[57] Professor Omori finds that the mean daily number of earthquakes y during the month x (reckoned from November 1891) may be approximately represented by the equation—
y = 16.9 / (x + 0.397);
or, taking the semi-daily earthquake numbers during the five days between October 29th and November 2nd, 1891, by the equation—
y = 440.7 / (x + 2.314),
where y denotes the number of earthquakes observed during the twelve hours denoted by x, the time being measured from the first half of October 29th. It is interesting to notice that, taking account of the mean annual frequency of earthquakes in ordinary years, the number of shocks observed at Gifu during the two years 1898-99 should, according to the latter formula, be 163; the actual number recorded was 160.
[58] The last violent shock before the end of 1893 occurred on September 7th, 1892, and its effects on the frequency of after-shocks is shown by the daily numbers recorded at Gifu during the first fortnight in September. These are—2, 2, 2, 3, 5, 5, 28 (on September 7th), 8, 8, 5, 4, 3, 2, 4, 3.
[59] The periodicity of after-shocks is discussed in the papers numbered 4, 12, 16, and 17 at the end of this chapter. In these, the existence of diurnal and other periods is clearly established. Professor Omori also shows that the mean daily barometric pressure is subject to fluctuations with maxima occurring on an average every 5½ days, and that earthquakes are least frequent on the days of the barometric maxima and minima, and more frequent in the days immediately preceding and following them.
[60] Of the Japanese earthquakes of 1885-92 originating beneath the land, twenty-six per cent. were accompanied by a recorded sound; but less than one per cent. of those originating beneath the sea and not more than ten miles from the coast.