A Study of Recent Earthquakes

by Charles Davison

Of the numerous time-records collected by Mr. Oldham, the best are those which were obtained from a few self-recording instruments, from the more busy telegraph offices, from the larger railway stations, and in some cases from private individuals. All records were in the first place subjected to a rigid process of selection; a large number were rejected on various grounds, and those only were retained which bore internal evidence of accuracy, due either to the conditions of the reporter's occupation or to the care taken by him to ensure exactness. To guard against any unconscious bias in making the selection, this process was carried out before the distances were calculated, and even before the position of the epicentral area was known.

The boundary of this area is shown by the continuous line A in Fig. 68. Its greatest length being about 200 miles from east to west, it is necessary in the first place to fix upon an equivalent centre within it, which may be regarded for this special purpose as the point of departure of the earth-waves. The more natural course perhaps would be to assume this point to coincide with the centre of the area. But, as the rate at which the initial movement spread over that area would probably differ little from the velocity of the earth-wave, and as all the time-stations lie towards the west, Mr. Oldham regards a point near the western boundary of the area (in lat. 25° 45' N. and long. 90° 15' E.) as a sufficiently exact approximation to the position of the equivalent centre.

The nearest place at which good time-observations were made is Calcutta, distant 255.5 miles from the assumed centre. One is indicated on the recording tide-gauge by a sudden rise of the water, while the others were obtained from the central telegraph office, the terminal railway stations, and from two careful readings by interested observers. They vary from 4h. 27m. 0s. to 4h. 28m. 37s. P.M., all being liable to an error of half-a-minute. The arithmetic mean for the beginning of the shock is 4h. 27m. 49s., and this is probably as accurate an estimate as the conditions allow.[70]

Bombay lies outside the disturbed area, 1208.3 miles from the equivalent centre; and, for the time of arrival in that city, we have to depend on the records of the barograph and the three magnetographs. The horizontal force magnet was set in motion two and a half minutes before the others, no doubt by the advance tremors. The times given by the barograph and the vertical force-instrument differ by only one minute, and the best result seems to be that obtained by taking their mean—namely, 4h. 35m. 43s., which is probably accurate to within a minute.

Assuming, then, that the time-interval between Calcutta and Bombay does not err by more than half-a-minute, it follows that the intervening velocity must lie between 2.8 and 3.2 kilometres per second, its probable value being 3 kilometres, or 2 miles, per second.

The remaining records, which are of less value than those obtained in these cities, fall into two groups, the first consisting of a number of stations along a line running north and south between Calcutta and Darjiling or within a hundred miles on either side of the same, and the second a long series of stations crossing Northern India in a nearly westerly direction. The observations made at the Burmese stations were unfortunately affected by an error arising from the retardation of the Madras time-signals through frequent repetition along the line.

Fig. 70.—Time-curve of Indian earthquake. (Oldham.)[ToList]

Individually, these records are not exact enough to be used in determining the velocity, but they may be employed collectively for the construction of the time-curve in Fig. 70. In this diagram, distances in hundreds of miles from the equivalent centre are represented along the horizontal line, and the time of occurrence in minutes past 4 P.M. along the perpendicular line. The small circles represent the observations at Calcutta and Bombay, the dots those at places lying nearly west of the origin, and the crosses those at places situated to the south or north-west. The continuous curve passes in an average manner through the series of points, and probably does not differ much from the true curve of the time of arrival of the shock at different places. The curve, it will be noticed, is at first concave, and afterwards convex, upwards; indicating that the times required to traverse successive equal distances at first increased, and then decreased. Thus, if the curve is an accurate representation of the facts, it would follow that the surface-velocity was subject to a continual decrease outwards from the centre, until it was a minimum at a distance of about 280 miles, after which it increased.