THE DEPTH OF AN EARTHQUAKE CENTRUM. 225 



Had the origin been more or less of a spherical 

 cavity, then the region of most violent disturbance upon 

 the surface would, according to a theorem we have 

 already mentioned, have been upon or near a circle of 

 about 8*8 miles in radius round the epicentrum. This 

 region, however, was found by observation to lie along a 

 curved band about forty miles in length, altogether on one 

 side of the epicentrum. 



To explain this anomaly Seebach followed Mallet, and 

 assumed that the origin was not a spherical cavity, but a 

 fissure. 



The depth and strike of this fissure was determined 

 by the observation that the area of greatest disturbance 

 was along a curved line lying radial to the epicentrum. 

 Such a condition it was assumed indicated that the 

 fissure of origin must be inclined towards this area of 

 greatest disturbance. A line was then drawn from this 

 area to the centrum, A second line at right angles to 

 this one gave the dip of the fissure. 



Hofer, when working on the earthquake of Belluno, 

 came to the conclusion that the disturbance Originated 

 from two faults meeting each other at an angle of 60°. 

 In this determination he was chiefly influenced by the 

 form of a certain homoseist which was of the form of an 

 elongated ellipse met on one side by a second ellipse, the 

 principal axes of the two ellipses giving the strike of the 

 two faults. 



