C. Davison — Great Japanese Earthquake, 1891. 23 



which have already been attempted. The palaeontologists of Argen- 

 tina must be content for many years more to supplement the pioneer 

 labours of Owen, Burmeister, and others, by the plodding detailed 

 study of the unique osteological collections at their disposal ; and 

 none but those who have seen the material can appreciate thoroughly 

 the overwhelming; nature of this task. 



IV. — On the Effect of the Great Japanese Earthquake of 



1891 ON THE Seismic Activity of the Adjoining Districts. 



By Charles Davison, Sc.D., F.G.S. ; 



King Edward's High School, Birmingham. 



THE problem I have tried to solve in this paper is, v^hether 

 the Mino-Owari, or great Japanese, earthquake of 1891 had 

 any effect on the seismic activity of the surrounding regions. The 

 effect we might anticipate would be either an increase or decrease 

 in the frequency of shocks in those regions soon after the occurrence 

 of the principal earthquake — a decrease if the stress were thereby 

 diminished over the whole district, an increase if the stress were 

 relieved in the central area and partially transferred to those 

 adjoining it. 



But, if such an increase, for example, were shown to take place, 

 it would not necessarily be a direct consequence of the great 

 earthquake. Both the central and the adjacent shocks, it is possible, 

 might result from a general increase of stress over a wide extent of 

 country, and the augmented frequency of the latter could not with 

 justice be regarded as an undoubted effect of the former, for they 

 might both be effects of the same cause. But, that the connection 

 in the present case is one of real dependence, is very probable for 

 the following reasons: — (1) An increase of stress cannot determine 

 the occurrence of an earthquake, unless it be sufficient to overcome 

 the resistance to displacement. Now, it is unlikely that the gradual 

 increase of stress should be so nearly proportioned everywhere to 

 the prevailing conditions of resistance as to give rise to a marked 

 and practically simultaneous change in seismic activity over a large 

 area ; whei'eas the paroxysmal occurrence of a strong earthquake 

 might alter the surrounding conditions with comparative rapidity, 

 and so induce a state of seismic excitement in the neighbourhood. 

 (2) The remarkable fault-scarp of the Mino-Owari earthquake was 

 at least foi-ty, and possibly seventy, miles in length ; the vertical 

 displacement reached 18 or 20 feet, and the horizontal displacement 

 as much as 5 or 6 feet. A more serious change was " the 

 permanent compression of ground, plots which were 48 feet in 

 length now measuring only 30 feet in length. It appears as if 

 the whole Neo Valley had become narrower.'" It is clear that 

 crustal distortions of this kind and magnitude could not be effected 

 without a very considerable change of stress in all the surrounding 

 country. 



My object now is to ascertain how far this change of stress 

 manifested itself by more frequent shocks, and for this purpose 

 1 Prof. J. Milne, Brit. Assoc. Rep. 1892, p. 117. 



