VOLCANIC EARTHQUAKES II9 
earthquake of 1906, are felt over an average area of 24,500 square 
miles. 
As will be seen from Figure 8, the rapid decline in the intensity 
of the shock is due to the shallowness of the focus. The curves 
marked A and B in Figure 8 represent the intensities of the shock 
at different distances from the origin, A corresponding to a focus 
+ of a mile in depth, B to one at a depth of 2 miles. The curves 
are drawn on the assumptions that the intensity of the shock at 
any point of the surface varies inversely as the square of the 
distance from the focus, and that the impulses are such that the 
intensity of the shocks vertically above both foci is the same. 
Fic. 8.—Diagram illustrating shallowness of foci in volcanic earthquakes 
On these assumptions it follows that the shock that would just 
be perceptible at a distance of 2 miles from the center for the 
shallow focus would be perceptible at a distance of 32 miles for 
the deeper one—that is, the disturbed area of the shock with the 
deeper focus would be 256 times the disturbed area of the other 
shock. 
It follows, then, that the foci of most volcanic earthquakes 
are situated at a very slight depth below the surface, and further 
that the foci vary in depth, the focus of an earthquake like the 
Nicolosi earthquake of 1901 or the Fondo Macchia earthquake of 
tgtzr being much nearer the surface than that of the Albano earth- 
quake of 1906. 
This inference, as to the shallowness of the foci in volcanic 
earthquakes, is supported by two other lines of evidence. In the 
