242 REPORTS ON THE STATE OF SCIENCE, ETC. 



the maximum M, which can be observed at great distances for even small shocks ; 

 but a recognisable P is another matter ; and an asterisk in column 4 marks cases 

 where P has been observed for A > 80°. But it must be frankly admitted that no 

 great precision has been attempted in either of these criteria, for they are in any 

 case rough, and to spend time on refinement would be undesirable if not 



impossible. 



4. The column headed ' Former Occasions ' is, it is hoped, an addition of some 

 value. It was left an open question for some years whether earthquakes were apt 

 to recur at precisely the same epicentre or merely in proximity to it ; and accordingly 

 independent determinations of epicentre were made for successive shocks in the 

 same neighbourhood. But it gradually became apparent that the hypothesis of exact 

 recurrence was often as good as any other, while the convenience of utilising the 

 calculations of A and azimuth already made was considerable. Accordingly the 

 habit of using old epicentres became gradually established ; and there is this to be 

 said in favour of it, that those who doubt the validity of the implied hypothesis may 

 be glad to have an easy reference to test cases. They may take such a case as that 

 of epicentre 43°-8 N. ll°-2 E. on 1920 Dec. 27d. 16h., and find the reference 

 back to 1920 Nov. 13, which again refers back to 1920 Sept. 16, and that (through 

 a previous shock on the same day) to Sept. 11, and so backwards for a series of 

 thirty-four shocks in all. To test the hypothesis of identity they must of course go 

 to the details in the International Summary ; but the present catalogue gives a 

 fair idea of the tendency to recurrence. A Ust of a dozen good series is given in the 

 Geophysical Supplement to the Mon. Not. R.A.S., vol. ii.. No. 1 (p. 70). 



5. The column ' Minor Ents. ' shows simply the number of observations relegated 

 to the notes, as cases where there is not sufficient material to give an epicentre. Many 

 of them are records at a single station only, unsupported by any independent observa- 

 tion. On some days there are only sporadic observations of this kind, with no serious 

 shock ; but no day in the seven years is completely blank, though on 1921 July 14 

 there is only one observation. It will be seen that the number of residual observations 

 of this kind is given, on days when there are also several considerable shocks, against 

 the last shock for that day. 



6. The daggers (t) in column 4 refer to notes collected at the end. Most of these 

 show the cases of anomalous focal depth, expressed in fractions of the earth's radius 

 and counted from the normal focal depth as reference depth. The great majority of 

 shocks come from approximately the same depth below the earth's surface, but whether 

 this normal depth is small or large is still somewhat uncertain. Most seismologists 

 are of opinion that the normal depth is about 50 km. or 30 miles or -008 radius, and it 

 must be admitted that the evidence in favour of some such figure is very strong. On 

 the other hand there seem to be cases (such as those on 1918 Sept. 7, 8, 12 ; 

 1919 May 6 ; 1922 Feb. 5 ; 1922 Oct. 17 ; 1923 Apr. 23) when there is evidence 

 for a focal height of 0-030 or even 0-040 above normal, so that the normal depth 

 should be of the order of 0040. The evidence for these cases is not nearly so strong 

 as that for the deep foci, down to 0-080 below normal, but it cannot be ignored ; and 

 if the normal depth is small some other explanation must be found for such cases 

 (suggesting heights above normal). 



7. As regards the cases of depth below normal, the case for them has been much 

 strengthened by an entirely independent investigation in Japan by Mr. Wadati, 

 published in the (Tokio) Geophysical Magazine, vol. i.. No. 4. Mr. Wadati identifies 

 the cases of deep foci in Japan from observations close round the epicentre, and 

 macroseismic information ; and his selection is practically identical with that made 

 by the observations at greater distances. For details reference must be made to the 

 International Seismological Summary. 



