6 42 SCIENCE PROGRESS 



Earthquakes are by no means rare in California, but many years 

 have elapsed since there was any considerable movement along 

 the fault in action in 1906. During this long interval we may 

 imagine the forces along the fault as gradually increasing until 

 in that year they were strong enough to overcome the resistances 

 opposed to them. Then with great rapidity, but certainly not 

 instantaneously, the sliding movement of each side took place. 

 Months passed before equilibrium was once more approximately 

 attained. Small shocks, each the result of a minor slip, were at 

 first comparatively frequent. A few still occur from time to 

 time, but we seem almost to have reached another period of 

 quiescence, during which the forces are slowly gathering which 

 in years to come will terminate in yet another violent shock. 



So far as the actual earthquakes are concerned, these periods 

 of quiescence and gradually increasing forces are apt to terminate 

 somewhat suddenly. But, before the critical moment arrived 

 when the crust gave way, its deformation must have already 

 begun. The imaginary line, referred to above as drawn during 

 the early days of quiescence at right angles to the fault, must 

 have shown signs of curvature before its severance along the 

 fault took place. It is not, of course, necessary to draw the 

 whole of this line. A few points upon it at definite intervals 

 apart would be ample. Even the two groups of four stone 

 pillars, erected two on each side of the fault after the earthquake 

 of 1906, would suffice. These pillars were actually placed so as 

 to afford a simple means of measuring fresh displacements along 

 the fault. But they may also be found to furnish evidence of a 

 coming earthquake by a slight and continual increase in the 

 distance between those on opposite sides of the fault. 



It will be obvious that this method of foreseeing earthquakes, 

 for which we are indebted to Prof. H. Fielding Reid, 1 is at 

 present in an early stage of development. Until another 

 earthquake occurs along the same fault we have no conception 

 of the time occupied by the process of preliminary curvature, 

 or whether the displacement occurs as a climax to a rapid 

 increase of curvature. The time involved may be too short 

 to be of practical service. But the method of forecast is well 

 worthy of examination and development. It is quite possible 

 that it may, in course of time, lead to valuable results. 



The California Earthquake of April 18, 1906, Report of the State Earthquake 

 Investigation Commission, vol. ii. 1910, pp. 31-2. 



