THE PREVISION OF EARTHQUAKES 645 



two years before the earthquake, they not only became relatively 

 more frequent, but were distributed with some approach to 

 uniformity over the entire fault-system. 



These two properties of the fore-shocks of the Mino-Owari 

 earthquake seem to be prophetic of the coming earthquake. 

 True it is that, so far, they are only known to foreshadow the 

 occurrence of one great earthquake. But there is reason to 

 suppose that they are not so confined. Think, for a moment, 

 of the probable cause of the fore-shocks. For at least four 

 or five years the forces which at last culminated in the great 

 crust-movement of 1891 had been gradually increasing. The 

 contest became one between these growing forces and the 

 resistance to motion along the fault-system. It is improbable 

 that the resistance to motion would be uniform throughout. At 

 different points there would be regions all along the faults 

 within which the resistance to motion was greater than else- 

 where. Until these areas of local resistance were cleared away, 

 there would be no displacement on a great scale. Here and 

 there, then, small slips along the fault would cause a fore-shock, 

 and the effect of the slips in all parts of the fault would be 

 to equalise over the entire fault-system the effective resistance 

 to motion. The further growth of the forces would then 

 precipitate the great displacement all over the fault-system and 

 give rise to the great earthquake to which the fore-shocks 

 pointed. 



Thus we have reason to believe that the increase in seismic 

 activity along a known fault, and the tendency to uniformity 

 in the distribution of that activity along the fault, may be 

 heralds of the great crust-movements which cause disastrous 

 earthquakes. The method, of course, can only be of service 

 in countries in which earthquakes are fairly numerous and 

 occasionally violent, and in those alone in which there exists an 

 efficient system for the observation of earthquakes. Such 

 conditions are satisfied at present in but one county. In the 

 empire of Japan about a thousand earthquakes occur every 

 year. They are so carefully studied that few, if any, escape 

 investigation. Every few years one of considerable violence 

 takes place. There can be no country, therefore, in which the 

 practical prevision of earthquakes can be more readily effected. 1 



1 Quart. Journ. Geol. Soc. vol. 53, 1897, pp. 1-15 ; Gerland's Reitrage zur 

 Geophysik, vol. 12, 1912, pp. 9-15. 



