16 ART. 1. S. KUSAKABE I FREQUENCY OF 



from being simihir to the first, are in quadrantal forms. In the 

 western ])art, indeed, wliere the curves lie within Quaternary 

 rocks, they are all i)arallel to each other ; but in the other three 

 directions they shiink in or swell out with all possible irregu- 

 hirities. 



These irregularities, however, become regular wlieii the geolo- 

 gical distribution of rocks in the corresponding regions is taken 

 into account. To express this in the form of a simple rule, the 

 curve swells out where Palœozoic, or better Archœan rock><, predo- 

 minate, and shrinks in where Cainozoic rocks pi'evail. This simple 

 law is sufficiently satisfied up to very minute portions, as the figure 

 ])roves most clearly. 



As a corollary, since the geological map indicates only the 

 surface distribution of rocks, we may conclude that the seismic 

 wave is mainly transmitted through the earth's surface, or more 

 probably, seismic action is mainly due to surface waves discussed 

 by Lord Rayliegh, and recently propounded by Lamb for isotropic 

 media. Any further discussion, however, as to the seismic wave 

 conductivity of different rocks requires moie precise quantitative 

 investigation of the amount of hysteresis for these rocks, which 

 may possibly prove a life-long problem. 



Seismic Frequency and Degree of Damage in a Given Region. 



It will not be entirely out of place to insert here discussions 

 on a topic relating to seismic frequency and the degree of damage 

 in a given region, in order that above statements may not be 

 misunderstood. Of the above conclusions, relating to frequency, 

 the one concerns the frequency of after-shocks at the central 

 region of a given earthquake, while the other relates to the case 



