42 Di\ C. Davison on Earthquake- Sounds. 



strength of the vibrations must be very much greater for deep 

 sounds than for high ones, and this is especially the case for 

 the deepest sounds of all. If, then, the sound is just audible 

 to any person, it will cease to be heard if the period of the 

 vibrations be increased without any change occurring in 

 their strength , or if the strength be diminished, while the 

 period remains unchanged. 



In the immediate neighbourhood of the epicentre, the 

 sound-vibrations are of such intensity that they are heard by 

 nearly all observers, at any rate in Great Britain. As we 

 recede from the epicentre the vibrations diminish in strength, 

 and at the same time, as we know from seismographic 

 records, the larger vibrations increase in period. On both 

 accounts, then, they tend to become inaudible. Thus the 

 percentage of persons who hear the sound should diminish as 

 the distance from the epicentre increases, until we reach the 

 boundary of the sound-area where no human being is capable 

 of hearing so deep a sound. 



If the vibrations which reach a particular observer are at 

 first very rapid, and then gradually increase in period without 

 an appropriate concomitant increase in strength, the sound 

 will become gradually deeper, and finally will cease to be 

 heard, the ending being more or less abrupt. Again, if the 

 period were to be subject to a succession of changes without 

 the proper change of strength, it is possible that to some 

 observers the vibrations might be inaudible during short 

 intervals when the period is great, and they would then hear 

 the sound as a succession of reports, a series of footfalls, &c. 



Further, the vibrations which reach the observers in one 

 place at any moment are probably of various amplitude and 

 period. One may hear many vibrations, and another only 

 some of the series. Thus to one observer the sound may be 

 like a rising wind, to another like a heavy traction-engine 

 passing ; one may hear the crashes which accompany the 

 principal vibrations, while a second will be deaf to them ; to 

 one the sound may become continually louder and cease 

 abruptly, to another it may increase to a maximum and then 

 gradually die away. 



As we recede from the epicentre, however, the vibrations 

 of every period tend to become inaudible. The limiting 

 vibrations of the whole series w T ill be the first to be lost, 

 especially those of longest period. Thus near the epicentre 

 sound-vibrations of many different periods will be heard, and 

 the sound will be more complex than it is elsewhere. It is 

 only in this region that the loud crashes will be heard with 

 the strongest vibrations. As the observer recedes from the 



