Correspondence — Br. S. J. Johnston- Lavis. 281 



tbeir production is almost simultaneous, but the smaller sound 

 vibrations travel at a greater rate than the larger mechanical ones. 



The fact that the more destructive the earthquake, the less marked 

 proportionally is the intensity of the sounds is easily explicable. 

 The sound vibrations are more quickly used up in traversing a given 

 thickness of rock, whilst the mechanical vibrations have hardly been 

 influenced in the short distance travelled in the shallow focussed 

 shocks that constitute the majority of the destructive earthquakes. 

 For the same reason of the more rapid destruction of the sound 

 vibrations by the rocks travei'sed the seismic area of sounds is 

 much more limited than that of the quakes. It must also be 

 remembered that during destructive earthquakes much of the noise 

 is due to cracking and falling buildings, shaking trees, etc. 



As to the cause of earthquake-sounds I believe they are very 

 various in different earthquakes, and even in any one earthquake. 

 Mr. Davison speaks only of fault friction, but rather neglects the 

 actual initial fracture, which we should expect would produce a very 

 loud noise. Next comes rock-crushing, such a common phenomenon 

 in any mountain region, especially along the central ridges and 

 troughs of anticlines and synclines. Then again we have to con- 

 sider the fracturing or splitting of rock by the formation of igneous 

 dykes, which may occur in a region free from surface volcanic 

 phenomena. Is it possible that the hundreds of dykes that rent the 

 old rocks of the northern counties of England and Scotland, and 

 most of which never reached the surface, were not accompanied in 

 their formation by earthquakes and earth- sounds. 



The origin of these sounds is no doubt the smaller vibrations 

 produced by the mechanical disturbances in fracturing and slipping 

 or grating in the tectonic earthquakes. In the case of volcanic or 

 plutonic shocks the sound is in the first place due to splitting and 

 fracturing of the solid rocks. It is then followed by the friction 

 of the injected fluid magma, and the sudden sharp arrest of this 

 against the walls of the cleft. The phenomenon is very similar to 

 the sounds produced by suddenly pumping water into a collapsed 

 leather hose-pipe, closed at the opposite outlet. We have in such 

 a case first a gentle rush followed by a sharp snack as the water 

 is arrested by the fully distended walls. Very similar sound- 

 phenomena may be heard on closing sharply a tap through which 

 water, under considerable pressure, is flowing. There is yet another 

 source of sound in such earthquakes, and that is the vesiculation of 

 any aquiferous magma when allowed to expand through a newly 

 formed fissure just filled by it.^ All these sounds are practically 

 simultaneously produced, and their combined effect with the pre- 

 dominance of one or another would explain the variable nature of 

 the audible phenomena of an earthquake. 



The mechanism of production of earthquake sounds I fully dis- 

 cussed years ago,^ whilst experimental researches on this question 



^ This may possibly explain the boiling cauldron sound so often mentioned in 

 earthquake descriptions. 



- See my monograph of the Earthquakes of Ischia, pp. 82, 89, and Proceed. 

 Eoy. Soc. Dublin, 1886, pp. 120, 124. 



