EARTHQUAKES 207 



attend many volcanic eruptions, and are attributable to the sudden 

 fracture and displacement of rock by the movements of lava, or by 

 the expansion due to heating. Quakes have also been attributed 

 to the sudden generation or cooling of steam in underground con- 

 duits, crevices, and caverns, and to the collapse of the roofs of 

 subterranean caves. 



Points of origin ; foci. It is probable that nearly all earthquakes 

 start within ten miles of the surface, and most of them within five. 



V\K. 20 v 1 )ia.gram illustrating by closed curves the different rates of propaga- 

 tion of seismic tremors from a focus F, and, by lines normal to these, the changing 

 directions of propagation of the wave-front. Propagation is least rapid toward 

 the surface where rocks are least elastic. The paths of propagation curve upwards 

 in approaching the surface. If the lines of emergence, as at E and E', are projected 

 downward in straight lines to F', the point of crossing will be below the true focus. 

 The line at the top of the Fig. represents the surface of the earth. 



The older calculations which placed some of the foci much deeper, 

 appear to be defective. 



The depth of the sources of disturbance is usually estimated by noting the 

 directions in which bodies at the surface are thrust during an earthquake, plotting 

 these directions, and projecting them backwards to their underground crossings 

 (lines Kl'' and ]'/ !'', Fig. 205). In the case illustrated by Fig. 205, this would 

 place the focus at F'. This method gives only a rude first approximation to the 

 location of the focus, which may be a point, a line, or a plane. The earthquake 

 \v;ivr l ravels out from the focus with unequal velocity in various directions. This 

 is because the rock.varies in density, elasticity, temperature, etc. The aggregate 

 effect of these variations is to make earthquake waves travel more slowly toward 

 the surface than in other directions, and more and more slowly as the surface is 

 approached. This is illustrated by Fig. 205, in which each closed curve connects 



