SEISMOLOGICAL INVESTIGATIONS 261 



For this earthquake, which had its epicentre in the New Hebrides, Mr. 

 Hughes gives in the Summary the focal depth 0-04 of the earth's radius, 

 or 250 km. Excellent confirmation is provided by a special investigation 

 of this earthquake by Father Stechschulte of St. Louis. 



High Focus Earthquakes. 



That earthquakes with deep foci occur is now well established, but the 

 significance of the observations which led Turner to attribute high foci to 

 certain earthquakes is not yet known . A good example of such an earthquake 

 was that of r 928 , January 6 , the epicentre of which was in East Africa , midway 

 between Mt. Kenia and Mt. Elgin. Mr. Hughes gives the height of the 

 focus as -CIS R or 100 km. It is certain that the focus of a normal earth- 

 quake is at a depth much less than 100 km., so that a height of 100 km. 

 above the normal is not to be taken literally. The difficulty in interpreting 

 the observations is that if the earthquake is treated as normal, the interval 

 between P and S phases is at most stations about 10 seconds greater than that 

 appropriate for the distance from the epicentre. The earthquake in 

 question is to be studied by Mr. E. Tillotson, who is collecting original 

 records, and it may be hoped that he will succeed in solving the mystery 

 of the ' High Focus.' 



The Surface Layers. 



It is by the study of near earthquakes that information must be sought as 

 to the usual depth of focus and as to the thickness of the layers of the earth's 

 crust. There is at present a remarkable difference in practice between 

 English investigators, who follow the method of Harold Jeffreys, and 

 most seismologists abroad, who keep to the procedure developed by 

 S. Mohorovi^id. Calculations by the Jeffreys method lead to estimates of 

 10 km. for the thickness of the granite (which is generally overlaid by a 

 kilometre or two of sedimentary rock) and 25 km. for the thickness of the 

 intermediate rock between the granite and the ultrabasic rock which trans- 

 mits the P and S waves. The alternative method has led to estimates of 

 about 60 km. for the thickness of the two upper layers. The nature of the 

 controversy as viewed by Jeffreys in 1928 is explained in the second edition 

 of his book The Earth. Following papers by Tillotson and Mourant, in 

 which the method of Jeffreys was used, there have been published in the 

 year under review two papers by A. W. Lee, which consolidate the evidence. 

 The success of the method depends on the detection of waves reflected at 

 the ground or at the upper surface of the granite layer. The method has 

 only been applied hitherto to small European earthquakes. It is to be 

 hoped that reflected waves will be investigated in other regions, so that 

 general agreement as to the merits of the alternative methods of inter- 

 pretation of the seismological evidence may be reached. 



MiCROSEISMS. 



In continuation of the work summarised in the last report, Mr. Lee has 

 investigated the theory of the propagation of surface waves over an area 

 where there is a known thickness of sedimentary rock over granite. It 

 appears that the larger microseisms are to be expected where the sedi- 

 mentary rocks are of greater thickness. As far as Great Britain is concerned 

 this conclusion is consistent with the geological evidence. Information is 

 now being collected at Kew as to the microseismic disturbance in all parts 

 of the world, so that the theory may be put to a thorough test. 



