190S] on Recent Earthqualces. 143 



however, may have given relief to seismic strain, or both effects may 

 arise from some common cause. 



Mass displacements accompanying a megaseismic effort must, 

 however, tend to produce some pole displacement, and thus set up 

 strains. From time to time these should find relief in the weaker 

 portions of the earth's crust. Large earthquakes should therefore 

 occur in pairs, triplets or in groups, after which we should expect a 

 period of quiescence. This idea is due to the Rev. H. V. Gill, S.J. 

 T find that the British Association registers lend considerable support 

 to the hypothesis. The author of the idea, however, goes a step 

 farther, and points out that if all matter within our globe or that 

 which constitutes its crust was equally free to move, the secondary 

 displacement should, with regard to the earth's axis of rotation, be 

 symmetrically located in regard to the position of the primary^ disturb- 

 ance. Out of 126 large earthquakes recorded between 1899 and 1905, 

 I find that 20 of these appear as 10 pairs, the members of each pair 

 being in symmetrically located districts. This may, or may not have 

 been a matter of chance. The observation that a marked relief of 

 seismic strain in one part of the world has frequently been followed 

 by a smaller relief in some distant region, also suggests the idea that 

 earthquake begets earthquake. In my own mind the relationship of 

 earthquake to earthquake has been fairly well demonstrated, but to 

 place the matter beyond the borderland of doubt large earthquakes 

 must be compared in regard to space and time with their kind, with 

 small earthquakes and with volcanic eruptions. All the volcanic 

 eruptions of the West Indies have closely followed on the heels of 

 great earthquakes which have originated, not in the West Indies, but 

 on the neighbouring coasts of Central and South America. One 

 general inference is that the faultings and freckles on the face of our 

 world should have a distribution as symmetrically disposed as wrinkles 

 are on the face of an elderly person. 



Already when speaking about the lengtli of faults which have been 

 created tit the time of large earthquakes, we have indicated at least 

 one dimension of the earth block which has been disturbed. For 

 instance, the earth block which was disturbed at the time of the San 

 Francisco earthquake may have had a length of 400 miles, its breadth 

 might be determined by the width of the country which had been 

 broken up by branching and parallel faults. Harboe suggests that in 

 a rneizoseismic area hidden faults may be assumed to exist along lines 

 drawn half way between pairs of groups of places which have been 

 struck at about the same time. E. D. Oldham attributes the Assam 

 earthquake of 1897 to the sudden shifting of 10,000 square miles of 

 territory over a thrust plain. The molar displacement determined by 

 the method suggested by Harboe would be that 50,000 square miles 

 had been disturbed. The fact that so many earthquakes shake the 

 whole world, or will agitate an ocean like the Pacific for many hours, 

 indicates that the initial impulse must have been dehvered over a 

 large area, or that sudden alterations have taken place in the contour 



