SUNSPOTS AND VOLCANIC AND SEISMIC PHENOMENA. 63 



Perhaps we may go a little further than Dr. Koto and 

 divide those earthquakes which he calls ' tectonic ' into (a) 

 those whose centre lie in areas undergoing Assuring through 

 cooling and contraction ; and, (6) those which originate in 

 areas undergoing expansion, folding and heating by rise of 

 isogeotherms ; that is areas in which the fracturing is due 

 to strain caused by folding more than the elasticity of the 

 strata can stand. 



In answer to this question a paper by Dr. P. Omori comes 

 to our aid. 1 Dr. Omori shows that Japan is divisable into 

 two distinct earthquake regions, in the one of which (A) 

 the earthquake frequency is greatest in winter, in the 

 other (B), it is greatest in summer. The (A) region is 

 shaken mostly by earthquakes of an inland origin, and the 

 (B) region chiefly by earthquakes of a submarine origin. 

 The B region comprises the present volcanic chain, or 

 fissure line, passing from the Kurile islands, through Japan 

 and the Biu-kiu (Lu-Tschu) Islands to Formosa. This 

 region becomes submarine a little north of Tokyo which 

 lies in the A region, and is continued in the eastern part 

 of Sikok. On comparing Dr. Omori's map (Fig. 7 of his 

 paper) with the geological map of the Japanese Empire 

 issued 1902, we see that the A region corresponds with 

 the older portions of Japan, adjoining the Senkungsfeld of 

 the Sea of Japan ; it is an area where elevation from rise 

 of isogeotherms has probably long ago ceased, in which 

 volcanic action is practically extinct, and in which ancient 

 granites, gneisses, palaeozoic sedimentaries and mesozoic 

 rocks have had time to become exposed. The B region 

 in Niphon and Yesso is an area of great Tertiary and 

 Quaternary uplift. Most of the late volcanic rocks are in 

 this region. 



1 "Annual and Diurnal Variation of Seismic Frequency in Japan" by 

 Mr. F. Omori, D. Sc, in " Publications of the Earthquake Investigation 

 Committee/' No. 8 ; also figure 7 in same. (Tokyo, 1902.) 



