Vol. 6] Eeid: The Elastic-Rebound Theory of Earthquakes. 417 



interesting observations and it will only be necessary to sum- 

 marize some of the most important which lead directly to a 

 clearer conception of the forces which produced the shock. 



A few days before the shock Professor Branner's students 

 had been working in the region of the San Andreas fault and 

 after the shock they quickly realized that there had been a new 

 movement on this line. Reports from other places along the 

 same fault showed that displacements had also occurred there, 

 and the more thorough exploration of the whole region brought 

 out the fact that at the time of the earthquake there had been 

 a slip on the San Andreas fault over a distance of 270 miles, 

 in which the two sides of the fault had been displaced relatively 

 to each other by amounts varying from a maximum of twenty- 

 one feet to an unascertained minimum, but which, of course, 

 must have disappeared entirely at the ends of the fault. The 

 movement was practically horizontal and although it is probable 

 that there was some vertical component, varying in different 

 parts of the fault, this has not been made out with satisfactory 

 accuracy. The discovery of this dislocation emphasized the 

 horizontal slip on faults, a fact which, although not unknown 

 before, had not received its merited attention. Text-books on 

 geology practically considered only movements in the vertical 

 plane at right angles to the fault, and gave methods of deter- 

 mining the vertical throw, but none to determine the horizontal 

 movement. The horizontal displacements on the San Andreas 

 fault were proved, without question, by the dislocations and 

 offsets in roads, fences and pipes, where they crossed the line 

 of the fault. Very naturally the cause of the earthquake was 

 ascribed to this sudden movement and we shall see later that it 

 accounts for the disturbance and supplies an abundant amount 

 of energy to explain all the effects produced. 



The field observations showed very clearly the dislocations 

 and offsets at the fault line itself, but they were not competent to 

 show how far from the fault the actual displacement extended. 

 That the displacement gradually became less and less, as the 

 distance from the fault-line increased, seemed probable, because 

 a thorough exploration of the region failed to discover any other 

 lines with offsets similar to those along the San Andreas fault. 



