Dr. C. Davison on Earthquake- Sounds. 69 



chiefly perhaps on the initial intensity of the vibrations from 

 its central portion. While the dimensions of the sound-area 

 should generally increase with those of the disturbed area, it 

 is evident that they should not increase in the same ratio. 

 With very strong shocks, the sound-area should be a compa- 

 ratively small district surrounding the epicentre. With very 

 slight ones, the marginal regions would be so great compared 

 with the central portion of the focus, that the sound-area 

 might overlap the disturbed area. In the limit, the central 

 portion of the focus would vanish, and an earth -sound would 

 be the only result sensible to human beings *. 



Several of the phenomena of earthquake-sounds depend on 

 the superficial form and inclined position of the focus, and on 

 the fact that the most sensible sound-vibrations come from the 

 upper and lateral margins. The isacoustic lines and the 

 boundary of the sound-area should not be concentric with the 

 isoseismal lines. Relatively to the latter the sound-area will 

 be shifted towards the fault-line, and also in a direction 

 parallel to it ; if one lateral margin should predominate over 

 the other. 



The time-relations of the sound and shock remain to be 

 considered. The sound- vibrations from the margin nearest 

 to the observer should be heard before the shock begins, those 

 from the upper margin, and possibly from parts of the central 

 portion, during the progress of the shock, and those from the 

 furthest margin after the shock ends. Moreover, the sound- 

 vibrations from the nearer lateral margin will be of greater 

 intensity than those from the further one; and thus the fore- 

 sound should be more generally heard than the after-sound, 

 and with Japanese and some other observers should be the 

 only one perceptible. The percentage of observations of the 

 after-sound should also diminish with the increasing distance 

 from the origin ; and the duration of the sound, especially- 

 near the epicentre, should be greater than that of the shock. 



At places near the epicentre, and also at places whose 

 distances are great compared with the dimensions of the 

 focus, the sound- vibrations which appear loudest will be those 

 which originate near the central portions of the focus ; and 

 thus the epochs of maximum intensity of the sound and shock 

 should coincide. If, however, the horizontal length of the 

 nearer focal margin be considerable, the sound which seems 

 loudest to observers at a moderate distance may come from or 



* It has "been suggested that the comparative smallness of the sound- 

 area may be due to the more rapid extinction of the sound-vibrations ; 

 but the explanation is clearly incomplete, for it fails in the ca^es in which 

 the sound-area overlaps the disturbed area. 



