Reports & Proceedings — Geological Society of London. 141 



bathyseisms, but aftershocks form fully three-quarters of the record, 

 and nearly a half consists of aftershocks of the Mino-Owari 

 earthquake of October 28, 1891. Taking these separately, we get 

 a curve of frequency similar to the Italian, except that the maximum 

 and minimum are reversed, the greatest number of shocks corresponding 

 to the period when the load is being lightened most rapidly, 

 indicating that these shocks are due to a general movement of 

 elevation rather than depression, a conclusion in accord with field 

 observations of other great earthquakes. In addition, the shocks 

 which occurred during the period 1885-90 were taken out, as 

 representing a more normal activity, though still one in Avhich 

 aftershocks form fully half of the record, and the curve was found, 

 as might have been expected from the character of the record, to 

 combine the features of the Mino-Owari aftershocks with those of 

 the Italian curve of frequency, of earthquakes prevailingly of the 

 so-called " tectonic" type. 



These results are of twofold geological interest. In the first 

 place they confirm the conclusion drawn from a study of the 

 Californian earthquake of 1906,' that the great earthquakes differ 

 from the ordinary, not merely in degree but in kind. They indicate 

 that in the latter the main stress is compressive, probably due to 

 settlement, and in the former to elevation or tension, a conclusion 

 which is in accord with the fact that, in those cases in which it has 

 been possible to compare accurate measurements made before and 

 after the earthquake, the comparison has indicated an expansion, 

 elevation, or both, of the area affected by the disturbance. 



The second point of interest is that the figures give a means of 

 estimating the rate of growth of the strain which produces earth- 

 quakes. If we accept the hypothesis that earthquakes, in the 

 limited sense of their orchesis, are due to the relief by fracture of 

 a growing strain when this has reached the breaking point, it can be 

 easily shown that a variable strain, acting in alternate periods in 

 increase or decrease of the general growth of strain, while leaving 

 the average rate unaltered, will give rise to a corresponding variation 

 in the frequency of shocks in each period ; and, besides that, there is 

 a simple relation between the magnitudes of the two stresses, to 

 which the strains are due, and the variations from the mean 

 frequency of earthquakes. A calculation on these lines shows that 

 the growth of strain, for Italy, is such that, accepting the published 

 estimates that an area of the earth's crust of the magnitude of Italy 

 would crush under its own weight if left unsupported to the extent of 

 4^0- of the force of gravity, the breaking strain would be reached 

 in about 3^ years, starting from a condition of no strain. The 

 aftershocks of the Mino-Owari earthquake give a little less than half 

 this figure, which is again reduced to from five to six months if 

 account is taken of the difference between the resistance of rock to 

 tension and to compression. These figures are given for what they 

 are worth; at the least, they are of interest as being the first 

 authentic estimate which it has been possible to make of the time 

 required to prepare for, and, thence, of the rate of growth of the 

 particular tectonic process involved in the production of earthquakes. 

 ^ Q.J.G.S., vol. Ixv, p. 14, 1909. 



