216 C. Darison — On Eavthquake- Sounds. 



Throughout the remainder of this paper, I shall conclude that 

 Prof. Milne's observations do, as he su(>;gest8, imply the existence of 

 preliminary vibrations short enough in period to give rise to the 

 plienoniena of earthquake-sounds ; and I shall endeavour now to 

 show how these vilirations originate, and at the same time to account 

 thereby for the different sound-phenomena described above. 



To give definiteness to the theory, I shall take the case of an 

 earthquake produced, as I believe most non-volcanic earthquakes 

 are produced, by the friction due to the slipping across one another 

 of the two rock-surfaces of a fault. 



The seismic focus, or slip-area, may be of very considerable 

 dimensions, sometimes fifty miles or more in length. The intensity 

 of a shock does not, however, depend so much on the size of a 

 slip-area as on the maximum amount and short dm-ation of the slip. 

 Now, it is evident that the amount of the slip must vary greatly 

 throughout the slip-area, but it will be sufficient to consider only 

 the simplest case, that in which the amount of slip is a maximum 

 in a certain central region, and diminishes gradually until it is zero 

 along the margin of the seismic focus ; though the faces of a fault 

 not being smooth planes, there will probably be several or many 

 such regions of maximum slip.' 



Now, since up to a certain point the period of a vibration increases 

 with its amplitude, and since the initial amplitude of the vibrations 

 must depend on the amount of slip producing them, there will, from 

 all parts of the slip-area considered, proceed vibrations varying, not 

 only in amplitude, but also in period ; and along the borders of the 

 slip-area, where the fault-slip dies away, these vibrations may be 

 small enough, and consequently rapid enough, to produce the 

 sensation of sound. 1 imagine, then, that the sound-phenomena ac- 

 companying earthqualces are produced by the minute vibrations coming 

 chiejiy from the upper ond lateral margins of the slip-area. 



For brevity, 1 will give the name of the " sound-focus " to that 

 part of the slip-area or seismic focus from which the sound- 

 vibrations come. 



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

 seismic focus is not a definite line. Its position ' varies with the 

 lower limit of audibility of eacli observer, so that at exactly the same 

 spot two observers might differently estimate the duration of the 

 sound. But, neglecting for our present purpose this personal 

 equation in the observers, it is evident that the position of the 

 boimdary-line referred to depends only on the position of the points 

 on the slip-area at which the amount of slip is just small enough to 

 produce vibrations which may be heard, that it is independent of 

 the maximum amount of slip within the seismic focus. Now, other 

 conditions being the same, the dimensions of the sound-area are 

 determined by the intensity of the vibrations which are just per- 

 ceptible as sound, and those of the disturbed area by the maximum 



1 The rumbling or rolling character of the sound, though arising partly no doubt 

 from interference, may also in part be due to the existence of several regions of 

 maximum slip. 



