266 T- Tkrada and T. Matuzawa 



Some sympathetic or contagions interrelation between the activit- 

 ies of distant earthquake zones has also been suspected by Omori^^^ 

 who cited many examples of Japanese destructive earthquak(S which 

 occurred within a short time interval after remarkable ones in other 

 parts of the world. He remarked the alternation of activities in the 

 submarine epicentral zone off the coast of Sanriku and that off the 

 southern part of the Pacific coast. He"^^^ also pointed out a tendency 

 of sympathetic occurrences of volcanic eruptions and great earthquakes. 

 On the occasion of the recent Tazima Earthquake, K. Suyehiro^"'^ 

 noticed the remarkable fact that the strong earthquakes in the Japan 

 Sea coast of San'indo occur mostly within a few years after the con- 

 spicuous destructive shocks in the Pacific zone. These facts, which 

 require of course further verification by ampler data, seem to afford 

 us valual)le material for the geophysical investigation of the actual 

 mechanisms of earthquakes. 



(2) Space Bistrihution of Earthquakes. 



(a) Earthquake zones. Omori^''^ and Imamura^'^^, during the long 

 years devoted to their seismic investigations, succeeded in sorting out, 

 from the chaos of abundant materials, complete sets of the principal 

 epicentral zones in Japan. Among these zones, the most conspicuous 

 are those two running parallel to the arc of the islands, one off the 

 entire Pacific coast and another along the Ja})an Sea coast. Other 

 groups of zones are those running across the arc with a more or less 

 definite angle of inclination to the axial line. The Sinanogawa 

 zone, the triangular zones surrounding Lake Biwa, a zone across 

 Kyusyu probably extending to the Inland Sea with a trend parallel 

 to the Pacific zone, the Formosan zones, etc., may be cited as the 

 most conspicuous ones. 



A certain statistical law of occurrence on alternative zones, or on 

 the different parts of the same zone, has been pointed out by Omori 

 on many occasions. Though such a law may j)robably exist, it seems 

 that a greater sufficiency of data must be accumulated, and the method 



(1) Ho., 57 (1906). 



(2) Bull., 2, No. 2 (1908.'. 



(3) A Eeport read before the E.I.C. ; not yet published. K. Suda also remarked 

 this fact, Special Keport on tlie Tazima Earthquake (1925), Imp. Mar. Obs. 



(4) Ho., 49 (1905); 54 (190G) ; 68B (1910); 88C (1920); 96 (1922); Bull., 1, No. 

 2 and 3 (1907); 2, No. 2 (1908); T.S.B.K., 4 (1907), 126; 4 (1908), 288. 



(5) Ho., 53 (1906); 70 (1910); 77 (1913); 82 (1915); 92 (1920); 95 (1922). 



