Diurnal Periodicity of Earthquakes in Japan. 909 



known as the method of overlapping means, which gives 

 results of sufficient accuracy, considering the nature of the 

 materials *. 



Annual Periodicity. 



In Table L, the resnlts, with certain stated exceptions, 

 refer to the whole country; the maximum epoch occurs 

 about the end of the month given ; for the semi-annual 

 period ; the earlier of the two epochs is given, the second of 

 course being six months later ; in estimating the ampli- 

 tude, the average monthly number of earthquakes in each 

 case is taken as unity. The amplitude, as Prof. Schuster 

 has shown, should exceed the value of ^/(7r/»), where n 

 is the number of earthquakes f, unless the epochs agree in 

 corresponding records. 



In Table II., similar results are given for the earthquakes 

 recorded at special stations J. 



Tables I. and IT. lead to the following results : — 



(i.) The maximum epoch of the annual period for the 

 whole country occurs from December to March for strong 

 earthquakes, and in September-October for slight earth- 

 quakes. In the destructive and slight earthquakes recorded 



Milne, J. (!•.). " A Catalogue of destructive Earthquakes, A.D. 7- 

 1899." Brit. Assoc. Report, 1911, pp. 649-740. 



Omori, F. (1). "On the After-shocks of Earthquakes." Journ. Coll. 

 Sci., Imp. Univ., Japan, vol. vii. pp. 111-200 (1894). 



Omori, F. (2). " Notes on the Earthquake Investigation Committee 

 Catalogue of Japanese Earthquakes." Journ. Coll. Sci., Imp. Univ. 

 Japan, vol. xi. pp. 389-437 (1899). 



Omori, F. (3). " Annual and Diurnal Variations of Seismic Fre- 

 quency in Japan." Publ. Earthq. In v. Com. No. 8, pp. 1-94, (1902). 



Omori, F. (4). Note on the Annual Variation of Seismic Frequency 

 in Tokyo and Kyoto." Bull. Earthq. Inv. Com. vol. ii. pp. 17-20 (1908). 



Omori, F. (5). " Note on the Annual Variation of the Height of 

 Level of Lake Biwa." Ibid., pp. 51-57. 



Omori, F. (6). "List of the stronger Japan Earthquakes, 1902- 

 1907." Ibid., pp. 58-88. 



* Phil. Trans. 1893 A, pp. 1108-1112. See also Boll. Soc. Sism. Ital. 

 vol. iv. pp. 89-100 (1898). In the present paper, I have not taken 

 account of the different lengths of the months, such refinement being 

 unnecessary. 



t A. Schuster, Koy. Soc. Proc. vol. lxi. pp. 455-465 (1897). 



+ The records are instrumental at Tokyo, Ilikone. Sapporo, Nagano 

 and Oita; non-instrumental at Numazu, Hamamatsu, and Kyoto; partly 

 instrumental at the other stations, the seismographic record beginning 

 with the year 1888 at Wakayama, 1891 at [shinomaki, 1891 at Niigata, 

 1895 at Hakodate, and during the omitted year in the remaining records. 



Phil. Mag. S. 6. Vol. 41. No. 246. June 1921. 3 O 



