SEISMOLOGICAL INVESTIGATIONS 255 



certain : the velocity Ss was taken as 2-9 km./sec, and the corresponding 

 density 2-5 gm./cm. 8 . On these assumptions the thickness of the sedi- 

 mentary layer can be calculated. The value of the thickness found, nearly 

 4 km., is double the thickness that Jeffreys estimates from the denudation 

 needed to account for the sodium in the ocean. 



Dr. Jeffreys has pointed out to me that, according to the work of the 

 Geophysical Laboratory at Washington, the rate of increase of the bulk- 

 modulus with pressure is very much greater for pressures less than 2 X io 9 

 dynes/cm. 2 (corresponding to a depth of about 8 km.) than for greater 

 pressures ; a corresponding increase in the rigidity probably goes with the 

 increase in the bulk-modulus, and if so, it may not be appropriate to take 

 for the upper part of the granitic layer the elastic constants determined from 

 near earthquakes for that layer as a whole. It may well be that, so far as 

 the velocity of surface waves is concerned, the upper part of the granitic 

 layer has to be reckoned as part of the sedimentary layer. Further, the 

 sedimentary layer almost certainly does not approach homogeneity, and 

 there is considerable doubt as to the density and the elastic constants that 

 should be chosen to represent it ; as the method is rather sensitive to 

 changes in the elastic constants, at any rate for the wave-periods used in 

 this investigation, an accurate determination of the thickness is not to be 

 expected. 



The Baffin Bay Earthquake of 1933, November 20. 



By Dr. A. W. Lee. 



This earthquake was chosen for study because the epicentre was in such 

 a position that the records at the numerous seismological stations of Europe 

 and America would provide material for determining more precisely the 

 travel-times for distances of the order 40 . 



A detailed investigation has now been completed and will be published 

 shortly. The records of ninety-nine observatories were collected and 

 examined at Kew Observatory ; over two-thirds of these observatories are 

 at epicentral distances between 25 and 50°. 



The epicentre is located as in latitude 73°- 3 N., 70°- 2 W., and the focus 

 at a depth of about 10 km. ; the time of occurrence of the shock is taken as 

 23I1. 21m. 31 -5s. G.M.T. 



Comparisons have been made between the observed travel-times for P 

 and S and the times calculated from various tables. The best representation 

 of the travel of the P waves from 25 to 50 is given by a table based upon 

 one published by Gutenberg and Richter ; in this modified table the apparent 

 velocity is uniform for epicentral distances from 25 to 40 and again from 

 45 to 50 , the velocity changing by 17 per cent, from 40 to 45 . There 

 are discrepancies between the observations of S and the tables of travel- 

 times hitherto available. A new table for S at distances from 25 to 50 

 has been computed from the travel-times for P on the assumption that 

 Poisson's ratio is constant for the rocks traversed by the waves. The 

 agreement between the observations and this table is satisfactory. 



Reappointment of the Committee. 



The Committee asks for reappointment, for the continuation of the normal 

 grant of £100 from the Caird Fund and for a special grant of £50 for the 

 maintenance of the International Seismological Summary. 



