SEISMOLOGICAL INVESTIGATIONS 251 



Montserrat. — The earthquakes in the island of Montserrat having con- 

 tinued, a small expedition to the island was organised by the Royal Society, 

 Mr. A. G. MacGregor being the geologist and Dr. C. F. Powell the physicist. 

 Prof. Jaggar of Hawaii and Sir Gerald Lenox-Conyngham also visited the 

 island. Dr. Powell has installed a Wiechert horizontal seismograph as 

 well as a number of Jaggar shock-recorders which were made at Kew 

 Observatory for the expedition. Seismic activity has been much less during 

 the first half of 1936 than during the previous two years. 



The International Seismological Summary. 



A Note by Mr. J. S. Hughes. 



The preparation of the International Seismological Summary for 1931 has 

 been completed. The sections for the first two quarters of the year have 

 been distributed and the other two sections are with the printer. As was 

 anticipated in the last Report, 1931 proved a very heavy year seismologically. 

 The number of epicentres identified was not exceptionally large, but the 

 earthquakes were more generally observed. For the first six months of 

 193 1 the number of pages required in the Summary was 283 as compared 

 with 197 in the previous year ; the earthquakes dealt with numbered 297 

 in 193 1, 284 in 1930. It is remarkable that after unusual seismic activity 

 on November 2, 1931, mainly connected with South Japanese shocks, there 

 was a sudden lull, and the spell of reduced activity lasted for about three 

 months. 



The only shock occurring in 193 1 which calls for special mention, although 

 there are many well-determined earthquakes in the year, is that of August 10. 

 The epicentre 46 -9 N. 90 -o E. (near the Great Altai Mountains, Mongolia) 

 is that used in the Seismological Summary of the British Association for the 

 earthquakes of 1917, July 31 and November 28, but was adopted only after 

 a separate calculation had shown the position accurate within o° • 1 . 



The P observations show very good fit with the determination made, but 

 the S readings are nearly all uniformly too large by 30 sec. This means 

 that in the preliminary calculations, when a T dependent on S-P was used, 

 the Japanese and European stations gave separate epicentres with the same 

 time at origin. According to the old routine when it was customary in the 

 Summary to keep the balance of S and P residuals at all costs, the shock 

 would probably have been entered as having a T of 2ih. 18m. 25s. with 

 a positive or high focus correction of 0-030 or so, leaving the interpretation 

 of the abnormality to seek. Now the abnormality is shown in a different 

 way. In a review of the International Seismological Summary, ' Nature,' 

 January 4, 1936, it was stated, ' It is interesting to notice that there were no 

 earthquakes to which it was found necessary to allot high focus. It appears 

 that with more reliable observations and more reliable standard tables the 

 anomalies which led Turner to assume high foci for certain Earthquakes 

 do not occur.' Here we have this anomaly turning up in a very pronounced 

 form ; had the differences been of the opposite sign there would have been 

 no hesitation in assuming considerable focal depth. 



It is always rather a question whether shocks occurring in the same 

 neighbourhood successively should be regarded as originating at the same 

 epicentre or whether the small differences which can sometimes be deduced 

 from the residuals have a real significance. An interesting case is that of 

 1 93 1 October 3 and the succeeding days, in which 21 shocks occurred 

 near the Solomon Islands, round about io° S. 162 E. In making the 

 determinations of these shocks, six separate epicentres were adopted, 



