[ 332 ] 



XXX. Proceedings of Learned Societies, 



GEOLOGICAL SOCIETY. 



[Continued from p. 228.] 



December 2nd, 1908.— Prof. W. J. Sollas, Sc.D., LL.D., F.R.S., 

 President, in the Chair.; 



HHHE following communication was read : — 



'The Geological Interpretation of the Earth-Movements asso- 

 ciated with the Calif ornian Earthquake of April 18th, 1906.' By 

 Richard Dixon Oldham, F.G.S. 



At the time of the San Francisco earthquake movement took 

 place along a fault, known as the San Andreas Fault, which can be 

 traced for a distance of about 200 miles. A remeasurement of the 

 primary triangulation in the region shaken by the earthquake 

 revealed considerable displacements, increasing in amount as the 

 fault is neared, and of such nature that places to the east of the fault 

 were shifted southwards while those to the west of it were shifted 

 northwards. The author points out that the extent and peculiar 

 distribution of these displacements negative the supposition that 

 the fault was the cause — it must rather be regarded as a con- 

 sequence of, or an incident in, the earthquake, this word being used 

 to denote the disturbance in its entirety. 



He also considers that the displacements cannot be explained in a 

 satisfactory manner on the supposition that they are the result of 

 strains affecting the crust of the earth as a whole, but may be 

 explained by the difference in character and behaviour of the 

 materials composing the greater part of it, where pressures are 

 great enough to produce the phenomena of solid flow, and of those 

 in the outer skin, where the pressures are not great enough to 

 produce any. material difference in the behaviour of rocks from that 

 which we associate with solidity, as experienced at the surface of 

 the earth. The surface-displacements constituting the earthquake, 

 as ordinarily understood, arise from disturbances in the outer skin ; 

 but in great earthquakes, like the one dealt with in the paper, 

 these may be the result of more deep-seated disturbances affecting 

 the whole crust of the earth. A distinction is drawn between these 

 two forms of disturbance, and the term bathyseism is proposed 

 for the deep-seated disturbance : the wave-motion which impresses 

 itself on distant seismographs and constitutes the teleseism or 

 world-shaking earthquake being the product of the bathyseism. 



The deep-seated cause, or bathyseism, of the San Francisco 

 earthquake is regarded as the result of a widespread strain, of the 

 nature of a shear, such as might have been produced by displace- 

 ments approximately parallel to the general direction of the coast- 

 line, and by forces which must have been very different from those 

 concerned in the formation of the San Andreas Fault. This fault 

 cannot, consequently, be regarded as the cause of the earthquake, 

 nor the earthquake as an incident in the growth of the fault. 



