68 Dr. C. Davison on Earthquake- Sounds . 



marginal regions, and especially from the upper and lateral 

 margins, that the more sensible sound-vibrations chiefly come. 

 I will now endeavour to show how the phenomena of earth- 

 quake-sounds can be explained on this theory. 



If there is no discontinuity in period between the large 

 vibrations which come from the centre of the area and the 

 sound-vibrations from the focal margins, it is evident that 

 among the vibrations must be included those which produce 

 the deepest sonnd that can be heard by the human ear. The 

 rumbling character of the sound may be due partly to its 

 neighbourhood to the lower limit of audibility, partly, no 

 doubt, to irregularities in the fault-surface producing local 

 variations in the initial disturbance. To the latter cause 

 may also be owing the intermittent sounds heard by some 

 observers. 



But whether there be any slight fluctuations in amplitude 

 and period, or not, it is evident that the average intensity of the 

 sound must gradually increase until the shock is felt ; and as 

 soon as it is over, or nearly over, must gradually die away. At 

 the same time the greater amplitude of the vibrations from 

 near the central part of the focus will render audible vibrations 

 of longer period than those which come from the margins ; 

 and thus the depth of the sound will increase and decrease 

 with the intensities of the sound and shock. Especially will 

 this be the case with the vibrations from the neighbourhood 

 of that part of the focus where the initial amplitude is greatest; 

 and the strongest vibrations and loud explosive crashes 

 audible to some persons should therefore be observed con- 

 currently. 



Similar reasoning leads to the conclusion that the intensity 

 of the sound should increase with that of the shock in dif- 

 ferent earthquakes, and that the sound should be deeper with 

 strong shocks than with weak ones. But while the marginal 

 vibrations are limited in period and amplitude, those from the 

 central parts of the focus have a much wider range. With 

 very weak shocks the sound should be the most prominent 

 feature ; with very strong ones the sound, though actually 

 louder, should be relatively insignificant ; and when, as in the 

 Hereford earthquake, the earthquake consists of two distinct 

 parts, the two shocks may differ widely in intensity, while the 

 accompanying sounds may be almost equally loud. 



With observers of given average auditory powers, the mag- 

 nitude of the sound-area depends on the limited strength of 

 the marginal vibrations, and on the dimensions of the seismic 

 focus, and therefore of its marginal regions. The magnitude 

 of the disturbed area depends partly on the size of the focus, 



