306 REID—SEISMOLOGICAL NOTES. [April 24, 



necessary to produce the phenomena which we know took place at 

 the time of the earthquake. It seems impossible to think that the 

 general shift was sudden ; for we cannot imagine what forces could 

 have produced a sudden displacement, amounting to four or five 

 feet, of a portion of the earth's surface covering thousands of square 

 miles. But we have indubitable evidence, in the foldings of the 

 rock common to all mountain chains, of the slow displacement of 

 large regions to considerable distances ; and unless such a displace- 

 ment were slow enough to allow the rock everywhere to flow 

 viscously and thus adjust itself to its new position, there would be 

 places where the elastic stresses would from time to time be greater 

 than the strength of the rock and ruptures would occur causing 

 earthquakes. 



This view of the case is so entirely in accord with the elastic 

 properties of rock, and with the slow movements of large regions, 

 familiar to geologists, that it commends itself strongly without 

 further argument ; but there is a consideration which seems almost 

 decisive in its favor. In the experiments described we saw that the 

 relative slip at the ruptured surface was exactly equal to the total 

 relative shift of the wooden blocks ; this, of course, was independent 

 of the slow or sudden nature of the shift. The slip on the fault- 

 surface at the time of the California earthquake was about 20 feet; 

 therefore the shift of the more distant regions which brought about 

 the break must have been as great ; but the surveys show that be- 

 tween II. and III., the shift was only 5.8 feet, and between I. and 

 11. , 4.6 feet; that is, in all, only about 10.4 feet since the earliest 

 surveys, some 50 years before the shock. We can therefore say, 

 definitely, that the shift which set up the elastic strains which finally 

 resulted in the earthquake, not only did not wholly take place at 

 the time of the rupture but that even fifty years earlier it had 

 already accumulated to about one half its final amount; that between 

 the I. and II. surveys it increased to about three-quarters of this 

 amount, and that the last quarter was added between the II. and 

 III. surveys. It is hardly possible, in view of this history not to be 

 convinced that the shift accumulated gradually. 



Since the general order of events, that is, the setting up of 

 elastic strains resulting in the rupture of the rocks which preceded 



