14 MR. K. D. OLDHAM. OS THE INTERPRETATION [Feb. 1909, 



power of changing its form without breach of continuity, which we 

 speak of as flow and regard as the most conspicuous characteristic 

 of a fluid. 



It is to the outer skin that we must look for the origin of the 

 greater part of the disturbances which we call earthquakes, and 

 usually to a sudden yielding, ordinarily of the nature of fracture, to 

 strains set up in it. Probably all local earthquakes originate in 

 the outer skin ; the strains may owe their origin to slow movements 

 of the underlying crust, but the abrupt yielding and sudden 

 displacements do not descend into it, and such earthquakes, though 

 occasionally of great violence near their origin, are characterized 

 by their localization and produce no impression on the most delicate 

 instruments at small distances outside the seismic area. 



In the case of great earthquakes, like the Californian one of 

 1906, the surface-disturbance is still the immediate result of 

 fracture and yielding of the outer skin, but these are the result and 

 accompaniment of an abrupt yielding of the underlying crust. It 

 is difficult to believe that this yielding can be precisely similar to 

 the fractures which may be and are produced in the surface-rocks; 

 but it is probably analogous in the sense that the ultimate result 

 is the same, and that there is a sudden yielding and displacement 

 of adjoining masses of matter relative to each other. On this 

 hypothesis we have, in great earthquakes, two closely connected and 

 yet distinct disturbances : there is first the dislocation of the outer 

 skin, which gives rise to the surface-shock, and secondly the deep- 

 seated displacement, or bathyseism, which gives rise to the 

 wave-motion, which, propagated to great distances, impresses itself 

 on suitable instruments all over the world and constitutes the 

 teleseism, or world-shaking earthquake. 



It is possible that the downward continuation of the San Andreas 

 fault may pass deep into or even right through the crust of the 

 earth, and have given rise to the deficiency in power of resistance 

 which resulted in a sudden yielding under strain and so produced 

 the Californian earthquake of 1906, but this is by no means 

 necessary. The distribution and variation in amount and direc- 

 tion of the surface-displacements suggest that the yielding of 

 the inner crust followed more or less closely the run of the coast- 

 line, and that the strains in the outer skin were, consequently, 

 greater along this zone than elsewhere : but there is no necessity, 

 either in this zone or farther aAvay, for an exact coincidence between 

 the surface-displacements (which must have been influenced, to a 

 large extent, in direction and amount by local irregularities in the 

 power of resistance of the superficial rocks) and the movements which 

 affected the crust as a whole. 1 



1 [In the description of the surface-displacements, contained in the Report of 

 the State Earthquake Investigation Commission, it is shown that where the out- 

 crop of the San Andreas fault is covered by alluvium, it frequently manifests 

 itself as a series of fractures arranged in echelon and each individually running 

 obliquely to the general direction of the main fault. According to the view 

 developed in the paper, the relation of the displacements in the bathyseism to 

 the San Andreas fault is very similar to that of the displacements along the 

 fault to the obliquely disposed surface-fractures.] 



