AFTER-SHOCKS AND SPACE-DISTRIBUTION OF SEISMIC WAVES. 3 



is to ascertain the cause of the after-shocks. It being a common 

 rule tliat even phenomena of one and the same category may 

 have different causes, so in the case of earthquakes probably 

 several distinct causes should be recognized, for it is hardly to 

 be supposed that all subterranean disturbances, differing as they 

 do so widely in intensity and in duration, should be referable to 

 any one common mechanical action. To what particular cause 

 or series of causes any earthquake may be due is not, however, 

 a question to be dealt with here. 



It is generally accepted, as a matter of course, that an earth- 

 (|uake depends upon a sudden inpulse due to the internal stress 

 of the earth. But the modus operandi whereby that internal 

 stress manifests itself in an impulse is a problem, which is by 

 no means of little importance but which unfortunately is neglected 

 in most cases where the wave motions of the earthcrust are dis- 

 cussed. Were the earth a cosmic body of perfect elasticity, as 

 it is generally assumed by clever mathematicians, we might surely 

 expect seismic waves to propagate after certain laws deduced by 

 their subtle analysis. 



But, in such a case as the above, how can the initial im- 

 pulse at the seismic centre be excited by the internal stress itself? 

 Ut tetisio, sic vis, and consequently no matter whether we adhere 

 to the Humboldt-Naumann volcanistic view, or to a tectonic 

 hypothesis, as Hoernes called it, or to R. Falb's sideric hypothesis, *■•' 

 so very gentle must be the changing of the earthcrust that, though 

 incessant from day to day, it can really be proved oiilv l.»v 

 means of careful observation. 



To cause an earthquake, the strain must inci'ease per mltum 



* Grundzüge zu einer théorie der Erdbeben und Vulkanausbrüche, 1869. 



