DEPTH OF THE EOCUS. 
47 
proposed by different authors can be applied in the present case, 
and that is the method proposed by Major C. E. Dutton. This 
depends upon the assumption that, in a uniform medium, the in- 
tensity varies as the square of the distance from the origin, and it 
is shown that the variation of surface intensity along a horizontal 
line drawn from the epicentre is most rapid at a particular point 
which depends upon the depth of the focus only ; a point also 
where the intensity must be ;| of the raaxinmm intensity at the 
epicentre. The relation between the distance of this point from the 
epicentre and the depth of the focus 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^3. 
On applying this formula to the present earthquake by making 
a section across the epicentral tract near Srimangal at right angles 
to its long axis, it appears that the intensity declines or varies, most 
rapidly at points situated about 6 miles from the epicentral line. 
The focus, therefore, under Balisera valley must lie at a depth below 
the surface of about 6^/3 miles, i.e., between 8 and 9 miles. 
Owing to the fact that buildings are few and scarce over the 
epicentral area it is impossible to fix the point where the surface 
intensity declines most rapidly, with any degree of accuracy, and 
the above estimate of the depth of the focus under the BaUsera 
valley is, at best, only vague ; nevertheless it is the nearest 
approximation that can be made. At the west-north-west end of 
the epicentral area the observations are too few to enable any cal- 
culation to be made, most of the country being low-lying rice-fields 
which at the time of the earthquake were inundated owing to the 
unusually heavy monsoon. 
Owing to the inaccuracy of practically all the observations in 
respect of the time at which the earthquake occurred at different 
places, it is impossible to calculate the depth of the focus by the 
method proposed by Dr. Aug. Schmidt based on the observed 
rates of travel.^ The times observed in different places are obvi- 
ously inaccurate and unreliable. The various Railway companies 
very kindly furnished me with the times at which the earthquake 
was recorded at railway stations, but unfortunately these records 
contain so many obvious inaccuracies that they are useless for the 
purpose required. Consequently the construction of a hodograph is 
impossible, 
i Jahr&shcfl Vcr. f. valcrl. Nalurk. in Wurtlemherg, XLIV, 227 (1890). 
