214 



EARTHQUAKES. 



angles of emergence, drawn in towards the seismic vertical, 

 Mallet found that twenty-three of these intersected at a 

 depth of 7g- geographical miles. The maximum depth was 

 ^8 geographical miles, and the minimum depth 2} geo- 

 graphical miles. 



The mean depth was taken at a depth of 5| geo- 

 graphical miles where, within a range of 12,000 feet, 

 eighteen of the wave paths intersected the seismic ver- 

 tical. 



The point where these wave paths start thickest is at 

 a depth not greater than three geographical miles, and 

 this is considered to be the vertical depth of the focal 

 cavity itself. 



For the Yokohama earthquake of 1880, from the in- 

 dications of seismometers, and by other means, certain 

 angles of emergence were obtained, leading to the conclu- 

 sion that the depth of origin of that earthquake might be 

 between 1^ and 5 miles. 



Possibly, perhaps, the earthquake may have originated 

 from a fissure the vertical dimensions of which was com- 

 prised between these depths. 



A source of error in a calculation of this description is 

 that the vertical motions may have been a component of 

 transverse motions or perhaps due to the slope of surface 

 waves. 



The following table of the depths at which certain 

 earthquakes have originated has been compiled from the 

 writings of several observers. 









111 feet 









Minimum 



Mean 



Maximum 



Rhineland . 



1846 (Schmidst) 





127,309 





Sillien . 



1858 (Schmidst) 





86,173 





Middle Germany . 



1872 (Seebach) 



47,225 



58,912 



70,841 



Herzogenrath 



1873 (Lasaulx) 



16,553 



36,516 



56,477 



Neapolitan . 



1857 (Mallet) 



16,705 



34,930 



49,350 



Yokohama . 



] 880 (Milne) 



7,920 



17,260 



26,400 



