354 ELEMENTARY GEOLOGY 
If the jar started from a point, the wave would move 
outward in all directions, maintaining a spherical form. 
In fact, this is never the case in nature, for the wave 
usually starts from a plane, and passes through rocks 
of varying density, so that from pomt to point it is 
liable to change its rate of progression. 
Assuming a wave to be nearly spherical, it passes 
through the rocks just as a jar would move through a 
piece of steel, suspended and 
heavily struck (Fig. 216). 
The place from which the 
wave starts, 1s the focus of 
the earthquake, and a point 
directly above this, where 
the wave first reaches the 
Fic. 216. 
Diagram to illustrate progression of surface, is called the epr- 
earthquake wave, from focus (fF). 
Epicentrum /; isoseismals AB’ 
and BA’. The shock reaches B 
centrum. 
This is the place 
of greatest violence; and if 
and A’ at exactly the same time. : 
we have the theoretical con- 
ditions, the violence gradually and uniformly decreases 
in all directions outward from this point. Also, start- 
ing from the epicentrum as the centre, and describing 
a circle with any radius, this will pass through points 
at which the shock simultaneously reaches the surface. 
This circle is the section of a spherical wave cut by the 
surface of the earth. 
In reality, this ideal condition is not exactly repro- 
