Vol. 63.J 



SWANSEA EARTHQUAKE OF 1906. 



357 



Nature of the Sound. 



The total number of descriptions of the earthquake-sound is 

 690. In 47 per cent, of these, the sound is compared with passing 

 traction-engines, etc. ; in 20 per cent., with thunder ; in 7, with 

 wind ; in 5, with loads of stones falling ; in 7, with the fall of a 

 heavy body ; in 13, with explosions ; and in 1 per cent., with 

 miscellaneous sounds. 



The variation in the nature of the sound with the distance from 

 the epicentre is shown in the following table (I), in which the 

 figures are percentages of comparison with the different types for 

 each of the districts mentioned : — 





in 







1 « 



>* 









-a 







o 

 -i-3 



c3 



2 





0D 



o 





© 







i ai 







<D 



Table I. 



Passing "v 



J 

 | 



E-i 



1 



xn ^3 



Fall of a 

 body. 



.9 

 o 



| 



r © 



o 



to 



Wi thin isoseistnal 8 



36 



12 



5 



! * 



10 



21 



4 



Between isos. 8 and 7 ... 



41 



21 



7 



6 



8 



15 



2 



„ 7 „ 6 ... 



53 



23 



6 



3 



7 



7 



1 



„ 6 „ 5 ... 



62 



20 



8 



... 



3 



6 



1 



„ „ 5 and the 

















boundary of the sound- 

















area 



53 



19 



12 



6 





3 





Whole sound-area 



47 



20 



7 



1 o 



"7 



13 



"i 



Average for strong shocks 



46 



24 



10 



5 



3 



7 



5 



The chief peculiarity exhibited by this table is the large per- 

 centage of comparison with sounds of an explosive character within 

 the isoseismal 8, due no doubt to the audibility of the deep crashes 

 which usually accompany the strongest vibrations in the neigh- 

 bourhood of the origin. At a little distance from the epicentre, 

 these crashes soon become inaudible. Thus, with increase of 

 distance from the epicentre, there is a rapid decrease in the per- 

 centage of reference to sounds of an explosive or abrupt character, 

 and a corresponding increase in the percentage of reference to 

 sounds of a smooth and monotonous type. 



Time-Relations of the Sound and Shock. 



In Table II (p. 358) the letters p, c, and / indicate the number 

 of records per cent, in which the beginning and end of the sound 

 preceded, coincided with, or followed, the corresponding epoch of 

 the shock ; the letters g, e, and I show the number of records 



