ON SEISMOLOGICAL INVESTlGATIOiNS. 

 Bulletins and Tables. 



239 



The International Seismological Summary up to the end of 1924 has been prinffed 

 and distributed, and the three months 1925 January-March are in the printer's hands. 

 From the Summary for the seven years 1918-1924 a simple list of epicentres and 

 times has been prepared which the British Association Council have agreed to publish. 

 The Summary itself reaches a rather limited public of those actively engaged in 

 seismological observation ; it is hoped that this list of epicentres and times may reach 

 and interest a wider public, including geographers and geologists. It seems probable 

 that seven years' systematic record of this degree of accuracy, now for the first time 

 available, should provide valuable material for systematic discussion. One or two 

 points may be mentioned by way of illustration : — 



(a) There is not a single day during the whole seven years on which no earthquake 

 was recorded, though there are one or two cases when a shock was recorded at one 

 observatory only, and a good many days when shocks were recorded by two observa- 

 tories only. 



(6) The foUowhig are the monthly counts of epicentres determined : — 



It will be seen that there is a sudden maximum in September. The effect of 

 September 1923 is no doubt exaggerated by the numerous aftershocks of the great 

 Tokyo earthquake ; but if we omit 1923 the mean value for September in the six 

 other years is 51, still much in excess of other months. So sudden a maximum cannot 

 be adequately expressed by harmonic analysis unless we use a great number of terms ; 

 but the phases of the first harmonic for the separate years are consistent, viz. : 

 245°, 202°, 223", 190", 178°, 232°, 221°. It was shown in the Geoph. Supp. to Monthly 

 Notices R.A.S., I, 5 (December 1924) that such ' annual ' variations are subject to slow 

 changes which indicate that the period is not accurately one year. 



Deep Focus. 



The h pothesis tliat in some cases the focus of an earthquake may lie -05 or perhaps 

 even -10 of the earth's radius below the earth's surface has been maintained in these 

 reports and in the International Seismological Summary for some half-dozen jears, 

 l)ut only recently has any independent testimony been forthcoming in favour of 

 this view, viz. in the (Tokio) Geophysical Magazine, Vol. I, No. 4 there is a paper 

 by Mr. K. Wadati on ' Shallow and Deep Earthquakes,' in which he examines 

 specially the earthquake of 1926 July fGd. 18h. .54m. 45s. epicentre 35-4° N., 136-4° E., 

 finding from observations 7iear the epicentre a depth of 343 km. = -054 of the earth's 

 radius. Most of the observations used by Mr. Wadati had not been made accessible 

 to us in Oxford until his paper appeared, but we had observations made at more 

 distant stations, including some near the Antipodes of the epicentre, and on apply- 

 ing the usual treatment to these observations a focal depth of -055 below normal was 

 readily deduced, in general confirmation of Mr. Wadati's result. Moreover he 

 indicates a number of other cases of deep focus, in all of which, mthout exception, 

 the usual reductions give results accordant with his, e.g. : — 



on 1924 April 3d. 2h. 30m. 30s. at 32-0° N., 1390° E. 



1925 April 19d. 15h. 46m. 36s. at 33-0° N., 137-5° E. 

 1925 May 27d. 2h. 29m. 54s. at 36-5° N., 1330° E. 



