OF THE DEPTH OP THE SEISMIC FOCUS. 69 



cal, this would give a depth of focus of not less than 80 miles, a result 

 not only absurd in itself, but so totally different from that given by trust- 

 worthy evidence elsewhere, that it cannot be the true angle of emergence 

 due to the distance from the seismic vertical. What then causes this 

 high angle of emergence ? 



It will be remembered that in the neighbourhood of Silchar, as else- 

 where, where fissures were formed, there was, at about the level of the 

 river, a thin bed of soft slimy ooze, on which lay a mass of clays 

 about 30 or 40 feet thick. Now when the earthquake*wave travelling 

 from the north, came to the river, all motion in the superficial stratum was 

 extinguished, and the shock was propagated through the deeper lyino* 

 clays under the river bed; but when the river was passed and the 

 wave once more emerged at the surface of the ground, motion had 

 to be transmitted to the upper beds through this layer of ooze. As, how- 

 ever, the clays possessed considerable inertia, and as the ooze could 

 oppose but a comparatively small resistance to change of shape, the full 

 amount of motion could not be transmitted at once, and so the velocity 

 of wave-particle could not, near the river, reach its full amount ; fur- 

 ther, this ooze, acting as a lubricant, would transmit less of the hori- 

 zontal than of the vertical component of the motion, and consequently 

 the angle of emergence would superficially and locally be increased. 

 Supposing the motion of the wave-particle to be resolved into two com- 

 ponents, one vertical and the other horizontal, and calling the first u and 

 the last h, then 



u : h : : sin. s : cos. e 



Now, in the case of an emergence of 4-5° 



u=h 

 and in the case of an emergence of 20°, which is close upon the true 

 angle of emergence as deduced from the depth of the focus and the 

 distance from the seismic vertical — 



u : h : : 342 : 940 



If we suppose the whole of the vertical component to be transmitted 



( 69 ) 



