 5 feet and the period 4 seconds—values, which are evidently inadmissible. Or, taking the maximum vertical component at 32 feet per second per second, the corresponding values would be 2 feet and 1½ seconds, that of the amplitude being still too great. Again, at Rambrai, the maximum velocity was found to exceed 16 feet per second. The other elements are unknown, but, if the amplitude were one foot, Mr. Oldham shows that the maximum acceleration would be 256 feet per second per second; or, taking the amplitude at the impossible amount of two feet, that the maximum acceleration would be 128 feet per second per second.

It follows, therefore, that only part of the high velocities at Rambrai and elsewhere can be due to the elastic waves provoked by the initial disturbances. The remaining portion must be attributed to a bodily displacement of the earth's crust within the epicentral area—a displacement of which the fault-scarps and other distortions of that region furnish ample evidence.

SOUND-PHENOMENA.

In the epicentral area, the sound that accompanied the earthquake was remarkable for its extraordinary loudness. At Shillong, the crash of houses falling within thirty yards was completely drowned by the roar of the earthquake.

The sound was generally compared to distant thunder, the passage of a train or cart, etc.; but, whatever the type may be, it always implies a sound of deep pitch, close to the lower limit of audibility—a continuous rumbling or rattling noise, as a rule gradually becoming louder and then dying away. There was the usual conflict in the evidence of different observers due to the depth of the sound. 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, noticed that the shock was preceded by a loud roar; while others were certain that there was no sound of any kind until the earthquake had become severe.

As in the case of the disturbed area, it is impossible to define the boundary of the region over which the sound was heard. Like the shock, also, it seems to have been observed farther to the west than towards the east. Leaving out of account records that are possibly doubtful, the sound was heard for a distance of 330 miles to the west and south-west, and 290 miles to the east of the epicentral area—that is, allowing for the dimensions of that area, it must have been perceptible over a region measuring not less than 800 miles from east to west.

VELOCITY OF THE EARTH-WAVES.

It is somewhat doubtful whether a more accurate estimate of the velocity is to be obtained from a violent earthquake or from one of moderate intensity. In the former case, the vast distances to which the shock is noticed lessen the effects of errors in the time-determinations, but this advantage is to a great extent compensated by the considerable duration of the shock and the consequent uncertainty whether all observers have timed the same phase of the movement. Also, in the Indian earthquake, there are further sources of error in the variety of standard times employed throughout the country and in the magnitude of the epicentral area.