MOVEMENTS AND DEFORMATIONS OF THE EARTH'S BODY. 533 



Sequences of vibrations. — Near the source, the main shocks are apt 

 to come suddenly and to be followed b}^ minor tremors. At a distance 

 there are usually " preliminary " vibrations followed by the main tremors, 

 and these by others of gradually diminishing value. This development 

 is assigned to different rates of propagation, and to refractions and 

 reflections not unlike the prolongation of thunder (see Fig. 448). 

 This deployment of the vibrations is notably developed in the shocks 

 that pass long distances through the earth. The vibrations of the first 

 phase are regarded as compressional, those of the second as distortional, 

 while the largest oscillations which arise still later perhaps come around 

 the surface, and may be undulatory, though their nature is not yet 

 determined. 



There is often, however, a true succession of original shocks caused 

 by a succession of slips or ruptures at the source. Sometimes these are 

 exceedingly persistent, running through days, weeks, or even months. 

 In such cases a slow faulting is probably in progress, and little slips 

 and stops follow in close succession. In one instance as many as 600 

 shocks in ten days have been reported.^ 



Gaseous emanations. — Vapors and gas frequently issue from earth- 

 quake rents, and are popularly made to serve as causes, but they are 

 usually merely the earth gases that are permitted to escape by the rend- 

 ing of the ground, or are forced out by readjustment of the shaken 

 beds. Like other subterranean gases, they are often sulphurous, and 

 they are sometimes hot, especially in volcanic regions. Where the 

 shocks are connected with eruptions, the gases may be truly volcanic. 



Distribution of earthquakes. — Over large portions of the globe, severe 

 earthquakes are exceedingly rare, but in certain regions the}^ are unfor- 

 tunately frequent. For the most part, these are volcanic districts, but 

 this is by no means a universal relation. Earthquakes and volcanoes 

 are only in part associates. In general, it may be said that earthquakes 

 are frequent where geologic changes are in rapid progress, as along 

 belts of young mountains, where the stresses are not yet adjusted, or 

 at the mouths of great streams, where deltas are accumulating, or about 

 volcanoes, where temperatures and strains are changing, or on the great 

 slopes, particularly the submarine slopes, where readjustments in 

 response to inequalities of surface stress are In progress. Not a few, 

 however, occur where the special occasion is not at all obvious. 



* Milne. The Geog. Jour., Vol. XXI, p. 1. See also Seismology, a more technical 

 work than the same author's Earthquakes. 



