MOVEMENTS AND DEFORMATIONS 339 



the most prevalent is probably the fracture of rocks and the slipping 

 of strata on each other in the process of faulting. To the same 

 class belong movements due to slumping, which is, in reality, super- 

 ficial faulting. Tremors often attend volcanic eruptions. Such 

 tremors are attributable to the sudden fracture and displacement 

 of rock by the movements of lava, or by the expansion due to rapid 

 and unequal heating. They are often attributed to the sudden 



Fig. 272. Diagram illustrating by closed curves the different rates of propa- 

 gation of seismic tremors from a focus F, and, by lines normal to these, 

 the changing directions of propagation of the wave-front. It will be 

 seen that the paths of propagation curve upwards in approaching the 

 surface. If the lines of emergence, as at E and E', be projected back- 

 wards, as to F', the points of crossing will be below the true focus. 



generation or cooling of steam in underground conduits, crevices, 

 and caverns. Earthquake vibrations also spring from the collapse 

 of the roofs of subterranean caverns. 



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

 quake movements start at depths of less than ten miles, and most 

 of them at depths of less than five. The earlier estimates which 

 placed some of the foci deeper, seem to be defective. The depth 

 of the sources of disturbance is usually estimated by noting the 

 directions in which bodies are thrust during an earthquake, plot- 

 ting these directions, and projecting them backwards to their 

 underground crossings (lines EF', Fig. 272). This only gives a 



