REPORTS OF RESEARCH COMMTTTEES. 299 



5. Speed of Main phase {Long Waves) under Ocean and. Confuioif. 



(a) The comparison of th© records of the Aleuten earthqiiake 

 l.V. 1915 and of the Kamschatka earthquake 31. VII. 1915 at two 

 stations of equal epicentral distance, at Apia and at Tiflis, shows 

 that longwaves are running at about 21 — 26 per cent, quicker 

 under the Pacific Ocean than under the Asiatic Continent. 



(b) A detailed analysis of the Apia records shews that the main 

 phase of these earthquakes begins with transverse waves ot" puiely 

 horizontal displacement at right angles to the direction of propa- 

 gation, which is followed by Rayleigh waves of horizontal move- 

 ment in the direction of propagation, and vertical movenient. The 

 ratio of velocity of both transverse and Rayleigh waves, being 0.91 

 for these earthquakes, is in very good accord with the theory which 

 demands 0.92. 



(c) The transverse, the Rayleigh waves, and the Coda being a 

 residual phenomencn confined to the- upper solid crust of the earth, 

 the transverse waves of the main phase have the) same velccit}^ of 

 propagation as S in the upper crust. The diffei'ence of velocity 

 under ocean and continent for S at Zl =6° where S is confined to 

 the upper crust and for transverse M'aves of the main phase are 

 very nearly equal. 



(r/) The results of observations of gravity over contments and 

 ocean suggest that the density of the solid earth crust under 

 ocean is greater than under continents, and compensates the 

 smaller weight of water. Therefore, we cannot attribute the 

 increase in velocity to a decrease in density, and we must assume 

 that the increase in velocity is caused by a higher rigidity under 

 ocean. 



6. Period of Transverse Waves under Ocean and ('outineni and 



Thickness of the Upper Crust. 



The comparison of records of Japan, Kamschatka, and Aleuten 

 earthquakes at Samoa, and at Gottingen and Tiflis, shows that the 

 periods of Coda waves are about 3 sec. longer under the Pacific 

 (17.5 sec.) than under the Asiatic continent (14.5 sec). If the 

 upper solid earth crust is resting on a sheet of plastic material 

 (Magma), a purely transverse wave may oscillate with the period 

 of the natural vibration of this crust, the thickness of this layer 

 being half the wave length. The main and Coda phase of earth- 

 quake records may contain such vibrations. 



Combining velocity and the period of the natural vibration, we 

 get a thickness of the upper eolid earth crust under the Pacific 

 Ocean of 41 klm., under the Asiatic continent of 28 kim. Accord- 

 ing to H. Faye, the better conducting over-lying water makes the 

 cooling of the upper earth crust under ocean progress quicker 

 than under continent. Therefore, he suggests the solid earth crust 

 may be thicker under ocean than under continent. 



