58 Dr. C. Davison on Earthquake- Sounds, 



Lastly, a very remarkable series of earth-sounds is that 

 described by Humboldt as the subterranean thunder of 

 Gruanaxuato, a city on the Mexican plateau far removed from 

 any active volcano. " The noise," he says, " began about 

 midnight on the 9th of January, 1784, and continued for a 

 month . . . , From the 13th to the 16th of January, it seemed 

 to the inhabitants as if heavy clouds lay beneath their feet, from 

 which issued alternate slow rolling sounds and short quick 

 claps of thunder. The noise abated as gradually as it had 

 begun. It was limited to a small space and was not heard in 

 a basaltic district at a distance of a few miles." " Neither 

 on the surface of the earth, nor in mines 1600 feet in depth 

 was the slightest shock to be perceived. No similar noise 

 had ever before been heard on the elevated table-land of Mexico, 

 nor has this terrific phenomenon since occurred there " *. 



From the above examples, and from others that might be 

 quoted, we may infer : (1) that earth-sounds especially cha- 

 racterize those districts where slight shocks are frequent ; 

 (2) that in the midst of a series of earth-sounds one or more 

 slight shocks, accompanied by precisely similar sounds, are 

 occasionally intercalated f; (3) that when moderate shocks 

 are felt, the earth-sounds are heard at places where the motion 

 is frequently vertical, and, when strong shocks are felt, at 

 places within the epicentral districts ; and (-1) that in any 

 one district, that of Coinrie for example, there is a complete 

 continuity from earthquake to earth- sound ; every stage of 

 the process is before us, from the strong earthquake in which 

 the disturbed area extends in all directions beyond the sound- 

 area, through the weak earthquake, in which the relations of 

 the areas are reversed, down to the earth-sound, when the 

 shock itself is imperceptible. We may therefore conclude 

 that earthquakes and earth-sounds are manifestations, differing 



* Cosmos (Bonn's edition), vol. i. pp. 203, 205-6. 



f It is worthy of notice that the sound in these cases is generally of 

 short duration. For instance, the earth-sounds following the Comrie 

 earthquake of 1839 are described by Drummond as "gas explosions.'' 

 Prof. Omori also remarks (Journ. Coll. Sci. Imp. Univ. Japan, vol. vii. 

 pt, 2, 1894, pp. 197-198) that " many of the after-shocks of the Mino- 

 Owari earthquake were attended with sounds, which were essentially of 

 two types, being either rushing feeble noises like those caused by winds, 

 or loud rumbling sounds like those caused by the fall of a heavy weight on 

 the ground, or by the discharge of a gun. The sounds of the second type, 

 which were sometimes like detonations of thunder, were most frequent 

 and distinct in the Neo Valley [the epicentral district], where, as I 

 believe, their origin really was. It is remarkable that tremblings of the 

 ground accompanying these sounds were invariably very feeble, and often 

 not to be felt at all." 



