1 66 ^- Imamura 



It is, how(;ver, quite different in tlie j^resent case. Not only was the 

 seismic focal area very wide, but another destructive earthquake 

 belonging to the same seismic zone but originating at a far distant 

 position took place on the next day and thenceforth it became neces- 

 sary to distinguish to which system each after-shock belonged (PI. 

 XXIII.). Consequently I have divided the area of the origin into the 

 following four different localities : — 



A. The area including the Bay and the Province of Sagami 

 as the main part. 



B. The Boso Peninsula and the vicinity of its S. E. coast. 



C. The drainage district of the Eiver Tone. 



D. Tokyo and its environs. 



Such a classification is based upon the following reasons : — 



(1). The destructive shocks, one on Sept. 1 and the otlier 

 Sept. 2 at llh 47m were of almost equal size but quite 

 different in their origins. (PI. XIII, XVI B.) 



(2). The geographical distributions of the after-shocks were 

 quite distinct in the two systems of Boso and Sagami. 



(3). The time distributions of the principal after-shocks 

 were quite distinct in the two systems. (Tables V — VII) 



(4). The earthquake which originated in the drainage area 

 of the River Tone is not to be treated as a pure after-shock 

 in the strict sense, and I am treating it so in the wider 

 sense only for the sake of convenience. 



(5). The geographical distribution of the earthquakes which 

 have originated in the vicinity of Tokyo during the years 

 1914 — 1922 coincides to a large extent with that of the 

 present after-shocks. This coincidence niay perhaps be due 

 to the geological construction of the stage. 



(6). Thousands of local shocks mostly accompanied by 

 sounds and felt over areas not greater than 100 sq. km. 

 were registered in different districts ; shocks of this category 

 observed at Tokyo include only a part so that they must 

 not be treated as of equal weight with ones belonging to 

 other categories. 



Table V shows observations made in Tokyo of the great earth- 

 quakes and their principal after-shocks. Of these, nos. 2, 4, 7 and 8 

 were semi-destructive in Kanagawa Prefecture, while n(»s. 12 and 14 

 were equally so in the southeastern part of the Boso Peninsula (PI. XVI 



