[134] In Table 8 the first column indicates the region; the second, the dates; and the third, the number of shocks. The fourth column gives the month in which the annual maximum occurs when the crude figures are smoothed by the use of overlapping six-monthly means. In other words, the average for each successive six months has been placed in the middle of the period. Thus the average of January to June, inclusive, is placed between March and April, that for February to July between April and May, and so on. This method eliminates the minor fluctuations and also all periodicities having a duration of less than a year. If there were no annual periodicity the smoothing would result in practically the same figure for each month. The column marked "Amplitude" gives the range from the highest month to the lowest divided by the number of earthquakes and then corrected according to Schuster's method which is well known to mathematicians, but which is so confusing to the layman that it will not be described. Next, in the column marked "Expected Amplitude," we have the amplitude that would be expected if a series of numbers corresponding to the earthquake numbers and having a similar range were arranged in accidental order throughout the year. This also is calculated by Schuster's method in which the expected amplitude is equal to the square root of "pi" divided by the number of shocks. When the actual amplitude is four or more times the expected amplitude, the probability that there is a real periodicity in the observed phenomena becomes so great that we may regard it as practically certain. If there is no periodicity the two are equal. The last column gives the number of times by which the actual exceeds the expected amplitude, and thus is a measure of the probability that earthquakes vary systematically in a period of a year.
[135] N. F. Drake: Destructive Earthquakes in China; Bull. Seism. Soc. Am., Vol. 2, 1912, pp. 40-91, 124-133.
[136] The only other explanation that seems to have any standing is the psychological hypothesis of Montessus de Ballore as given in Les Tremblements de Terre. He attributes the apparent seasonal variation in earthquakes to the fact that in winter people are within doors, and hence notice movements of the earth much more than in summer when they are out of doors. There is a similar difference between people's habits in high latitudes and low. Undoubtedly this does have a marked effect upon the degree to which minor earthquake shocks are noticed. Nevertheless, de Ballore's contention, as well as any other psychological explanation, is completely upset by two facts: First, instrumental records show the same seasonal distribution as do records based on direct observation, and instruments certainly are not influenced by the seasons. Second, in some places, notably China, as Drake has shown, the summer rather than the winter is very decidedly the time when earthquakes are most frequent.
[137] A comparison of tropical hurricanes with earthquakes is interesting. Taking all the hurricanes recorded in August, September, and October, from 1880 to 1899, and the corresponding earthquakes in Milne's catalogue, the correlation coefficient between hurricanes and earthquakes is +0.236, with a probable error of ±0.082, the month being used as the unit. This is not a large correlation, yet when it is remembered that the hurricanes represent only a small part of the atmospheric disturbances in any given month, it suggests that with fuller data the correlation might be large.
[138] Ellsworth Huntington: The Geographic Work of Dr. M. A. Veeder; Geog. Rev., Vol. 3, March and April, 1917, Nos. 3 and 4.
[139] Frank Schlesinger: Variations of Latitude; Their Bearing upon Our Knowledge of the Interior of the Earth; Proc. Am. Phil. Soc., Vol. 54, 1915, pp. 351-358. Also Smithsonian Report for 1916, pp. 248-254.
[140] Harold Jeffreys: Causes Contributory to the Annual Variations of Latitude; Monthly Notices, Royal Astronomical Soc., Vol. 76, 1916, pp. 499-525.
[141] John Milne: British Association Reports for 1903 and 1906.
[142] C. G. Knott: The Physics of Earthquake Phenomena, Oxford, 1908.
[143] A. C. Lawson: The Mobility of the Coast Ranges of California; Univ. of Calif. Pub., Geology, Vol. 12, No. 7, pp. 431-473.