CHAPTEE XL 



THE DEPTH OF AN EAKTHQUAKE CENTRUM, 



The depth of an earthquake centrum — Greatest possible depth of an 

 earthquake — Form of the focal cavity. 



Depth of centrum. — The first calculations of the depth at 

 which an earthquake originated were those made by Mallet 

 for the Neapolitan earthquake of 1857. ' These were made 

 on the assumption that the earth wave radiated in straight 

 lines from the origin, and, therefore, at points at diflferent 

 distances from the epicentrum it had different angles of 

 emergence. These angles of emergence were chiefly calcu- 

 lated from the inclination of fissures produced in certain 

 buildings, which were assumed to be at right angles to the 

 direction of the normal motion. If we have determined 

 the epicentrum of an earthquake and the muzoseismal 

 circle, and make either the assumption that the angle of 

 emergence in this circle has been 45° or 54° 44' 9" (see 

 page 54), it is evidently an easy matter by geometrical 

 construction to determine the depth of the centrum, 

 Hofer followed this method when investigating the earth- 

 quake of Belluno. 



Other methods of calculation which have been employed 

 are based on time observations, as, for instance, the method 

 of Seebach, the method of co-ordinates, the method of hyper- 

 boloids or spheres (see pages 200-212). 



By means of a number of lines parallel to twenty-six 



