f56 Dr. C. Davison on Earthquake- Sounds . 



Table VIII. 



Earthquake. 



Cornwall, 1892 , 

 1898 

 Exmoor, 1894 

 Leicester, 1893 

 Pembroke, 1892 



1893 

 Hereford, 1896 



Total 



Percentage 



Duration of Sound with reference to that 

 of Shock. 



Greater. 



5 

 22 

 G9 

 49 

 86 



0/ 



Equal. 



Less. 



1 





3 





8 



1 



11 



4 



11 



4 



40 



12 



74 



21 



18 



5 



Doubtful. 



6 

 63 



84 

 20 



Origin of Earthquake -Sounds. 



In any earthquake there are generally three well-defined 

 stages of motion — the preliminary tremor, the principal 

 portion, and the end portion. In the first, the characteristic 

 movements are small in amplitude and short in period ; the 

 second and most active part of the disturbance consists chiefly 

 of vibrations of larger amplitude and longer period ; and these 

 are followed by the feeble movements which constitute the 

 end portion. Slow undulations, with a period of about 

 one-half to two seconds, may, however, be present in all three 

 stages ; while the ripples, with a period of one-tenth of a 

 second or more, may be superposed on the slow undulations 

 in the second and third stages as well as in the first. The 

 average period of the ripples, it is important to notice, is 

 slightly greater during the principal portion than during the 

 other two stages *. 



During the earlier part of the movement there is a distinct 

 increase in the period of the vibrations ; and this has led some 

 seismologists f (all at the time living in Japan) to suggest 



* See a valuable paper by F. Omoii and K. Hirata, " Earthquake 

 Measurement at Miyako," Journ. Coll. Sei. Imp. Univ., Tokyo, vol. xi. 

 (1899) pp. 161-195. 



t J. A. Ewing, "Earthquake Measurement," Mem. of the Sci. De- 

 partment, Univ. of Tokio, No. 9 (1883), p. 11 ; J. Milne, Japan. Seismol. 

 Soc. Trans, vol. xii. (1888) pp. 58-59 ; F. Omori, Journ. Sci. Coll. Imp. 

 Univ., Tokyo, vol. xi..(1899) p. 147. 



