Vol. 6] Eeid: The Elastic-Rebound Theory of Earthquakes. 431 



of no strain under the action of their own elastic stresses, the 

 amount of the sudden rebound diminishing as the distance from 

 the fault-line increased, and being no longer measurable at a 

 distance of less than six miles from it. 1 



The summary above describes all the mass movements that 

 occurred at the time of the earthquake in a zone about fifty-six 

 miles wide, and excludes from that zone the movements of blocks 

 of the crust as a whole. The gradual diminution in intensity 

 of the disturbance to the eastward of the zone, with the exception 

 of alluvial tracts, where the terrane caused an increase, indicates 

 that no fractures occurred to the eastward; and the intensity 

 at the Farallon Light House shows that no fracture occurred for 

 some distance west of it. As far, therefore, as negative evidence 

 goes, no block movements occurred ; and, indeed, at the time of 

 the earthquake all the known movements can be accounted for 

 without assuming them. Moreover, the lack of dislocations on 

 other faults not far from the San Andreas fault is a very strong 

 argument, from an observational standpoint, against block move- 

 ments at the time of the California earthquake. 



A great amount of energy was set free at the time of the 

 earthquake; the law of the conservation of energy points out 

 that it was not created at that time, but must have existed earlier 

 in a potential form. The sudden earth movements were practi- 

 cally horizontal and, therefore, it could not have been in the 

 form of gravitational energy, the energy which would have been 

 liberated if a block of the earth's crust had suddenly sunk. But 

 we have seen that the earth movements were merely elastic re- 

 bounds, and therefore the energy must have been in the form 

 of potential energy of elastic strain ; a form nearly related to 

 the energy stored in the spring when a clock is wound up, or 

 the energy in a bow when the arrow is drawn back, ready to 

 fly. It is an easy matter to calculate the amount of energy 

 contained in the rock in the form of elastic deformation by the 

 work done as the two sides of the fault flung back to positions 

 of equilibrium. This equals the total elastic force multiplied 



i This conception of the causes of the California earthquake of 1906 was 

 first stated by Professor Lawson in the Report of the California Earthquake 

 Commission, vol. 1, pp. 147-151. It was developed further in the second 

 volume of the report. 



