154 



EEPORT — 1895. 



earthquakes is dependent upon influences which simultaneously affect 

 Japan as a whole, and in the latter case to determine how far their 

 frequency may be related to phenomena of a more local character. 



As an example of an influence wliich affects Japan as a whole, the 

 difference in the summer and winter barometrical gradients crossing the 

 country may be taken, while tidal loads along the coast would be expected 

 to produce effects in difterent districts at diti'erent times. 



Not only is it open for us to determine effects due to external 

 influences, but these, so far as possible, must be distinguished from effects 

 resulting from internal conditions. The great frequency in District 7 was 

 entirely due to the shocks succeeding a terrible disturbance which took 

 place on October 28, 1891 ; and if these after shocks, which at first 

 occurred at the rate of 1,700 per month, and which ajjparently result 

 from the settlement of disjointed strata, are included in any general list, 

 it is clear that they might accentuate or destroy any law respecting a long 

 period frequency. What is true for District 7 is also true for District 11. 

 By themselves they yield information about the rate at which an enonnous 

 quantity of broken-up strata settles to a state of equilibrium, and because 

 the district around the ejiicentrum is for some time after the primary 

 disturbance in an extreme state of seismic sensibility, it is quite possible 

 that there may be fluctuations in the rate at which quiescence is ap- 

 proached, due to external influences. Other problems which suggest 

 themselves are the possible relationships between the seismic activity 

 of the various districts, the times taken for different areas under the 

 influence of secular movement to attain varying degrees of seismic sensi- 

 bility, and the connection between earthquake occurrence and the 

 geotectonic character of the country. If the object of an analysis is 

 to discover a relationship between earthquake frequency and exogenous 

 phenomena which recur at long intervals, it would .seem advisable to omit 

 long lists of after shocks, and only to take into consideratifin disturbances 

 which occur in districts whei'e seismic activity is in a normal state. On 

 the contrary, should we seek a relationship between the occurrence of 

 earthquakes and phenomena which recur at intervals of not more than 

 a few days, as, for example, barometrical fluctuations or the rising and 

 falling of the tide, this precaution is hardly necessary. 



