NATURE OF FOCUS, DEPTH, ETC. 



333 



drawn from the epicentre, is most rapid at a particular point which 

 depends on the depth of the focus only, a point also where the intensity 

 must be f of the maximum intensity at the epicentre. The relation 

 between the two is exhibited by the formula x~q tan 30°, where x is 

 the horizontal distance of the place from the epicentre, and q the depth 

 of the focus. If x is known, then q-=*x v 3. In the diagram, fig. 45, 

 A B represents the surface of the ground, the centrum, and the 

 vertical lines are proportional to the intensities at the several points on 

 A B. The resulting curve is steepest at the point where it touches the 

 I intensity line. 



Fig 45. 



Now, it seems that we may apply this formula to the present earth- 

 quake by making sections across the epicentral tract at right angles to its 



