G6 OLDHAM: THE CACHAR EARTHQUAKE OF 10TH JANUARY 1869. 



CHAPTER V. 



OF THE DEPTH OF THE SEISMIC FOCUS. 



As the determination of the depth of the seismic focus, or in other 

 words the depth at which the shock originated, is of great, and indeed 

 primary, importance in the study of seismology, it is to be regretted that 

 the data from which this point can be determined are of the scantiest, 

 a fact which, as will presently be seen, is the more to be regretted since 

 there is great reason for believing that the seismic focus was situated 

 at an exceptionally great distance below the surface. 



On account of this small number of trustworthy indications of 

 the angle of emergence, it has been deemed advisable to describe and 

 discuss the observations in a separate chapter, and not in the general 

 description under the heads of the places at which they were obtained. 



We will begin by considering the indications at Yeddo, as it is 

 nearest to the seismic vertical, and because there the evidence is most 

 complete and satisfactory. At this station the native hospital, maga- 

 zine, and several bungalows were more or less severely cracked. The 

 cracks in the native hospital building give an angle of emergence of 

 38°, but in one of the bungalows this is as low as 30°, the mean 

 emergence being 34° at this station. These observations are quite the 

 best that are accessible, and may be said to be good, as the buildings were 

 well suited for seismometric purposes, and as there does not seem to have 

 been any disturbing element which would augment or decrease the 

 obliquity of emergence at this place. 



The distance of the station of Yeddo from the seismic vertical 

 being 46 miles by applying the formula — 



D=C tan E. 

 (where D is the depth of focus, C the distance from the seismic vertical, 

 and E the angle of emergence), we obtain the depth of the seismic focus— 



for an emergence of 38° 



D=359 miles. 

 for an emergence of 30° 



D=26'5 miles, 

 and for the mean value 34° 



D=31 miles, 



( 66 ) 



