298 KEPORTS OF RESEARCH COMMITTEES. 



SECTION A. 



4. Seismological Committee. 



Summary of Report on Pacific Earthquakes. 



{By G. Angenheister, Samoa : communicated by Dr. J. M. Baldwin, 

 M.A., Secretary to the Committee.) 



1. Ejyicenfre of Tonga Earthqitakcs. 



The epicentres of all main Tonga earthquakes, 13 Megaseisms 

 (1907-1919)', 77 larger, &c., ones (1913-1919) are situated all on 

 the Tonga-plateau, on the western side of the Tonga-depth. 

 According to observations the intensity of gravity at the Tonga- 

 plateau shows a surplus of -I- 0.17 cm. Over the Tonga-depth the 

 intensity is — 0.25 cm. too low. Therefore, the pressure of masses 

 must be directed from the Tonga-plateau to the Tonga-depth, just 

 wliere the epicentres of the earthquakes are h^ing. 



2. T'llt'nni of the (jioiiikI duiinci Eai-lhqual-cs. 



. Bv star observations with a meridian transit it could be proved 

 that 8 mill, after the beginning of a very strong earthquake of 

 degree VIII. (Mercalli), the vibrations did not contain any tilting 

 of the ground of 2" or more. 



3. The Time Curve. 

 The time curve of the first preliminary tremor P could be con- 

 tinued to 145" distance from the enicentre (for A = 145° is 

 P = 1,040 sec). For A = 100" PP is divided into two parts, the 

 quicker part arrives at Zl =177" after 1.210 sec. 



4. Speed of P> eliminari/ Tremor iiiu/i r Oeean nnd Continent . 



The comparison of the first (P) and second (S) preliminary 

 tremor of Tongan earthquakes recorded in Apia with P and S 

 derived from contiiiental earthquakes recorded in Europe shows 

 that P and S are running quicker under the Pacific than under 

 the European co,ntinent. 



The differences between sub-Pacific and sub-continental seismic 

 waves are the greatest for about A =6", being there 13 sec. for 

 P and 25 sees, for S ; for A =18" these differences are not greater 

 than 2 sec. For A = 6" the maximum depth (Scheiteltiefe) to 

 which P and S submerge into the earth is about 50 klm. So the 

 differences between sub-oceanic and sub-continental P and S are 

 at a maximum for rays entirely running in an upper crust of 

 50 klm. thickness which, therefore, may be the thickness of the 

 upper solid earth crust. The mean velocity of S under the Pacific 

 Ocean is about 18 per cent, greater than under the European con- 

 tinent. 



