C. Davison — On Earthquake- Sounds. 217 



intensity of tlie initial earthquake-vibrations ; and therefore the 

 extent of the sound-area must he independent of that of the disturbed 

 area. 



It is possible that from the lower margin of the slip-area sound- 

 vibrations may proceed ; but, so far as regards the sounds heard 

 at the surface, the vibrations proceeding from the upper and lateral 

 margins must be most perceptible. The centre of intensity of the 

 sound-focus must therefore, as a rule, be within the upper margin 

 of the seismic focus, i.e. the sound-area is not concentric with the 

 disturbed area, and the centre of the former is nearer to the fault- 

 line than the centre of the latter. 



Throughout the greater part of the sound-area, the vibrations first 

 perceived must be those from the upper or lateral margin of the 

 seismic focus, i.e. the beginning of the sound must generally precede 

 the beginning of the shock. There will, however, be a small part of 

 the disturbed area, that immediately surrounding the point whei-e 

 the normal to the slip-area meets the surface, where the shock may 

 be felt first : unless, indeed, the fault-slip does not take place 

 instantaneously, but, commencing very slowly, initiates a series of 

 short-period vibrations from the whole slip-area before the true 

 earthquake vibrations are produced. In this case, however, the 

 excentricity of the sound-area would be hardly perceptible. 



At most places within the sound-area, then, the sound will be first 

 heard, due to vibrations proceeding from the nearer lateral margin 

 of the seismic focus. The sound will become gradually louder and 

 deeper until its intensity is a maximum, the vibrations then coming 

 from the boundary-line between the sound-focus and the rest of the 

 seismic focus. Soon after this, the sensible shock will be felt, 

 due to vibrations proceeding from that part of the focus where the 

 amount of slip is greatest, the sound continuing for all or part of 

 the time, owing to the arrival of vibrations from the upper margin 

 of the slip-area. And, lastly, after the sensible shock ceases to be 

 felt, will be heard the sound corning from the further lateral margin 

 of the slip-area, provided that margin be not too distant, the sound 

 becoming higher as it dies away. 



In the neighbourhood of the boundary of the sound-area, the 

 sound-vibrations from the further lateral margin of the seismic focus 

 will be imperceptible, and the sound will be heai'd only preceding, 

 or preceding and accompanying, the shock ; and this may in part 

 account for the comparative rarity of the records of the subsequent 

 sound-phenomena. 



Again, in most British shocks, the part of the focus from which 

 the sensible vibrations come is of small magnitude, so that only one 

 or two principal vibrations are produced, and these are felt just at the 

 time when the sound is loudest. 



If the sound- vibrations first or last perceived be those which come 

 from the boundary between the sound-focus and the rest of the 

 seismic focus, the sound will begin or end abruptly. But observa- 

 tions of such a phenomenon must be rare. 



The diiferent relations between the dimensions of the sound-area 



