C. Bauson — On Earthquake- 8ounds. 215 



distance on tlie upthrow side of the same fault, is the centre of the 

 sound-area ; but its position cannot be determined with any great 

 accuracy, for I know of no places where the sound was certainly 

 not heard. The evidence obtained is, I believe, sufficient to show 

 that the earthquake was caused by a slip of the fault referred to, 

 the slip being one that increased its throw ; that the centre of 

 intensity of the seismic focus was at a depth of about 3;| miles ; 

 that the horizontal length of the area over which the slip took place 

 was short, perhaps less than a mile. Remembering the uncertainty 

 in the position of the centre of the sound-area, I think we may infer 

 that, in this earthquake also, the sound- vibrations originated at a 

 part of the fault nearer the surface than the centre of the seismic 

 focus. 



C. Origin of Earthquake-Sounds. 



Within the last few years, the numerous seismographic records 

 made in Japan by Profs. Milne, Ewing and Sekiya have thrown 

 considerable light on the nature of earthquake- vibrations. It is 

 chiefly, however, that part of the series including, and bordering 

 on, the sensible vibrations which has been studied : for, as Prof. 

 Milne remarks, " many earthquakes, like the solar spectrum, have 

 extremities which are ditflcult to investigate." 



The records referred to show that earthquakes usually begin with 

 a series of very small and very rapid tremors, from six to eight 

 occurring every second ; that, after lasting perhaps for many seconds, 

 they become less rapid, and then, without any break of continuity, 

 follow the sensible vibrations of larger amplitude and longer period, 

 at the rate of about three to five per second. One or more of these, 

 attaining an amplitude still greater and having a period of one 

 or two seconds each, constitute what are generally known as the 

 principal shock or shocks. The earthquake closes with vibrations 

 of smaller amplitude, but " which are so long in period, that the 

 pointers and steady points of our seismographs do not give a relative 

 movement, but follow these back and forth movements as a whole, 

 and no record is obtained." On the other hand, they fail to register 

 the commencing tremors of the earthquake on account of their ex- 

 tremely small amplitude. 



Now, for the part of the series preceding and including the 

 principal shocks, the period of the vibrations increases with the 

 amplitude ; and it is therefore not unreasonable to conclude, as 

 Prof. Milne has done, that the first tremors recorded are " the con- 

 tinuation of still smaller and more rapid movements, which on 

 account of want of sufficient multiplication in our instruments have 

 never yet been rendered visible." And it is to these supposed very 

 rapid vibrations, which form the front portion of an advancing earth- 

 quake, that Prof. Milne attributes the origin of the earthquake- sounds. 

 Summing up, he says : " The majority of earthquake-sounds are 

 produced by short period surface vibrations of the earth and these 

 vibrations are portions of and continuous with the earthquake that 

 accompanies the sound." ^ 



^ Japan Seismal Soc. Trans, vol. xii. pp. 107 and 60. 



