Vol. 65.] THE CALIFORNIA]* EARTHQUAKE OF 1906. 11 



frame, fitting closely but without adhesion, and capable of distor- 

 tion out of the square for the purpose of causing compression 

 along one diagonal, while the concomitant lengthening of the other 

 diagonal allows room for expansion of the indiarubber block. In 

 the position represented by fig. 4 the frame is slightly distorted 

 so as to produce compression along the diagonal E-W, the block 

 being free to expand along the diagonal N-S, as shown by the 

 gaps at these corners between the block and the frame ; and in 

 this view may be seen a narrow white band, extending in a straight 

 line across the block between the centres of the two longer sides of 

 the block. On either side of the narrow white band is a broader 

 black band, the purpose of which will become apparent in fig. 5 ; 

 in fig. 4 it is broken at the slit, the upper half being shifted to the 

 left and the lower to the right. Fig. 5 represents the block in 

 another condition, compressed along the diagonal N-S, and free to 

 expand along E-W. The black band, which was broken at the slit 

 in fig. 4, now forms a continuous band across the block, while the 

 white line in the middle of it has become broken. There was no 

 appreciable movement of the outer ends of these bands as between 

 the two extreme positions, so that the edges of the black and white 

 bands, when continuous, represent straight lines j oining two stationary 

 points, on either side of and at a distance from the slit, and a com- 

 parison of the two views illustrates the manner in which the applica- 

 tion of shearing stresses to the block as a whole can produce positive 

 displacements in opposite directions along a line of weakness. A 

 comparison of the two figures shows that, as a result of the alteration 

 of shape from that represented in fig. 4 to fig. 5, the upper half has 

 moved to the right and the lower to the left, the amount of displace- 

 ment increasing progressively as the slit is approached, and this is just 

 what took place along the San Andreas fault-line in California. 



We have, then, an explanation and an illustration of the dis- 

 placements connected with the earthquake of 1906. Neither 

 explanation nor illustration is complete in every respect, but they 

 indicate the kind of strain which preceded and gave rise to the 

 earthquake ; they explain the occurrence of detached areas of 

 increased violence of shock by the formation or movement of 

 independent fissures ; and finally they show that this fault was not, 

 as has been generally thought, the cause, but a consequence, of the 

 earthquake, if the word be used in its fullest sense as covering the 

 whole of the disturbance. 1 



i [In ' Nature,' May 28th, 1908 (vol. lxxviii, p. 78), I pointed out that two distinct 

 forms of disturbance are covered by the ordinary use of the word earthquake, 

 namely — (I) the vibratory movement, of the nature of an elastic wave, which is 

 due to molecular disturbance, and leaves the ground as it was before the earth- 

 quake, apart from damage to buildings or disturbance of the surface-layers 

 which may be produced as a secondary result of the molecular displacements 

 involved in the propagation of the wave-motion ; and (2) molar displacements 

 of solid rock, which are not the result of the wave-motion, and are permanent 

 in the sense that the masses affected do not return to their original position 

 after the earthquake has passed. I proposed that the word earthquake 

 should be limited to the first sense, and that the word earths hake should be 



