May 20, 1887.] 



SCIJEJNCE. 



493 



the surface intensity varies along a line radiating 

 from the epicentre. 



The first noteworthy featm-e of this curve is the 

 contrast between the rapidity with which the in- 

 tensity diminishes near the epicentre, and the 

 slowness with which it diminishes at remote dis- 

 tances. Thus, at a distance from the epicentre 

 equal to the depth of the focus, the intensity has 

 fallen to one-half, at twice this distance it has fal- 

 len to one-fifth, and at three times the distance to 

 one- tenth, of the intensity at the epicentre. This 

 suggests at once the possibility of making an ap- 

 proximate estimate of the depth of the focus, 

 based upon the rate at which the intensity of the 

 shock at the surface diminishes in the neighbor- 



the same, while the depth of the focus varies. 

 The first series of curves (fig. 2) will enable us to 

 make a comparison of the effect of two or more 

 shocks of the same total energy, but originating 

 at different depths. The intensity at the epicentre 

 being inversely proportional to the square of the 

 depth, the shallower shock would be much more 

 energetic than the deeper one ; while at a great 

 distance from the epicentre the two would be ap- 

 proximately equal in their effects. The rate of 

 diminution of intensity would be correspondingly 

 varied, and we might commit large errors in es- 

 timating these ratios on the ground, while the 

 error of the depth deduced for the focus would 

 be less than our errors of estimate. In short, the 



3 s 



-Energy constant, depth varying in ratios 1, 2, 3, and 4. 



Fig. 4.— Depth and energy both variable, 

 9 but with constant intensity at the 

 epicentre. 



hood of the epicentre. If we were able to con- 

 struct uj)on any arbitrary scale whatever a series 

 of iooseismal curves around the central parts of 

 the earthquake with any approach to accuracy, 

 this depth would follow at once from the relations 

 of these isoseismals to each other. In the case of 

 a very powerful earthquake in a region which is 

 so fiat and uniform in its features as the vicinity 

 of Charleston, this can be done with a rough ap- 

 proach to accuracy. 



To appreciate more fully the validity of this 

 mode of reasoning, let us take a series of these 

 intensity curves, and vary the values of the con- 

 stants. And first let us suppose the total energy 

 of the shock, measured by the constant a, remains 



method is not sensitive to small or moderate errors 

 of observation. 



The second series of curves (fig. 3) is conditioned 

 upon the assumption that the depth remains con- 

 stant, while the energy of the shock varies. In 

 these curves, the ordinates corresponding to any 

 abscissa are proportional to each other in a simple 

 ratio. In the first series they are proportional to 

 each other in a duplicate ratio. 



The third series (fig. 4) represents the effect of 

 varying both the energy and the depth in such a 

 way that the intensity at the epicentre is con- 

 stant. 



It will appear, therefore, that every shock must 

 have some characteristic intensity curve, depend- 



